Le Positivisme anglais/1/8

1864 (p. 69-85).

◄ VII

IX ►

bookLe Positivisme anglaisHippolyte Taine1864ParisVTaine - Le Positivisme anglais, 1864.djvuTaine - Le Positivisme anglais, 1864.djvu/1069-85

VIII

Ce sont là des formules, un fait sera plus clair. En voici un : vous allez voir les méthodes en exercice ; il y a un exemple qui les rassemble presque toutes. Il s’agit de la théorie de la rosée du docteur Well. Je cite les propres paroles de Mill ; elles sont si nettes, qu’il faut vous donner le plaisir de les méditer.

« Il faut d’abord distinguer la rosée de la pluie aussi bien que des brouillards, et la définir en disant qu’ « elle est l’apparition spontanée d’une moiteur sur des corps exposés en plein air, quand il ne tombe point de pluie ni d’humidité visible.^[1] » La rosée ainsi définie, quelle en est la cause, et comment l’a-t-on trouvée ?

« D’abord, nous avons des phénomènes analogues dans la moiteur qui couvre un métal froid ou une pierre lorsque nous soufflons dessus, qui apparaît en été sur les parois d’un verre d’eau fraîche qui sort du puits, qui se montre à l’intérieur des vitres quand la grêle ou une pluie soudaine refroidit l’air extérieur, qui coule sur nos murs lorsqu’après un long froid arrive un dégel tiède et humide. — Comparant tous ces cas, nous trouvons qu’ils contiennent tous le phénomène en question. Or, tous ces cas s’accordent en un point, à savoir, que l’objet qui se couvre de rosée est plus froid que l’air qui le touche. Cela arrive-t-il aussi dans le cas de la rosée nocturne ? Est-ce un fait que l’objet baigné de rosée est plus froid que l’air ? Nous sommes tentés de répondre que non, car qui est-ce qui le rendrait plus froid ? Mais l’expérience est aisée : nous n’avons qu’à mettre un thermomètre en contact avec la substance couverte de rosée, et en suspendre un autre un peu au-dessus, hors de la portée de son influence. L’expérience a été faite, la question a été posée, et toujours la réponse s’est trouvée affirmative. Toutes les fois qu’un objet se recouvre de rosée, il est plus froid que l’air^[2].

« Voilà une application complète de la méthode de concordance : elle établit une liaison invariable entre l’apparition de la rosée sur une surface et la froideur de cette surface comparée à l’air extérieur. Mais laquelle des deux est cause, et laquelle effet ? ou bien sont-elles toutes les deux les effets de quelque chose d’autre ? Sur ce point, la méthode de concordance ne nous fournit aucune lumière. Nous devons avoir recours à une méthode plus puissante : nous devons varier les circonstances, nous devons noter les cas où la rosée manque ; car une des conditions nécessaires pour appliquer la méthode de différence, c’est de comparer des cas où le phénomène se rencontre avec d’autres où il ne se rencontre pas^[3].

« Or la rosée ne se dépose pas sur la surface des métaux polis, tandis qu’elle se dépose très-abondamment sur le verre. Voilà un cas où l’effet se produit, et un autre où il ne se produit point.... Mais, comme les différences qu’il y a entre le verre et les métaux polis sont nombreuses, la seule chose dont nous puissions encore être sûrs, c’est que la cause de la rosée se trouvera parmi les circonstances qui distinguent le verre des métaux polis^[4]... Cherchons donc à démêler cette circonstance, et pour cela employons la seule méthode possible, celle des variations concomitantes. Dans le cas des métaux polis et du verre poli, le contraste montre évidemment que la substance a une grande influence sur le phénomène. C’est pourquoi faisons varier autant que possible la substance seule, en exposant à l’air les surfaces polies de différentes sortes. Cela fait, on voit tout de suite paraître une échelle d’intensité. Les substances polies qui conduisent le plus mal la chaleur sont celles qui s’imprègnent le plus de rosée ; celles qui conduisent le mieux la chaleur sont celles qui s’en humectent le moins^[5]] : d’où l’on conclut que « l’apparition de la rosée est liée au pouvoir que possède le corps de résister au passage de la chaleur. »

« Mais si nous exposons à l’air des surfaces rudes au lieu de surfaces polies, nous trouvons quelquefois cette loi renversée. Ainsi le fer rude, particulièrement s’il est peint ou noirci, se mouille de rosée plus vite que le papier verni. L’espèce de surface a donc beaucoup d’influence. C’est pourquoi exposons la même substance en faisant varier le plus possible l’état de sa surface (ce qui est un nouvel emploi de la méthode des variations concomitantes), et une nouvelle échelle d’intensité se montrera. Les surfaces qui perdent leur chaleur le plus aisément par le rayonnement sont celles qui se mouillent le plus abondamment de rosée.^[6] On en conclut « que l’apparition de la rosée est liée à la capacité de perdre la chaleur par voie de rayonnement. »

« A présent l’influence que nous venons de reconnaître à la substance et à la surface nous conduit à considérer celle de la texture, et là nous rencontrons une troisième échelle d’intensité, qui nous montre les substances d’une texture ferme et serrée, par exemple les pierres et les métaux, comme défavorables à l’apparition de la rosée, et au contraire les substances d’une texture lâche, par exemple le drap, le velours, la laine, le duvet, comme éminemment favorables à la production de la rosée. La texture lâche est donc une des circonstances qui la provoquent. Mais cette troisième cause se ramène à la première, qui est le pouvoir de résister au passage de la chaleur, car les substances de texture lâche sont précisément celles qui fournissent les meilleurs vêtements, en empêchant la chaleur de passer de la peau à l’air, ce qu’elles font en maintenant leur surface intérieure très-chaude, pendant que leur surface extérieure est très-froide^[7].

» Ainsi les cas très-variés dans lesquels beaucoup de rosée se dépose s’accordent en ceci, et, autant que nous pouvons l’observer, en ceci seulement, qu’ils conduisent lentement la chaleur ou la rayonnent rapidement, — deux qualités qui ne s’accordent qu’en un seul point, qui est qu’en vertu de l’une et de l’autre le corps tend à perdre sa chaleur par sa surface plus rapidement qu’elle ne peut lui être restituée par le dedans. Au contraire, les cas très-variés dans lesquels la rosée manque ou est très-peu abondante s’accordent en ceci, et, autant que nous pouvons l’observer, en ceci seulement, qu’ils n’ont pas cette propriété. Nous pouvons maintenant répondre à la question primitive et savoir lequel des deux, du froid et de la rosée, est la cause de l’autre. Nous venons de trouver que la substance sur laquelle la rosée se dépose doit, par ses seules propriétés, devenir plus froide que l’air. Nous pouvons donc rendre compte de sa froideur, abstraction faite de la rosée, et, comme il y a une liaison entre les deux, c’est la rosée qui dépend de la froideur ; en d’autres termes, la froideur est la cause de la rosée^[8].

» Maintenant cette loi si amplement établie peut se confirmer de trois manières différentes. Premièrement, par déduction, en partant des lois connues que suit la vapeur aqueuse lorsqu’elle est diffuse dans l’air ou dans tout autre gaz. On sait par l’expérience directe que la quantité d’eau qui peut rester suspendue dans l’air à l’état de vapeur est limitée pour chaque degré de température, et que ce maximum devient moindre à mesure que la température diminue. Il suit de là déductivement que, s’il y a déjà autant de vapeur suspendue dans l’air que peut en contenir sa température présente, tout abaissement de cette température portera une portion de la vapeur à se condenser et à se changer en eau. Mais, de plus, nous savons déductivement, d’après les lois de la chaleur, que le contact de l’air avec un corps plus froid que lui-même abaissera nécessairement la température de la couche d’air immédiatement appliquée à sa surface, et par conséquent la forcera d’abandonner une portion de son eau, laquelle, d’après les lois ordinaires de la gravitation ou cohésion, s’attachera à la surface du corps, ce qui constituera la rosée… Cette preuve déductive a l’avantage de rendre compte des exceptions, c’est-à-dire des cas où, ce corps étant plus froid que l’air, il ne se dépose pourtant point de rosée : car elle montre qu’il en sera nécessairement ainsi, lorsque l’air sera si peu fourni de vapeur aqueuse, comparativement à sa température, que môtae, étant un peu refroidi par le contact d’un corps plus froid, il sera eucore capable de tenir en suspension toute la vapeur qui s’y trouvait d’abord suspendue. Ainsi, dahs un été très-sec, il n’y a pas de rosée, ni dans un hiver très-sec de gelées blanches^[9].

» La seconde confirmation de la théorie se tire de l’expérience directe pratiquée selon la méthode de différence. Nous pouvons, en refroidissant la súrface de n’importe quel corps, atteindre en tous les cas une température à laquelle la rosée commence à se déposer. Nous ne pouvons, à la vérité, faire cela que sur une petite échelle ; mais nous avons d’amples raisons pour conclure que la même opération, si elle était conduite dans le grand laboratoire de la nature, aboutirait au même effet.

« Et finalement nous sommes capables de vérifier le résultat, même sur cette grande échelle. Le cas est un de ces cas rares où la nature fait l’expérience pour nous de la même manière que nous la ferions nous-mêmes, c’est-à-dire en introduisant dans l’état antérieur des choses une circonstance nouvelle, unique et parfaitement définie, et en manifestant l’effet si rapidement, que le temps manquerait pour tout autre changement considérable dans les circonstances antérieures. On a observé que la rosée ne se dépose jamais abondamment dans des endroits fort abrités contre le ciel ouvert, et point du tout dans les nuits nuageuses ; mais que, si les nuages s’écartent, fût-ce pour quelques minutes seulement, de façon à laisser une ouverture, la rosée commence à se déposer, et va en augmentant. Ici il est complètement prouvé que la présence ou l’absence d’une communication non interrompue avec le ciel cause la présence ou l’absence de la rosée ; mais puisqu’un ciel clair n’est que l’absence des nuages, et que les nuages, comme tous les corps entre lesquels et un objet donné il n’y a rien qu’un fluide élastique, ont cette propriété connue, qu’ils tendent à élever ou à maintenir la température de la surface de l’objet en rayonnant vers lui de la chaleur, nous voyons à l’instant que la retraite des nuages refroidira la surface. Ainsi, dans ce cas, la nature ayant produit un changement dans l’antécédent par des moyens connus et définis, le conséquent suit et doit suivre : expérience naturelle conforme aux règles de la méthode de différence^[10]. »

↑ “We must separate dew from rain, and the moisture of fogs, and limite the application of the term to what is really meant, which is, the spontaneous appearance of moisture on substances exposed in the open air when no rain or visible wet is falling.”
↑ “Now, here we have analogous phenomena in the moisture which bedews a cold metal or stone when we breathe upon it ; that which appears on a glass of water fresh from the well in hot weather ; that which appears on the inside of windows when sudden rain or hail chills the external air ; that which runs down our walls when, after a long frost, a warm moist thaw comes on.” Comparing these cases, we find that they all contain the phenomenon which was proposed as the subject of investigation. Now “all these instances agree in one point, the coldness of the object dewed, in comparison with the air in contact with it.” But there still remains the most important case of all, that of nocturnal dew : does the same circumstance exist in this case ? “Is it a fact that the object dewed is colder than the air ? Certainly not, one would at first be inclined to say ; for what is to make it so ? But... the experiment is easy ; we have only to lay a thermometer in contact with the dewed substance, and hang one at a little distance above it, out of reach of its influence. The experiment has been therefore made ; the question has been asked, and the answer has been invariably in the affirmative. Whenever an object contracts dew, it is colder than the air.”
↑ Here then is a complete application of the Method of Agreement, establishing the fact of an invariable connexion between the deposition of dew on a surface, and the coldness of that surface compared with the external air. But which of these is cause, and which effect ? or are they both effects of something else ? On this subject the Method of Agreement can afford us no light : we must call in a more potent method. We must collect more facts, or, which comes to the same thing, vary the circumstances ; since every instance in which the circumstances differ is a fresh fact : and especially, we must note the contrary or negatives cases, i.e., where no dew is produced : for a comparison between instances of dew and instances of no dew is the condition necessary to bring the Method of Difference into play.
↑ “Now, first, no dew is produced on the surface of polished metals, but it is very copiously on glass, both exposed with their faces upwards, and in some cases the under side of a horizontal plate of glass is also dewed. ” Here is an instance in which the effect is produced, and another instance in which it is not produced ; but we cannot yet pronounce, as the canon of the Method of Difference requires, that the latter instance agrees with the former in all its circumstances except in one ; for the differences between glass and polished metals are manifold, and the only thing we can as yet be sure of, is, that the cause of dew will be found among the circumstances by which the former substance is distinguished from the latter.
↑ “In the cases of polished metal and polished glass, the contrast shows evidently that the substance has much to do with the phenomenon ; therefore let the substance alone be diversified as much as possible, by exposing polished surfaces of various kinds. This done, a scale of intensity becomes obvious. Those polished substances are found to be most strongly dewed which conduct heat worst, while those which conduct well, resist dew most effectually.”
↑ The conclusion obtained is, that, cœteris paribus, the deposition of dew is in some proportion to the power winch the body possesses of resisting the passage of heat ; and that this, therefore (or something connected with this), must be at least one of the causes which assist in producing the deposition of dew on the surface.

“But if we expose rough surfaces instead of polished, we sometimes find this law interfered with. Thus, roughened iron, especially if painted over or blackened, becomes dewed sooner than varnished paper : the kind of surface, therefore, has a great influence. Expose, then, the same material in very diversified states as to surface” (that is, employ the Method of Difference to ascertain concomitance of variations), “and another scale of intensity becomes at once apparent ; those surfaces which part with their heat most readily by radiation, are found to contract dew most copiously.”
↑ The conclusion obtained by this new application of the method is, that, cœteris paribus, the deposition of dew is also in some proportion to the power of radiating heat ; and that the quality of doing this abundantly (or some cause on which that quality dépends) is another of the causes which promote the deposition of dew on the substance.

“Again, the influence ascertained to exist of substance and surface leads us to consider that of texture : and hère, again, we are presented on trial with remarkable differences, and with a third scale of intensity, pointing out substances of a close firm texture, such as stones, metals, etc., as unfavourable, but those of a loose one, as cloth, velvet, wool, eiderdown, cotton, etc., as eminently favourable to the contraction of dew. The Method of concomitant Variations is here, for the third time, had recourse to ; and, as before, from necessity, since the texture of no substance is absolutely firm or absolutely loose. Looseness of texture, therefore, or something which is the cause of that quality, is another circumstance which promotes the deposition of dew ; but this third cause resolves itself into the first, viz. the quality of resisting the passage of heat : for substances of loose texture are precisely those which are best adapted for clothing or for impeding the free passage of heat from the skin into the air, so as to allow their outer surfaces to be very cold, while they remain warm within.”
↑ It thus appears that the instances in which much dew is deposited, which are very various, agree in this, and, so far as we are able to observe, in this only, that they either radiate heat rapidly or conduct it slowly : qualities between which there is no other circumstance of agreement, than that by virtue of either, the body tends to lose heat from the surface more rapidly than it can be restored from within. The instances, on the contrary, in which no dew, or but a small quantity of it, is formed, and which are also extremely various, agree (so far as we can observe) in nothing, except in not having this same property. This doubt we are not able to resolve. We have found that, in every such instance, the substance must be one which, by its own properties or laws, would, if exposed in the night, become colder than the surrounding air. The coldness therefore, being accounted for independently of the dew, while it is proved that there is a connexion between the two, it must be the dew which depends on the coldness ; or in other words, the coldness is the cause of the dew.
↑ The law of causation, already so amply established, admits, howewer, of efficient additional corroboration in no less than three ways. First, by deduction from the known laws of aqueous vapour when diffused through air or any other gas ; and though we have not yet come to the Deductive Method, we will not omit what is necessary to render the speculation complete. It is known by direct experiment that only a limited quantity of water can remain suspended in the state of vapour at each degree of temperature, and that this maximum grows less and less as the temperature diminishes. From this it follows, deductively, that if there is already as much vapour suspended as the air will contain at its existing temperature, any lowering of that temperature will cause a portion of the vapour to be condensed, and become water. But, again, we know deductively, from the laws of heat, that the contact of the air with a body colder than itself, will necessary lower the temperature of the stratum of air immediately applied to its surface ; and will therefore cause it to part with a portion of its water, which accordingly will, by the ordinary laws of gravitation or cohesion, attach itself to the surface of the body, thereby constituting dew. This deductive proof, it will have been seen, has the advantage of proving at once causation as well as coexistence ; and it has the additional advantage that it also accounts for the exceptions to the occurrence of the phenomenon, the cases in which, although the body is colder than the air, yet no dew is deposited ; by shewing that this will necessarily be the case when the air is so undersupplied with aqueous vapour, comparatively to its temperature, that even when somewhat cooled by the contact of the colder body, it can still continue to hold in suspension all the vapour which was previously suspended in it : thus in a very dry summer there are no dews, in a very dry winter no hoar frost.
↑ The second corroboration of the theory is by direct experiment, according to the canon of the Method of Difference. We can, by cooling the surface of any body, find in all cases some temperature (more or less inferior to that of the surrounding air, according to its hygrometric condition), at which dew will begin to be deposited. Here, too, therefore the causation is directly proved. We can, it is true, accomplish this only on a small scale ; but we have ample reason to conclude that the same operation, if conducted in Nature’s great laboratory, would equally produce the effect.

And, finally, even on that great scale we are able to verify the result. The case is one of those rare cases, as we have shown them to be, in which nature works the experiment for us in the same manner in which we ourselves perform it ; introducing into the previous state of things a single and perfectly definite new circumstance, and manifesting the effect so rapidly, that there is not time for any other material change in the pre-existing circumstances. It is observed that dew is never copiously deposited in situations much screened from the open sky, and not at all in a cloudy night, but if the clouds withdraw even for a few minutes, and leave a clear opening, a deposition of dew presently begins, and goes on increasing… Dew formed in clear intervals will often even evaporate again, when the sky becomes thickly overcast. The proof, therefore, is complete that the presence or absence of an uninterrupted communication with the sky causes the deposition or non-deposition of dew. Now, since a clear sky is nothing but the absence of clouds, and it is a known property of clouds, as of all other bodies between which and any given object nothing intervenes but an elastic fluid, that they tend to raise or keep up the superficial temperature of the object by radiating heat to it, we see at once that the disappearance of clouds will cause the surface to cool ; so that Nature, in this case, produces a change in the antecedent by definite and known means, and the consequent follows accordingly : a natural experiment which satisfies the requisitions of the Method of Difference.

[1] “We must separate dew from rain, and the moisture of fogs, and limite the application of the term to what is really meant, which is, the spontaneous appearance of moisture on substances exposed in the open air when no rain or visible wet is falling.”

[p71-2] “Now, here we have analogous phenomena in the moisture which bedews a cold metal or stone when we breathe upon it ; that which appears on a glass of water fresh from the well in hot weather ; that which appears on the inside of windows when sudden rain or hail chills the external air ; that which runs down our walls when, after a long frost, a warm moist thaw comes on.” Comparing these cases, we find that they all contain the phenomenon which was proposed as the subject of investigation. Now “all these instances agree in one point, the coldness of the object dewed, in comparison with the air in contact with it.” But there still remains the most important case of all, that of nocturnal dew : does the same circumstance exist in this case ? “Is it a fact that the object dewed is colder than the air ? Certainly not, one would at first be inclined to say ; for what is to make it so ? But... the experiment is easy ; we have only to lay a thermometer in contact with the dewed substance, and hang one at a little distance above it, out of reach of its influence. The experiment has been therefore made ; the question has been asked, and the answer has been invariably in the affirmative. Whenever an object contracts dew, it is colder than the air.”

[3] Here then is a complete application of the Method of Agreement, establishing the fact of an invariable connexion between the deposition of dew on a surface, and the coldness of that surface compared with the external air. But which of these is cause, and which effect ? or are they both effects of something else ? On this subject the Method of Agreement can afford us no light : we must call in a more potent method. We must collect more facts, or, which comes to the same thing, vary the circumstances ; since every instance in which the circumstances differ is a fresh fact : and especially, we must note the contrary or negatives cases, i.e., where no dew is produced : for a comparison between instances of dew and instances of no dew is the condition necessary to bring the Method of Difference into play.

[4] “Now, first, no dew is produced on the surface of polished metals, but it is very copiously on glass, both exposed with their faces upwards, and in some cases the under side of a horizontal plate of glass is also dewed. ” Here is an instance in which the effect is produced, and another instance in which it is not produced ; but we cannot yet pronounce, as the canon of the Method of Difference requires, that the latter instance agrees with the former in all its circumstances except in one ; for the differences between glass and polished metals are manifold, and the only thing we can as yet be sure of, is, that the cause of dew will be found among the circumstances by which the former substance is distinguished from the latter.

[5] “In the cases of polished metal and polished glass, the contrast shows evidently that the substance has much to do with the phenomenon ; therefore let the substance alone be diversified as much as possible, by exposing polished surfaces of various kinds. This done, a scale of intensity becomes obvious. Those polished substances are found to be most strongly dewed which conduct heat worst, while those which conduct well, resist dew most effectually.”

[6] The conclusion obtained is, that, cœteris paribus, the deposition of dew is in some proportion to the power winch the body possesses of resisting the passage of heat ; and that this, therefore (or something connected with this), must be at least one of the causes which assist in producing the deposition of dew on the surface.

“But if we expose rough surfaces instead of polished, we sometimes find this law interfered with. Thus, roughened iron, especially if painted over or blackened, becomes dewed sooner than varnished paper : the kind of surface, therefore, has a great influence. Expose, then, the same material in very diversified states as to surface” (that is, employ the Method of Difference to ascertain concomitance of variations), “and another scale of intensity becomes at once apparent ; those surfaces which part with their heat most readily by radiation, are found to contract dew most copiously.”

[7] The conclusion obtained by this new application of the method is, that, cœteris paribus, the deposition of dew is also in some proportion to the power of radiating heat ; and that the quality of doing this abundantly (or some cause on which that quality dépends) is another of the causes which promote the deposition of dew on the substance.

“Again, the influence ascertained to exist of substance and surface leads us to consider that of texture : and hère, again, we are presented on trial with remarkable differences, and with a third scale of intensity, pointing out substances of a close firm texture, such as stones, metals, etc., as unfavourable, but those of a loose one, as cloth, velvet, wool, eiderdown, cotton, etc., as eminently favourable to the contraction of dew. The Method of concomitant Variations is here, for the third time, had recourse to ; and, as before, from necessity, since the texture of no substance is absolutely firm or absolutely loose. Looseness of texture, therefore, or something which is the cause of that quality, is another circumstance which promotes the deposition of dew ; but this third cause resolves itself into the first, viz. the quality of resisting the passage of heat : for substances of loose texture are precisely those which are best adapted for clothing or for impeding the free passage of heat from the skin into the air, so as to allow their outer surfaces to be very cold, while they remain warm within.”

[8] It thus appears that the instances in which much dew is deposited, which are very various, agree in this, and, so far as we are able to observe, in this only, that they either radiate heat rapidly or conduct it slowly : qualities between which there is no other circumstance of agreement, than that by virtue of either, the body tends to lose heat from the surface more rapidly than it can be restored from within. The instances, on the contrary, in which no dew, or but a small quantity of it, is formed, and which are also extremely various, agree (so far as we can observe) in nothing, except in not having this same property. This doubt we are not able to resolve. We have found that, in every such instance, the substance must be one which, by its own properties or laws, would, if exposed in the night, become colder than the surrounding air. The coldness therefore, being accounted for independently of the dew, while it is proved that there is a connexion between the two, it must be the dew which depends on the coldness ; or in other words, the coldness is the cause of the dew.

[p82-9] The law of causation, already so amply established, admits, howewer, of efficient additional corroboration in no less than three ways. First, by deduction from the known laws of aqueous vapour when diffused through air or any other gas ; and though we have not yet come to the Deductive Method, we will not omit what is necessary to render the speculation complete. It is known by direct experiment that only a limited quantity of water can remain suspended in the state of vapour at each degree of temperature, and that this maximum grows less and less as the temperature diminishes. From this it follows, deductively, that if there is already as much vapour suspended as the air will contain at its existing temperature, any lowering of that temperature will cause a portion of the vapour to be condensed, and become water. But, again, we know deductively, from the laws of heat, that the contact of the air with a body colder than itself, will necessary lower the temperature of the stratum of air immediately applied to its surface ; and will therefore cause it to part with a portion of its water, which accordingly will, by the ordinary laws of gravitation or cohesion, attach itself to the surface of the body, thereby constituting dew. This deductive proof, it will have been seen, has the advantage of proving at once causation as well as coexistence ; and it has the additional advantage that it also accounts for the exceptions to the occurrence of the phenomenon, the cases in which, although the body is colder than the air, yet no dew is deposited ; by shewing that this will necessarily be the case when the air is so undersupplied with aqueous vapour, comparatively to its temperature, that even when somewhat cooled by the contact of the colder body, it can still continue to hold in suspension all the vapour which was previously suspended in it : thus in a very dry summer there are no dews, in a very dry winter no hoar frost.

[p85-10] The second corroboration of the theory is by direct experiment, according to the canon of the Method of Difference. We can, by cooling the surface of any body, find in all cases some temperature (more or less inferior to that of the surrounding air, according to its hygrometric condition), at which dew will begin to be deposited. Here, too, therefore the causation is directly proved. We can, it is true, accomplish this only on a small scale ; but we have ample reason to conclude that the same operation, if conducted in Nature’s great laboratory, would equally produce the effect.

And, finally, even on that great scale we are able to verify the result. The case is one of those rare cases, as we have shown them to be, in which nature works the experiment for us in the same manner in which we ourselves perform it ; introducing into the previous state of things a single and perfectly definite new circumstance, and manifesting the effect so rapidly, that there is not time for any other material change in the pre-existing circumstances. It is observed that dew is never copiously deposited in situations much screened from the open sky, and not at all in a cloudy night, but if the clouds withdraw even for a few minutes, and leave a clear opening, a deposition of dew presently begins, and goes on increasing… Dew formed in clear intervals will often even evaporate again, when the sky becomes thickly overcast. The proof, therefore, is complete that the presence or absence of an uninterrupted communication with the sky causes the deposition or non-deposition of dew. Now, since a clear sky is nothing but the absence of clouds, and it is a known property of clouds, as of all other bodies between which and any given object nothing intervenes but an elastic fluid, that they tend to raise or keep up the superficial temperature of the object by radiating heat to it, we see at once that the disappearance of clouds will cause the surface to cool ; so that Nature, in this case, produces a change in the antecedent by definite and known means, and the consequent follows accordingly : a natural experiment which satisfies the requisitions of the Method of Difference.

[1]

[2]

[3]

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