whofe own religious opinions appear to be very different, is an additional prefumption in favour of the determination itfelf, and, a circumftance that fets the candour of the writer in a very amiable and refpectable view. With refpect to the subject of the controversy, we prefume not to decide. Fair arguments and charitable fentiments, from whatever party they may come, will always, as fuch, command our approbation and refpect; but falfe reafoning, accompanied by the illiberal cant of fanctified intolerance, even though it should appear in fupport of our own opinions, we cannot help beholding with indignation, and treating with contempt. The caufe of religious liberty is infinitely dearer to us, and of more importance to mankind, than any particular decifion concerning difputable points of theology; nor can we better exprefs the rule which, as Reviewers, we wish. to follow, than by adopting the language of Cicero: Defen-, dat quod quifque fentit: funt enim judicia libera: nos inflitutum tenebimus, nulliufque unius difciplinæ legibus adftricti, quibus in philofophia neceffario pareamus, quid fit, in quaque re, maxime probabile, femper requiremus.

To Dr. JAMES HUTTON, F. R. S. Edinb. &c. &c. SIR,

RETURN,

your paper, intitled, Answers to the Objections of Mr. De Luc with regard to the Theory of Rain, printed in the Tranfactions of the Royal Society of Edinburgh *; and as the fubject itlelf is very important in the prefent ftate of experimental philofophy, I am glad of an opportunity of again difcuffing it with you.

1. The fundamental HYPOTHESIS of your THEORY OF RAIN is this: "When two maffes of air, of different temperature, are mixed together, the humidity of the new mais, is greater, than the mean between the humidities which the two males had feparately." This you maintain to be a phyfical truth; though you have made no experiment to afcertain it: but you refer to fome natural phenomena, which you conceive to be the effects of that supposed law of nature. I have objected, not only to thefe examples, as proots of your HYPOTHESIS; but alfo to the fufficiency of the HYPOTHESIS telf, for the EXPLANATION OF RAIN: you think you have aniwered my objections; and this fuppofition is now

to be examined.

* See Rev. vol. lxxix. p. 32.

Y y 4

2. The

2. The first example given, for the fupport of your bypothefis, was, "The mist which appears at the moment of the expiration of animals, when they breathe in cold air." In this cafe you find the mixture of two maffes of air of different temperature, one of which is very moift: to thefe circumstances you attribute the mit which appears; and you think I have granted it. "M. DE Luc" (you fay) " admits all that I could poffibly propofe to draw from this example, viz. that moist air, breathed from the lungs of an animal into the colder atmosphere, produces a condensation of water." However, you could only fay, that I acknowleged the production of a mift in that cafe; which is a well-known fact, but I did not grant, that the product of the expiration of animals was moist air only; by which is to be understood, air containing, at moft, moiffure to its maximum: whereas my opinion is; that the mift here in contemplation, or the greatest part of it, exifts already in the expired air. The reafons I gave for that opinion are of two forts; à priori and à pofteriori. The fi:ft are, 1. That it appears to me, that, if the quantity of moisture which comes conftantly out of the lungs of animals, proceeded from the evaporation of a conftant supply of water in their air-veffels, thefe would be obftructed, and the ani mals thereby fuffocated, when they breathe in warm and moist air: for then, the water in the lungs could not evaporate. 2. That the undoubted effect of a conftant evaporation in the lungs of animals, would be the cooling of that organ: while it appears to be the principal, if not the only fource of animal heat.

3. The reafon à pofteriori was this: All the facts difcovered respecting the effects produced on the atmospherical air by breathing, and the analogy of thefe effects, with thofe of the combuftion of vegetable and animal fubftances, lead me to think, that, as in this laft cafe there is team produced with fixed-air; the fame happens in the act of breathing. In both cafes alfo, though in different proportions, the quantity of fleam produced, is more than the adjacent air can contain in a tranfparent state; which occafions the decompofition of part of it, whence heat and a precipitation of water. This caufe of moisture in the expired air, ap pears to me more probable, than the evaporation of water conftantly fupplied in the lungs; and I concluded from it, that the mift, which we fee coming out of the noftrils of fome animals, and of our own, in winter, exifted in great part in the lungs, previously to any communication with the external air. in winter, alfo, animal-heat requires a greater recruit than in fummer; and there is more mift coming out of the lungs; a circumftance which agrees with my hypothefis: however, I do not pretend to explain the caufe of that correfpondence; especially, as the contact of the external air, more or lefs warm, has, in the phenomenon, a fhare (to which I fhall come hereafter) which

makes

makes it difficult to determine, and, perhaps, to ascertain, the effect of the other cause of increase of mist.

4. You may fee now, Sir; that, inquiring after the cause of that humidity, was not, as you lay, leaving the fubject in hand: that, an argument founded upon this fuppofed operation of the lungs, is more to the purpose, than if I had mentioned the heart, the liver, or the kidneys, which you think had done as well: and that I did not "evade making a direct answer to that question; Does the moist air, expired in breathing, form a condenfation of water, in being mixed with cooler air fufficiently faturated with humidity?" fince I refused to admit the fact, thus expreffed. There is a mift; but, in my opinion, it is not produced by the caufe to which you there allude.

5. Your fecond example is, "The mift which appears over water boiling in open air :" and my objection to it was, that the cafe of pure fteam, brought into contact with cooler air, is very different from that, of mixing together two masses of air: for, at the inftant that the fteam lofes any part of its fenfible heat, by communicating with colder air, it is destroyed by the pressure of the atmosphere, and its water is precipitated under the form of a mist, which is diffeminated in the ambient air, with the latent heat of the decompoled fteam. To which you answer:

"Here M. DE Luc confiders tranfparent fteam, when coming in contact with cooler air, as cooled by the air, without noticing, that it proportionably beats the air by which it is cooled. This overfight in another perfon but M. DE LUC might have been natural; it might even in M. DE Luc himself have been more excufable, had he been lefs converfant with the important theory of latent heat which Dr. BLACK difcovered long ago. But first, to confine our attention to the cooling of the fteam, and then to explain the appearance of condenfation from this cooling alone, is a species of reafoning that one would not have expected from the author of the Modifications of the Atmosphere."

Soon after, however, you find, that I had not forgotten the latent heat of the fteam.

"M. DE Luc" (you fay), "though he had recourse to the cooling of the team alone, to account for the mist which inftantly appears-does not lofe fight of the beat, which he knows is not left; but he brings it into action again, for the evaporation of that mist."

Here you find fault again. "This explanation" (you fay) "which M. DE LUC has offered to account for the evaporation of the vifible mift, appears to be inconfiftent with his theory refpeating the condenfation of the fteam. For, if the condensation of the fteam be the effect of its being cooled by the air; how could the former ftate of things be restored, wi hout an affignable caufe? That is to fay: how could the air teftore to the water, that heat which it had received by communicating with the team ?"

The contradiction you find here, proceeds from your fuppofing, that there is a former fate of things to be restored; be

caufe

caufe the mist difappears. But the former ftate was, a mass of pure fteant; and the fubfequent is, an invisible vapour diffeminated in the air: I will fhew you very foon, the difference between thefe two cafts. You are alfo miftaken when you think, that I attribute the decompofition of the fteam, to its cooling alone; and that mistake is the more furprifing, as you have taken notice of the efficient caufe I affigned of that decompofition; I mean, the preffure of the atmosphere. But this you look upon as no cause. This condenfation" (you fay)" does not happen in confequence of the fteam being expofed to any preffure which it had not sustained before." But, Sir, when the fteam fuftained the preffure of the atmosphere, its fenfible beat was that of boiling water; and at the inftant it lofes a fmall part of that beat by communicating with the air, it is fuddenly crushed by that fame preffure. This overfight is the foundation of all your criticifm. For, if you had examined how the team was deftroyed, you would not have confidered that deftruction, and the mift produced, as an example favourable to your hypothefis; nor would you have wondered at my attributing a more rapid evaporation of the mjt in the ambient air, to the latent heat of the detroyed fteam. On this part of the phenomenon, you fay:

"It requires ftrict attention to many circumftances, in order to fee, in a just light, that atmospheric operation, which had led a natural philofopher to make a fuppofition of that kind."

And, after having explained, in a very undetermined manner, the evaporation of the mit in the ambient air, you conclude:

Therefore M. DE LUC has endeavoured to explain the evaporation of the mist in that cafe upon a falje principle; by not taking into confideration the quantity of under-faturated air which the ascend, ing vapour meets with in the atmosphere."

By vapour, I fuppofe you here understand mist.

Is it for not having ufed the expreffion under faturated air, that my principle is falfe? But furely you could not imagine, that, after having explained, in its proper place, the phenomena of evaporation, I could mean here, that the mifl was evaporated in faturated air. However, fince we are to compare principles, 【 will, first, fhew you, how far mine may lead me in the determination of that quantity of under faturated air, which you call only a certain quantity: after which I will advert to your principles.

6. From fome experiments of Mr. NAIRNE (Ph. Tranf. 1777), the elaftic fluid produced by the evaporation of water in vacuo, the thermometer being at 57° of Fahrenheit, exercifes a preffure equal to of that of the atmosphere, the barometer being at 30 inches. In fome experiments of M. M. LAVOISIER and DE LA PLACE, the evaporation of water in the torricellian vacuo, deprefled the column of mercury inch, the thermometer being 54", which is about the fame refult (Id. fur la

Met. 20): Mr. JAMES WATT, by the fame method, has found the quantity fomething lefs (Ibid). By experiments of M. DE SAUSSURE, nearly the fame effect is produced from the evaporation of water in air previoufly reduced to extreme dryness (Ibid. § 628). From Mr. WATT's experiments, the fpecific gravity of aqueous vapour, or fleam, is to that of air, the barometer being about 30 inches, as 4 to 9 (Ibid. § 636); from which, and the precedent determinations, the mafs of evaporated water, at its maximum, in air of that denfity and temperature, is about 1% of the mafs of that air (Ibid. § 12). Lastly; from my obfervations, the quantity of water diffeminated in the air of the plain, is feldom half of that which conftitutes its maximum.

7. Let us now fuppofe, that, at a certain moment, the temperature of the air is fuch, as to fix to 6 of its mafs, the quantity of water that would faturate it; but that the actual quantity is only : and let us fet afide, for a moment, the latent heat of the team that will be decompofed in that air. From thefe data, 108 cubic inches of the defcribed air, will be fufficient for the total evaporation of the mift produced by the decompofition of cutic inch of pure team. For the quantity of water wanting in that mais of air, to raise it to the maximum of moisture, was al of itfelf, or the mass of of a cubic inch of air; and that is the mais of water in I cubic inch of team. If we were fufficiently informed of the comparative capacities for heat, of water and air, we might go one ftep farther in that determination: knowing, from M. WATT'S very accurate experiments, that the latent heat of team could raife 9:32, the temperature of a mafs of liquid water equal to that which it contains in an elastic form, we might determine how much the temperature of 108 cubic inches of air would be railed by the latent heat of 1 cubic inch of team. Laftly, if we knew with a fufficient certainty, the ratio between the correfpondent increafes, of the heat in the air, and of the maximum of water that it can contain in a transparent ftate,; we might determine, finally, in how much less than 1c8 cubic inches of the defcribed air, the mist produced by the decompofition of I cubic inch of fteam would totally difappear. It would here be ufelefs to examine the degree of certainty of the known experiments on these two laft points; as the indication of the principles from which the determination which I have begun above, muft proceed (principles which are all explained, and frequently applied, in my work), is fufficient to fhew you, that, if I had thought it neceffary to enter into more particulars in fpeaking of the evaporation of the mist in the prefent cafe, I might have done it confiftently with my theory. Let us now fee, if you are confiftent in your examples; which