ATMOSPHERE. 



241 



production ; and also another fact that a number of 

 uascs, of very different natures, are capable of holding 

 exactly the same quantity of water in vapour. In 

 opposition to the first and apparently most conclusive 

 of these proofs, it is stated, that vapour is produced 

 in the exhausted receiver, only because the pressure 

 of the air on the surface of the water is taken off at 

 the same time the air itself is removed; but that 

 when the air is removed, and a pressure equal to its 

 weight continued, which would be exactly the state 

 of things in nature if the air had no agency in evapo- 

 ration, the evaporation does not go on. In attempt- 

 ing to turn the edge of this objection, Dalton, not- 

 withstanding all his abilities, failed. There are other 

 objections, more in the way of common observation : 

 evaporation is not always rapid in proportion to the 

 heat, for on some intensely cold days, when there is 

 a strong wind, it goes on with great activity ; and 

 those east winds which, in the early part of the season, 

 not only dry the surface of the earth, but parch and 

 wither the vegetation, are cold. Still, it is true that 

 evaporation always produces cold ; and therefore, 

 though the action of heat is not the sole cause of it, 

 it is certainly one of the causes, or one of the ele- 

 ments of the compound cause. 



The other and more popular theory of evaporation 

 attributes it to the chemical solution of water in the 

 air, by means of an attraction or affinity existing be- 

 tween the two fluids ; and though it has been urged 

 against this theory that evaporation does not go on 

 most rapidly when the air is densest, which, accord- 

 ing to the theory, it ought to do, yet the effect of 

 pressure in preventing, in the exhausted receiver, 

 that evaporation which goes on freely and readily 

 when there is no pressure, shows that, beyond a cer- 

 tain point of density, the ascertaining of which is of 

 course matter of experiment, the pressure of the air 

 checks and diminishes the process. 



That some kind of attraction subsisting between 

 air and water is, like heat, though not the sole cause 

 of evaporation, yet, like that, one of the causes, or 

 one of the elements of the compound cause, seems 

 unquestionable. But it is doubtful whether this at- 

 traction should be called a chemical attraction, or the 

 diffusion of vapour through the air a chemical solution. 

 It is true that, in the formation of vapour, there is 

 cold produced, or a certain degree of the action of 

 heat is absorbed, and absorbed, no doubt, in the for- 

 mation of the vapour ; it is also true, that when the 

 vapour is again condensed and falls in rain, there 

 appears to be heat produced, or a certain degree of 

 the action of heat which was previously impercepti- 

 ble or latent, is rendered sensible ; and it is also true 

 that similar changes, with regard to heat, take place 

 in many chemical solutions and their reversals. But 

 a chemical solution of one substance in another, or 

 which is the very same, a chemical combination, is 

 never formed without some effort : the two substances 

 so united to a certain extent become one, that one 

 has new properties which did not belong to any of 

 the two singly, and the combination cannot be dis- 

 solved without some action. 



It would be contrary to the observed facts, as well 

 as to the analogy of nature (the only guide which we 

 have where our mechanics and chemistry fail us), to 

 suppose that the water which the atmosphere holds 

 in the state of vapour, is combined with the atmo- 

 sphere by any chemical process. One chemical attrac- 

 tion cannot ue overcome but by another and more 



NAT. HIST. Vui.. I. 



powerful attraction of the same kind, and no chemical 

 compound can be dissolved without more powerful 

 chemical action than that by which it was formed 

 and is held together. But we find the atmosphere 

 parting with its humidity to the smallest animal, or 

 the most tender leaf, with no more effort than when 

 it parts with its oxygen in the process of respiration. 

 Analogy, indeed, points out to us, with a force of rea- 

 son not easily to be resisted, that no other substance 

 can combine more intimately with the atmosphere, than 

 its two grand component elements, oxygen and nitro- 

 gen, combine with each other ; and it has been shown, 

 in the article AIR, that these can always be separated 

 without any thing approaching to the nature of a 

 chemical process. The atmosphere is the grand 

 messenger of nature, and whatever is carried free 

 to all the productions of nature, without labour or 

 exertion on their part. Chemical compounds are 

 never formed in nature, unless the compound body 

 has some functions to perform as a whole. Such at 

 least is the case with all compounds formed according 

 to our chemistry. But our chemistry, extensive and 

 important as it is as a science, and valuable as its 

 applications are in the arts, is a very partial, and 

 therefore doubtful and deceptive agent, when we 

 bring it to the philosophy of nature. It applies to 

 dead matter only, and therefore when we attempt to 

 use it either in the explanation of any one function of 

 life, or of that upon which life depends, it fails, or 

 leads us into error. Now the atmosphere is preemi- 

 nently the element of life ; and therefore we can hope 

 for little aid from chemistry in our investigation of the 

 natural properties and agencies of the atmosphere. 



That there does exist some sort of attraction 

 between the atmosphere and the water which it takes 

 up in the process of evaporation, cannot bfe doubted ; 

 as little can it be doubted, from the short distance to 

 which foreign substances accompany the union, that 

 this attraction is for water in its pure and simple state, 

 for water, not as it holds only one or two substances 

 in solution, and is fit only for the particular operations 

 in which those substances are agents, but in its simple 

 state, equally fit for all purposes to which water can 

 be applied. Of what particular nature this attrac- 

 tion is, we are ignorant, and it belongs to that class of 

 natural actions, respecting which we have little chance 

 of gaining any information, further than observing the 

 results. Perhaps we shall not err very much, if, to 

 distinguish it from every thing chemical, and from all 

 those attractions in which the action of heat, under 

 any of its modifications, is the agent, we call it a 

 corpuscular attraction, an attraction belonging to the 

 same class with that by which one liquid mixes with 

 another without change of temperature or action of 

 any kind; that by which fluids wet the surfaces of 

 solids, and many others which are not chemical, 

 neither are they mechanical, as they do not depend 

 upon gravitation. It appears, from the fact that there 

 is always air between substances which prevents 

 them sticking together, unless in the case of what 

 appears to be a stronger corpuscular attraction of the 

 substances, that the air has a corpuscular attraction 

 for every known substance ; and that all substances 

 whatever might rise in vapour in the atmosphere, 

 provided their cohesion were sufficiently overcome by 

 the action of heat, or by any other means. This 

 property, which is verified by the facts as far as these 

 go, points at a most extensive, almost a universal use 

 of the atmosphere in the operations of nature. 



This 



