260 PHILOSOPHICAL TRANSACTIONS. [aNNO 1792. 



deeomposed by a certain degree of pressure, or cooling, according to determined 

 laws. 



After enumerating several other laws or circumstances, Mr. D. adds, the whole 

 theory of hygrology appears to be comprehended in the foregoing propositions, 

 founded on facts. The objects of that science are in general the cause of evapora- 

 tion, and the modifications of the evaporated water. The common source of the 

 water thus disseminated in the atmosphere, is the surface of the earth ; whence, 

 in spontaneous evaporation, both in air and in vacuo, as well as in ebullition, we 

 see that water fly off with latent fire. If we collect that product in a close space, 

 it acts in the same manner as a new quantity of expansive fluid. We know from 

 experience, that an expansive fluid is really produced by ebullition, and by eva- 

 poration in an exhausted vessel : there is no reason why the cause of evaporation, 

 and its product, should change in any case, only by the presence of air ; and in 

 examining what may happen in open air, we find no particular cause of the de*, 

 struction of that expansible fluid, nor any difficulty in conceiving its dissemination 

 in every part of the atmosphere. 



But here we lose sight of steam, for it is as transparent as air itself: here also 

 its mechanical action is as little perceivable as that of any set of scattered particles 

 of air: and though its specific gravity is much less than that of air, its quantity 

 existing in the atmosphere is most times so inconsiderable, that it can hardly be 

 discovered by that means, on account of other causes which also afl^ect the spe- 

 cific gravity of a given mass of free air. Therefore, notwithstanding our experi- 

 ments on the formation of steam and its effects in our vessels, we should be igno- 

 rant of its functions in the atmosphere, if it were not for its property of producing 

 moisture, by which we may trace it wherever it is, and determine its quantity. 

 Here then a new field is open for experiments and observations ; since by con- 

 necting hygrometry with hygrology, the hygrometer is for us in the atmosphere, 

 what the manometer is in close vessels. The particular experiments which he 

 has to relate have that connection in view ; as they will show, that in a close 

 vessel, either filled with air, or free from it, the product of evaporation afl^ects, 

 at the same time, the hygrometer and the manometer ; the former by moisture, 

 the latter by pression. 



On the laws of hygrometry, Mr. D. remarks, that the science of hygrometry 

 derives its origin from the cause why the density of steam has different maxima, 

 according to the temperature. That hygroscopic substances are of 3 distinct 

 classes. Some seize on the water of steam, by a chemical affinity with that 

 liquid ; among these are acids, salts, and calces. Some only imbibe it by its 

 tendency to propagate itself in capillary pores; but, from their nature they re- 

 ceive no sensible increase in their bulk by its introduction ; in the number of 

 these are porous stones. Lastly, some substances, which also only imbibe a cer- 



