1821.] Causes of Calorific Capacity, Latent Heat, fyc. 376 



little, the evaporation from the water will fall short of the conden- 

 sation of the vapour, or the re- evaporation of the vapour, as it 

 condenses on the sides of the vessel will be less than the con- 

 densation ; and thus the apparent condensation will increase 

 until the vapour is reduced to a proper tension. 



By these views and the theorem given towards the end of the 

 preceding corollary, it is easy to ascertain from the celerity of 

 evaporation, the quantity of aqueous vapour at any time in the 

 atmosphere. 



This theory demonstrates the absurdity of the doctrine of 

 atmospheric saturation by aqueous vapour. If we admit such a 

 doctrine as this, we must likewise admit the saturation of a 

 vacuum ; for even a vacuum at a given temperature can only 

 contain a definite " dose." But if we are determined to have 

 saturation, to make the thing agree with phenomena, we must 

 introduce the saturation of space. We must grant what I fear 

 will not easily be acknowledged, that space can be saturated 

 with oue thing, and at the same time admit any quantity of 

 another without inconvenience or confusion. Such an hypo- 

 thesis as this carries, perhaps, too much the face of improbabi- 

 lity ; but if we do not discard saturation, 1 cannot see how we 

 can reject it. 



Cor. 3. — When the megethmerin of the vapour in the atmo- 

 sphere is equal to that which is due to the tension of the vapour 

 at that temperature, the condensation upon any non-absorbent 

 will just balance the re-evaporation ; and the sides of any vessel 

 containing such an atmosphere would neither appear remarkably 

 damp or remarkably dry, but any absorbent in such an atmosphere 

 would be completely damp. If, however, the air was compressed 

 but ever so trifling, the megethmerin of the vapour would become 

 too great, some of the vapour would condense, and the surface 

 of the body would appear to be covered with a slight dampness 

 or dew. Were the compression carried further, the quantity of 

 dew would of course increase, and would be proportional to the 

 degree of compression from the point where the condensation 

 and evaporation balance. On the contrary, the aridity of the 

 sides or air would also be increased in proportion to the rarefac- 

 tion from the same point of balance or equilibrium. We can, 

 therefore, extract vapour of any kind from a body by simply 

 rarefying the air in which it is placed. And if this rarefaction 

 at proper intervals be carried to a sufficient length, we may pro- 

 duce an effectual desiccation. 



By the application of this principle, we arrive at a simple 

 method of obtaining an almost perfect vacuum. If a body which 

 evaporates with tolerable ease containing a sufficient quantity of 

 humidity be introduced into a receiver by means of an apparatus, 

 by which it may be easily enclosed or exposed, a thing which it 

 is no difficult matter to contrive, the air bung as much as possi- 

 ble exhausted, and the damp body exposed, the evaporated 



