376 Mr. Hei'apath on True Temperature^ and* the [Nov. 



vapour will mix with the remaining atmosphere and increase its 

 elasticity ; so that after a little time the pump will again act, and 

 a considerable portion of the remaining atmosphere may be 

 extracted with the vapour. This process repeated at proper 

 intervals, the atmosphere may be exhausted to almost the last 

 particle. The damp body being now withdrawn, and a desicca- 

 tive exposed, either by letting off mercury with which it may 

 have been covered, or otherwise, the whole of the vapour might 

 be absorbed, and nearly a Torricellian vacuum obtained. A 

 process something like this has been used by Smeaton and some 

 of the French philosophers, but being unacquainted with the 

 true nature and laws of vapours, they could not bring their ideas 

 to that practical perfection which it is hoped the laws here deve- 

 loped will enable them to do. 



A phenomenon of the kind 1 have just been mentioning has 

 been remarked by M. Biot, which often puzzles young philoso- 

 phers. When the air has been extracted by the air-pump until 

 it has no longer strength to raise the valves, we find, if the appa- 

 ratus be let stay for some time, that the pump will act again, and 

 extract a considerable quantity of air, though we are certain the 

 apparatus was so tight that none could have introduced itself to 

 the interior. This arises from the slow and gradual conversion 

 into vapour of the humidity which almost always adheres to the 

 interior and sides of the apparatus. 



Since the megethmerin of any vapour may be so increased by 

 compressing the air in which it is contained that the momentary 

 condensation shall at length exceed the re-evaporation ; and 

 since we know that this will be the case when this megethmerin 

 exceeds by ever so trifling a degree that which is due to the 

 proper tension of the vapour at the temperature of the atmosphere, 

 we can evidently, by compressing the atmosphere until a slight 

 dew just begins to appear, and by determining the amount of that 

 compression, ascertain the dampness or quantity of vapour in the 

 atmosphere. Let mf be the megethmerin of the vapour at its 

 proper tension, and in that of the vapour in the atmosphere ; and 

 let the dew just begin to appear when the atmosphere is com- 

 pressed in the ratio of r to 1. Then the megethmerin m must by 

 this compression be increased in the ratio of 1 to r, and becomes 



r m, and must be equal to m' . Whence m = - . Therefore, m' 



and r being known, m is known, that is, the quantity of vapour in 

 a given volume of the atmosphere. 



Cor. 4. — There is another method of determining the quantity 

 of aqueous vapour at any time in the atmosphere by pouring into 

 a glass vessel water at different temperatures inferior to that of 

 the atmosphere, and observing at what temperature of the water 

 the vapour of the atmosphere just begins to be deposited in the 

 form of dew on the sides of the glass. At this temperature, the 

 megethmerin of the vapour in the atmosphere must evidently be 



