On Atmospheric Density and Pressure. 425 



necessary to produce the conversion of a liquid into a ga$ 

 (of evaporation) must depend on the pressure of the air; the 

 greater the pressure, the greater the heat necessary from its 

 greater resistance to its escape, and vice versa. 



Thus, in the application of a heat (nearly boiling) to a 

 fluid, its particle.-> being rendered so rare and min :tu acquire 

 a disposition to ascend, from want of gravity. In this dis- 

 position to ascend they come into contact with the particles 

 of the air pressing downwards, and are thrown down again, 

 their force of ascent being inferior to the force of descent 

 possessed by the atmospheric particles. The body is there- 

 fore still kept liquid in this degree of heat: if, however, it 

 is increased a little further, the particles are forced to rise 

 up and conquer the force of pressure, and thus evaporation 

 takes place. 



In this state then a body is balanced between three 

 powers, two of which may be said to be quiescent, and the 

 third divelle7it; if the united sum of the quiescent forces, 

 pressure and cohesion, amounts to more than the divellent 

 caloric, its liquidity remains; if less, evaporation ensues. 



Then, in a case of evaporation, to know the force or power 

 with which it takes place, add the sums of the quiescent 

 powers, cohesion and pressure, (numerically expressed,) de- 

 duct their united amount from the sum of the divellent ca- 

 loric, and the remainder wUl be the sum required. Thus, 

 suppose a body possessed with the three powers so, the qui- 

 escent A and B, and the divellent C, thus A with the force 

 of 6, B with the force of 2, and C with the force of io, 

 then we have A6+ B c 2 = 8 — C 10 = 2, the force with 

 which the body evaporates. If the forces are equal, and 

 that there is no remainder, the body of course will remain 

 as it is. 



On the other hand, in the reconversion of a gas into a 

 liquid (or condensation) where the powers are reversed, to 

 know the force with which the body condenses, subtract 

 -the quiescent caloric from the united sum of the divellents, 

 pressure and cohesion, the remainder is the sum required. 

 'J nus, suppose a body with the powers to C, caloric with 

 the force of G, B pressure with the force of 8, and A cohe^i'.n 



with 



