MECHANICS. 



In a liquid, as we have already ex- 

 plained, the repulsive energy of the ca- 

 loric nearly balances the cohesive force 

 of the particles. Now, if we increase 

 the quantity of caloric considerably, its 

 repulsive effect will predominate over 

 the cohesive attraction of the particles ; 

 and instead of being nearly in a state of 

 indifference as to mutual attraction, we 

 may expect to find that they strongly 

 repel each other, and that the liquid 

 will assume a form in which it will be 

 necessary that it should be closely con- 

 fined in order to prevent its total dissi- 

 pation by the dispersion of the particles 

 owing to this repulsive effect. 



Accordingly we find that if a liquid 

 be exposed for a considerable time to 

 the action of fire, it will be gradually 

 converted into steam ; a fluid totally 

 differing in its mechanical character 

 from a liquid. If the liquid be inclosed 

 in any vessel, it will press with no other 

 force upon the surface which confines 

 it, than with those pressures which are 

 produced by its weight ; but when the 

 same liquid is converted into vapour 

 and confined in a close vessel, it will 

 press on the confining surface with its 

 elastic force, which is quite independent 

 of its gravity, and arises from the effort 

 which its particles exert to repel each 

 other; and which, therefore, exert a 

 bursting pressure on the inner surface 

 of the vessel which contains it. 



The degree of elasticity or the pres- 

 sure which confined steam exerts upon 

 the surface of the vessel which contains 

 it, will, according to our theory, and 

 which is confirmed by experience, in- 

 crease with the heat which is communi- 

 cated to the steam ; and, on the other 

 hand, according as the temperature of 

 the steam is lowered, the elastic pres- 

 sure will be proportionally diminished. 



The evaporation of liquids, however, 

 is effected by a force, whose effects are 

 not sensible in the liquefaction of solids. 

 The atmospheric pressure has an effect 

 in binding together the particles of a 

 liquid ; and, therefore, conspires with 

 the cohesive force in opposing the 

 effects of the caloric. When the caloric 

 has been communicated to a body in 

 such a quantity as to form a balance 

 for the cohesive force; the body, ac- 

 cording to our theory, ought to be in 

 a state in which the slightest increase of 

 caloric would convert it into an elastic 

 vapour. Under these circumstances, 

 however, the atmospheric pressure op- 

 poses the change, and is the means, and 

 the only means, by which the particles 



are bound together and retained in a 

 state of liquidity. In proof of this, we 

 have only to remove the atmospheric 

 pressure, and many bodies which are 

 now held in the liquid state, by the 

 mechanical action of that pressure, wil) 

 evaporate. 



Let water at 180 of temperature, or 

 alcohol, or ether, be placed under the 

 receiver of an air-pump, and they will 

 boil and evaporate on removing or 

 rather diminishing, by rarefaction, the 

 pressure of the air on their surfaces. 

 Indeed, ether will evaporate if exposed 

 to the atmosphere without any diminu- 

 tion of pressure. 



On the other hand, it would follow 

 from this theory, that if the pressure be 

 increased, the evaporation will be re- 

 sisted by it ; and such, in fact, we find 

 to be the case. Water under the at- 

 mospheric pressure, when the barometer 

 is at 30 inches, will boil and vaporize 

 at 212 of Fahrenheit's thermometer; 

 but if the same water be submitted to 

 increased pressure, it will not boil or 

 evaporate until it has reached much 

 higher temperatures. 



If a sufficient quantity of heat be 

 withdrawn from the vapour which has 

 been raised from a liquid, it will be re- 

 stored again to its liquid form ; and it 

 is a very important fact, that, in this 

 case, its bulk is reduced in a very high 

 ratio. A cubic inch of water, when 

 converted into steam at the usual 

 atmospheric pressure, will form a cubic 

 foot of steam ; and it follows, therefore, 

 that if the caloric be withdrawn from a 

 cubic foot of such steam, by application 

 of cold bodies,, or otherwise, it will be 

 reconverted into a cubic inch of water. 

 Now this property is rendered subser- 

 vient to the production of a very im- 

 portant mechanical agent. If a cubic 

 foot of steam be inclosed in a vessel, 

 and that the vessel be cooled until the 

 steam be condensed or reconverted into 

 water, we shall have 1727 cubic inches 

 of empty space or vacuum ; for the 

 steam, which, before condensation, filled 

 a cubic foot, or 1 728 cubic inches, will, 

 after condensation, be reduced to one 

 cubic inch, leaving 1727 cubic inches 

 void of any material substance. 



The condensation of vapour, or its 

 reconversion into the liquid state, is 

 thus rendered an easy and effectual 

 method of producing a vacuum, and is 

 free from the objections to the mechani- 

 cal method of producing the same 

 effect alluded to in Chap. VI., inasmuch 

 as a vacuum may thus be obtained 



