6 M. R. Clausius on the Moving Force of Heat, 



steaui ; the second is, not only as its name impoi-ts, hidden from 

 our perceptions, but has actually no existence -, during the altera- 

 tion it has been converted into work. 



We must introduce another distinction still as regards the 

 heat expended. The work produced is of a twofold nature. In 

 the first place, a certain quantity of work is necessaiy to over- 

 come the mutual attraction of the particles, and to separate them 

 to the distance which they occupy in a state of vapour. Secondly, 

 the vapour during its development must, in order to procure 

 room for itself, force back an outer pressure. We shall name 

 the former of these interior work, and the latter exterior work, 

 and shall distribute the latent heat also under the same two heads. 



With regard to the interior work, it can make no difference 

 whether the evaporation takes place at /q or at /j, or at any other 

 intermediate temperature, inasmuch as the attraction of the par- 

 ticles must be regarded as invariable*. The exterior work, on 

 the contraiy, is regulated by the pressure, and therefore by the 

 temperature also. These remarks are not restricted to the ex- 

 ample we have given, but are of general application ; and when 

 it was stated above, that the quantity of heat necessary to bring 

 a body from one condition into another depended, not upon the 

 state of the body at the beginning and the end alone, but upon 

 the manner in which the alterations had been carried on through- 

 out, this statement had reference to that portion only of the 

 latent heat which corresponds to the exterior work. The re- 

 mainder of the latent heat and the entire amount of sensible heat 

 are independent of the manner in which the alteration is effected. 



WTien the vapour of water at t^ and t?, is reconverted into 

 water at /q, the reverse occurs. Work is here expended, inasmuch 

 as the particles again yield to their attraction, and the outer 

 pressure once more advances. In this case, therefore, heat must 

 be produced ; and the sensible heat which here exhibits itself does 

 not come from any retreat in which it was previously concealed, 

 but is newly produced. It is not necessary that the heat developed 

 by this reverse process should be equal to that consumed by the 

 other ; that portion which corresponds to the exterior work may 

 be greater or less according to circumstances. 



We shall now turn to the mathematical treatment of the sub- 

 ject, confining ourselves, however, to the consideration of per- 



* It must not be objected here that the cohesion of the water at t^ is less 

 than at f^, and hence requires a less amount of work to overcome it. The 

 lessening of the cohesion implies a certain \york performed by the warming 

 of the water as water, and this must be atlded to that produced by evapo- 

 ration. From this it follows, that of the heat which the water receives from 

 without, only one portion must be regarded as sensible, while the other 

 portion goes to loosen the cohesion. This view is in harmony with the 

 fact, that water possesses a so much greater specific heat than ice, and pro- 

 bably than steam also. 



