204? Prof. Claitsius on the Internal Work 



the specific heat of water in its three states of aggregation is 

 caused by great differences in the proportion which is consumed 

 as work, and that this proportion is considerably greater in the 

 liquid state than in the other two states*. We must, accord- 

 ingly, here distinguish between the observed specific heat and 

 the true specific heat with which the alteration of temperature 

 dT must be multiplied, in order that we may obtain the corre- 

 sponding increase of the quantity of heat actually present ; and, 

 in accordance with the above theorem, I believe we must admit 

 that the true specific heat of water is the same in all three states 

 of aggregation ; and the same considerations as apply to water 

 must naturally also apply in like manner to other substances. 

 In order to determine experimentally the true specific heat of a 

 substance, it must be taken in the form of strongly overheated 

 vapour, in such a state of expansion, in fact, [that the vapour 

 may, without sensible error, be regarded as a perfect gas ; and 

 its specific heat must then be determined under constant 

 volume. 



Ilankine is not of my opinion in relation to the specific heat 

 of bodies in different states of aggregation. At page 307 of his 

 ' Manual of the Steam-Engine/ he says, <c The real specific heat 

 of each substance is constant at all densities, so long as the 

 substance retains the same condition, solid, liquid, or gaseous ; 

 but a change of real specific heat, sometimes considerable, often 

 accompanies the change between any two of these conditions." 

 In the case of water in particular^ he says, on the same page, that 

 the true specific heat of liquid water is (( sensibly equal " to the 

 apparent specific heat ; whereas, according to the view above put 

 forth by myself, it must amount to less than half the apparent 

 specific heat. 



If Rankine admits that the true specific heat may be different 

 in different states of aggregation, 1 do not see what reason there 



* I have already enunciated this view in my first memoir on the Mecha- 

 nical Theory of Heat, having, in fact, inserted the following in a note 

 (Poggendorff's Annalen, vol. lxxix. p. 3/6), which has reference to the 

 diminution of the cohesion of water with increase of temperature :—~ 

 "Thence it follows, also, that only part of the quantity of heat which 

 water receives from without when heated, is to be regarded as heat in the 

 free state, while the rest is consumed in diminishing the cohesion. In 

 accordance with this view is the circumstance that water has so much higher 

 a specific heat than ice, and prohably also than steam." At that time the 

 experiments of Regnault on the specific heat of the gases were not yet 

 published, and we still found in the text-books the number 0*847, obtained 

 by De la Roche and Berard, for the specific heat of steam. I had, however, 

 already concluded, from the theoretical grounds which are the subject of 

 the present discussion, that this number must.be much too high ; and it is 

 to this conclusion that the concluding words, "and probably also than 

 steam," refer. 



