Tltcovy of Surface Forces, 473 



nature of this heat, of which we hear so much. It seems 

 hopeless to combine thermodynamics with a merely statical 

 view of the constitution of matter. 



On these grounds I find it difficult to attach a meaning to 

 such a theorem as that enunciated in the following terms* : — 

 " La derivee partielle du travail mecanique interne prise 

 par rapport au volume egale 1 'attraction par metre carre qu' 

 exercent l'une sur Fautre les deux parties du corps situees 

 desdeux cotes d'une section plane," viz. the intrinsic pressure. 

 In the partial differentiation the volume is supposed to vary 

 and the temperature is supposed to remain constant. The 

 difficulty of the first part of the supposition has been already 

 touched upon ; and how in a fundamental theory can we 

 suppose temperature to be constant without knowing what it 

 is ? It is possible, however, that some of these theorems 

 may be capable of an interpretation which shall roughly fit 

 the facts, and it is worthy of consideration how r far they may 

 be regarded as applicable to matter in a state of extreme cold. 



With respect to the value of K, Young's estimate of 23,000 

 atmospheres for water has already been referred to. It is 

 not clear upon what basis he proceeded, but a chance remark 

 suggests that it may have been upon the assumption that 

 cohesion was of the same order of magnitude in liquids and 

 solids. Against this, however, it may be objected that the 

 estimate is unduly high. Even steel is scarcely capable of 

 withstanding a tension of 23,000 atmospheres. 



So far as I am aware, the next estimates of K are those of 

 Dupre. One of them proceeds upon the assumption that for 

 rough purposes K may be identified with the mechanical 

 equivalent of the heat rendered latent in the evaporation of 

 the liquid, that in fact evaporation may be regarded as a 

 process of disaggregation in which the cohesive forces have 

 to be overcome. This view appears to be substantially sound. 

 If we take the latent heat of water as 600°, we find for the 

 work required to disintegrate one gram of water 



600x4*2xl0 7 C.G.S. 

 One atmosphere is about 10 s C.G.S. ; so that 



K = 25,000 atmospheres. 

 The estimates of his predecessors w r ere apparently unknown 

 to Van der Waals, who (in 1873) undertook his work mainly 

 with the object of determining the quantity in question. He 

 finds for water 11,000 atmospheres. The application of 

 Clausius's equation of virial to gases and liquids is obviously 



* Loc. cit. p. 47. 

 Phil Mag. S. 5. Vol. 30. No. 187. Dec. 1890. 2 L 



