12 Prof. S. U. Pickering. Determinations to test [Dec. 11, 



is lowered, the decrease per degree being equal to the difference 

 between the heat capacities of the liquid and solid, and that, therefore, 

 there most be a certain temperature at which the heat of fusion will be 



nil, this temperature being given by t , in which t is the melting 



u c 



point of the substance, I its heat of fusion at t, and C and c its heat 

 capacity in the liquid and solid conditions respectively. At this tem- 

 perature a liquid could not freeze, since there would be no difference 

 between it and the solid, and Person argued that there would then be 

 no heat at all in it, and that this temperature was the absolute zero. 

 He then made determinations with various substances, which tended 

 to show that this temperature was the same for all bodies, and was 

 situated at -160C. 



The analogy between this zero and that deduced for gases is, how- 

 ever, very imperfect ; the total heat in a gas is measured by its tem- 

 perature reckoned from 273, whereas it is only the difference 

 between the total heat in a liquid and solid that is measured by its tem- 

 perature reckoned from 160, and, instead of considering the latter 

 as the absolute zero, it is preferable to regard it as the critical tem- 

 perature for the solid-liquid conditions (see ' Chem. Soc. Trans.,' 1889, 

 p. 32) ; and indeed, since we have now succeeded in obtaining liquids 

 at temperatures below 160, it is quite impossible to regard 160 as 

 the absolute zero, or to believe that the heat capacities of all bodies 

 would indicate this same temperature for that of no solidification. 



Guldberg (' Bidrag til Agarnernes Molekylar Theorie,' ch. v, p. 484) 

 has defined the critical point of the solid-liquid states as that at which 

 the volumes of the liquid and solid are identical, and at which the 

 heat of fusion is nil, a certain pressure, as well as a certain tem- 

 perature, being required to fulfil these conditions. It appears to me, 

 however, that the question of pressure may practically be left out of 

 consideration; pressure will, of course, affect the temperature m 

 question, but to such a small extent that the ordinary atmospheric 

 pressure, under which the data necessary for the calculations are 

 obtained, may be regarded as nil, and it also appears to me that the 

 definition depending on the heat of fusion being nil includes the idea 

 of equal volumes, for it seems hardly possible to conceive two con- 

 ditions of the same substance, each possessing the same kinetic and 

 potential energy, which could yet differ from each other in volume or 

 any other property. 



The analogy, however, between this temperature and the critical 

 temperature for the liquid gaseous conditions is at best but an imper- 

 fect one. If we start with a crystalline solid below this temperature 

 and heat it, it could never pass by insensible degrees into a liquid ; 

 the molecules in a crystal possess a definite arrangement, those of a 

 liquid an indefinite arrangement, and, between these two, no inter- 



