564 SCIENCE PROGRESS 



doubt on the hypothesis by experiments on ammonium chloride 

 which had been dried with exceptional care. The new fact 

 which this investigation has established is that the really dry 

 substance does undergo the Scheffer transition. Wegscheider's 

 hypothesis, therefore, falls to the ground if we identify his 

 allotropic forms with those discovered by Scheffer. What is 

 obviously required, as indeed the authors point out, is to make 

 simultaneous determinations of vapour density and transition 

 points, to see whether a given sample undergoes transition 

 and at the same time gives rise to ^^dissociated vapour. 



The Chemical Constant. — It is well known that the ther- 

 modynamic theory of chemical reactions has for its object the 

 expression of the equilibrium constant as a function of the 

 temperature. The classical theory permits us to calculate 

 all the necessary terms with one exception, the integration 

 constant. In order to determine this quantity it was necessary 

 to make at least one experimental determination of the equilib- 

 rium constant itself. The great merit of the heat theorem of 

 Nernst lies in the fact that it enables us to obtain a value for 

 the integration constant without actually measuring the 

 equilibrium, provided we have made measurements of the 

 vapour pressure of each separate substance at different tempera- 

 tures. Each single vapour pressure expression contains a term 

 called the " chemical constant " of the substance, and the 

 integration constant referred to above in connection with the 

 chemical equilibrium is simply the algebraic sum of such 

 chemical or individual constants, obtainable from the vapour 

 pressure data. 



Attempts have naturally been made to analyse still further 

 the chemical or individual constant of any single substance 

 with the object of expressing it in a form which shall contain 

 a term independent of the nature of the substance and 

 another term or terms characteristic of the substance. There 

 is, however, difference of opinion regarding the dimensions of 

 the chemical constant, one view being that it has the dimensions 

 of the logarithm of a pressure, the other that it has the di- 

 mensions of the logarithm of a pressure divided by a tem- 

 perature to the power 5/2. The attempt to distinguish between 

 these two possibilities has been made recently by Lindemann 

 {Phil. Mag., Jan. 1920). It is not possible to enter closely 

 into the argument. 



Suffice it to say that the problem involves a choice between 

 two possible values to be ascribed to the atomic heat of a 

 monatomic gas in the neighbourhood of absolute zero. A 

 so-called " degradation " theory has been advanced in recent 

 years, according to which the atomic heat of such a gas will 

 diminish to zero. The classical theory leads us to expect, 



