PHYSICAL CHEMISTRY 193 



but only by the photocatalyst. The photocatalyst must con- 

 tain the same elementary atoms as the reactants and thus have 

 common frequencies in the infra-red with the reactants. 



Alkali Metals in Liquid Ammonia. — Kraus {J.A.C.S., 1921, 

 43, 749) has continued his investigations into the nature of the 

 conduction processes occurring in solutions of the alkali metals 

 in ammonia. The alkali metals do not form compounds in 

 ammonia, and since the energy changes accompanying the 

 solution are inconsiderable there does not appear to be very 

 much complex formation. The electrical conductivity gives a 

 minimum value at about 0-05 N, rising in the case of the stronger 

 solutions to very high values. The nature of the ions carrying 

 the electric current varies with the strength of the solution. In 

 dilute solutions alkali metal is deposited on the cathode, but 

 the cation only possesses about one-seventh of the speed of 

 the anion. In the neighbourhood of N solutions, however, the 

 speed of the anion is some hundred times greater than that of 

 the cation. The anion is the same for all of the alkali metals. 

 Solution of metals in ammonia forms a connecting link between 

 electrolytic and metallic conduction. The cation is un- 

 doubtedly the metal, and the negative ion, being sub-atomic, 

 is probably the electron. In dilute solutions the electron is 

 probably associated with ammonia molecules, and hence its 

 speed is comparable with that of the cation, but in concentrated 

 solutions the complexes break down and the conductivity 

 becomes metallic. 



The Mass Effect in the Entropy of Solids and Gases. — Sackur 

 first showed that the entropy of a perfect monomolecular gas 

 is given by the relation 



S = |R /w 7w + IR /w T — R In P + So 



where So is a constant independent of the substance and m is 

 the molecular weight. Latimer {J.A.C.S., 1921, 43, 819) 

 proposes a simpler formula for the entropy of elements in the 

 solid state, which holds when its specific heat has reached the 

 Dulong and Petit value of 6 per gram atom. Thus : 



S298 = |R ^^ at. wt. + S^o 



holds where S^o = — 0*94. This value is calculated from the 

 value of S398 = 297-0 for KCl. Assuming that the entropy of a 

 compound is given by the sum of the entropies of the elements, 

 values are obtained agreeing fairly well with those from specific 

 heat and thermochemical data. An equation is also proposed 

 for the mass effect in diatomic gases. Tolman {J.A.C.S., 192 1, 

 43, 866) brings forward the objection to this formula that it is 

 not in agreement with the theory of similitude. 



Colour Changes in Disperse Systems. — ^Weigert {Kolloid Zeit., 



