36 ROYAL SOCIETY OF CANADA 
CuC1,.2CuO than to any other of the eight or nine that have been 
described by previous investigators.” 
In the absence of any clearly defined principle of selection much 
must of necessity be left to individual taste, and as there is wide room 
for difference of opinion, flat contradictions are not uncommon in the 
literature of the subject. In Dammer’s “Handbuch der anorganischen 
Chemie,” for instance, after detailed descriptions of seven compounds 
of lead chloride with ammonium chloride, comes the announcement 
that “according to Randall, none of André’s compounds exist”; while 
with reference to Cross and Suguira’s basic oxychlorides of lead, 
Pb,,CL,,0,, Pb,CL,0, and Pb,CLO, the editor himself ventures on the 
criticism, “ These were obviously impure, and probably badly analysed.” 
But neither André nor Randall, Cross nor Dammer adduces conclusive 
evidence in support of his contentions. | Mere complexity in the 
formula is in itself no:bar to the genuineness of a chemical compound 
—Mr. Allan’s experiments, referred to below, establish beyond question 
the existence of a basic nitrate of bismuth with the formula 
Bish, .N-O.: | | 
Even in the case of so important an article of commerce as white 
lead, it is still wholly uncertain whether the different varieties are to be 
regarded as distinct chemical species, as mixtures of a few individuals, 
or as more or less saturated (solid) solutions of carbon dioxide in lead 
oxide or in some basic carbonate. It is consequently hardly surprising 
that success in the manufacture of such substances is conditioned by 
strict adherence to empirical recipes, any deviation from which may 
seriously affect the properties—and the value—of the product. 

Application of the Phase Rule. 
Difficult though it might appear to find any rational basis of 
classification for these precipitates, there is one large group—viz., all 
cases where equilibrium is attained between precipitate and mother- 
liquor—for which the problem is completely solved by two theorems 
due to Professor Willard Gibbs, which form part of what is commonly 
known as his “Phase Rule.” 
These theorems may bestated as follows:—Assuming that the sys- 
tem has arrived at equilibrium, at “arbitrary” temperature and pressure, 
(i) The system can in general consist of no more phases than it has 
components, (11) A solution can form one of a group of n phases (n is 
the number of the components) only if it has attained a certain compo- 
sition dependent on the temperature and pressure, and on the chemical 
and physical nature of the other phases present. 
