24 THE CONSTITUTION OF THE SILICATES. [bull. 125. 
union. Following Leniberg, lie regards sodalite as a molecular com- 
pound of nephelite with sodium chloride, and taking his series of 
compounds throughout, he looks upon the sodium salts which have 
been added to the fundamental silicate as equivalent in function to 
water of crystallization. In favor of this view he cites many arguments, 
some of which are entitled to considerable weight. Thus, when sodalite 
is ignited NaCl is driven off, whereas if the chlorine were united with 
aluminum A1C1 3 should be expelled. Similarly, by the action of water 
alone, sodium chloride can be split off from the sodalite molecule, thus 
indicating a looser form of union than the proposed structural formulae 
show. 
But what is molecular union? To this question there is no satisfac- 
tory answer, and even in the case of water of crystallization the term 
is only a confession of ignorance. Unless we assume the existence of 
two kinds of chemical union, it means merely that the structural link- 
ing is unknown, and that the problem is laid on one side, conveniently 
labeled for future reference. The constitutional formulae here adopted 
for sodalite and cancrinite are intended to give a provisional solution 
of the juoblein in their particular cases, and to express the genetic 
relationships Avith nephelite on the one hand and the crystallographic 
analogy with garnet on the other. The objections to them raised by 
Thugutt are serious, but not absolutely conclusive. When sodium 
chloride is split off from sodalite the mechanism of the reaction is 
quite unknown, and the relative affinities in the molecule are quite 
unstudied. Until these are understood the objections raised by Leni- 
berg, Thugutt, and others are not fatal. Furthermore, the presence of 
a group = A1 — CI does not imply, as Thugutt supposes, the splitting 
off of AICI3 by heat. To effect such a decomposition three molecules 
of sodalite would have to be broken up, and there is no probability 
that such a disintegration could occur. At all events the formuhe 
proposed fulfill a definite purpose, even though they are not finally 
established. They express known relations, but not necessarily all the 
relations which the future may reveal. The facts that the sodalite- 
cancrinite minerals are derivable from nephelite, and that nephelite is 
again derivable from them, are unquestionable. 
Taking now the formula of a typical garnet, Al 2 (Si0 4 ) :3 Ca3, the ques- 
tion of its molecular structure remains to be considered. Regarding it 
as a derivative of the normal salt Al 4 (Si0 4 )3, it may be written in at 
least two ways, thus : 1 
1. 2. 
/ 4= I Ca A1 ^f 
Al-Si0 4 ™ i >Si0 4 =Ca 
AI< 
Ca 3 
IfAl 
\\SiO 4 =0a 
That is, isomerism is possible; and of the two species partschinite and 
spessartite, one may belong to one type and the other to the other. 
