28 RICHARDS—SOLVENT IN CRYSTALS. [April 4, 
THE INCLUSION AND OCCLUSION OF SOLVENT 
IN CRYSTALS. 
An Insipious SourcE OF ERROR IN QUANTITATIVE CHEMICAL 
INVESTIGATION. 
BY THEODORE WILLIAM RICHARDS. 
‘(Read April 4, 1903.) 
Perhaps the most frequent and the most insidious cause of error in 
quantitative chemical research is the unsuspected presence of water. 
The disturbing effect of this impurity is frequent, because water is 
one of the most plentiful of substances; and insidious, because 
there is usually ne easy quantitative or qualitative test for small 
quantities of it. 
The object of this paper is to recount several experiments indi- 
cating the prevalence and magnitude of inaccuracy from this cause, 
to show how many published results have been vitiated by it, to 
emphasize the theoretical aspect of the phenomenon, and especially 
to point out the methods of reducing the inaccuracy to a minimum. 
In a number of isolated cases it has been shown by various chem- 
ists that substances crystallizing from a solution enclose within 
their crystals small quantities of the mother-liquor. It is very well 
known, for example, that crystals of common salt explode or 
decrepitate upon being heated, because of the vaporization of the 
enclosed water. Thus when common salt is weighed some water 
is weighed with it. This water is not combined as water of crys- 
tallization, of which common salt has none; it is entirely acci- 
dental. Following the nomenclature of the mineralogists, a liquid 
imprisoned in this way is best called ‘‘ Included solvent.’’ The 
mother-liquor is thus imprisoned, in addition to combined water, 
also in salts which contain this latter as an essential part of their 
crystal structure ; in these cases it is even more difficult to detect 
and more generally overlooked, because of the simultaneous pres- 
ence of the combined water. 
Although these facts have been thus occasionally pointed out, 
the frequency of the occurrence of this cause of error has not often 
been realized by quantitative analysts. It is no careless exagger- 
ation to state that in all my chemical experience I have never yet 
obtained crystals from any kind of solution entirely free from acci- 
dentally included mother-liquor; and, moreover, I have never 
found reason to believe that anyone else ever has. Whether the 
