32 RICHARDS—SOLVENT IN CRYSTALS. [April 4, 
tend to increase this new cause of error by increasing the field of 
its action. 
Thus the irregularity of curve shown by the results with baric 
chloride is easily explained, as well as the abiding presence of an 
excess of water. In the effort to escape the Scylla of inclusion the 
chemist has run foul of the Charybdis of adsorption. 
Adsorption of aqueous vapor can be eliminated by greatly rais- 
ing the temperature or by greatly reducing the tension of the 
surrounding aqueous vapor. ‘These means may be used either with 
anhydrous salts or with the sundry chemical vessels used for 
containing substances while weighing ; but unfortunately either of 
these changes drives out also the crystal-water contained in a 
hydrated salt. 
For these reasons tt seems to me impossible to determine with the 
exactness demanded in the most accurate work the true weight of any 
salt containing water of crystallization. 
In the case of anhydrous salts, which may be heated and placed 
in a perfectly dry atmosphere without decomposition, it is easy to 
eliminate adsorbed moisture, as already stated. It is not by any 
means so easy, however, to drive off the included solvent 
imprisoned in hidden cells. Some means must be used which dis- 
integrates the walls of these cells; and the means adopted depends 
upon the nature of the substance being studied. 
Either mechanical, thermal or chemical influences may be used 
to effect the disintegration. The mechanical means, pulverization, 
need not be further discussed. The application of heat first tends 
to vaporize the imprisoned solvent, if it is volatile, causing great 
pressure in the small space. ‘This pressure often causes the enclos- 
ing cell to explode, and thus the solvent is set free. It is not by 
any means certain, however, that all the cells are thus able to dis- 
charge their contents, especially in tenacious substances ; and in 
many cases hours of intense heating are necessary to drive off every 
trace of water, as with silica and iron rust. In the case of metals 
precipitated as such from aqueous solutions, temperatures not far 
from the melting-points must be used for drying, in order that their 
condition may be soft enough to yield to the internal pressure of 
the enclosed solvent. This precaution has usually been overlooked 
by physicists determining electro-chemical equivalents, although 
Lord Rayleigh pointed it out in the special case of silver twenty 
years ago. 
