262 AQUEOUS SOLUTIONS—MACGREGOR. 
being filled with distilled water of known temperature, from 
measuring vessels whose volumes were known. The one used 
in calibrating the tubes was so divided that changes in the 
volume of the water it contained could be read to 0:05 ce. The 
water, with which the bottles were thus filled, had been freed 
from air under the receiver of an air-puimp. 
To test the tightness of the stoppers, the dilatometers were 
filled, until the upper surfaces of the water were near the tops 
of the tubes. The stoppers were thus subjected to as great 
pressures as they would be during the experiments. After the 
bottles had taken the temperature of the bath, I observed the 
variation of the height of the water in the tubes from time to 
time, until I had satisfied myself that there was no leak,—a 
return to a formerly observed height in one bottle being accom- 
panied in all cases by a similar return in the others. 
I next satisfied myself that differences of temperature between 
the bottles, greater than any which could arise during the 
experiments through the dissolving of salt in some bottles and 
not in others, would vanish in less than the time that was to 
intervene between successive measurements. 
As the dilatometers could not be kept at constant temperature, 
and as any change of volume of their contents must therefore be 
partially due to change of temperature, it was necessary to know 
the relative apparent thermal expansion of their contents. For 
this purpose, both at the outset when all the bottles contained 
water, and at intervals during the series of experiments when 
some of them contained solutions, the temperature of the bath 
was varied, and the heights of the water or solutions in the dif- 
ferent tubes were observed when the bottles had assumed the 
temperature of the bath. These results were tabulated for pur- 
poses of correction. 
The solutions, whose volumes were measured, were formed by 
the addition of known masses of anhydrous salt to the water in 
the bottles. The salt was simply dropped little by little down 
the tubes of the dilatometers. Occasionally the salt was found 
to cake at the surface of the liquid. In that case various 
expedients were adopted to hasten the solution ; but the greatest: 
