AQUEOUS SOLUTIONS--MACGREGOR. 263 
care was taken to prevent the !oss, either of any of the salt 
which had been weighed out for solution, or of any of the liquid 
in the bottles. When the desired amount of salt had been 
added to a bottle, the upper end of the tube was closed with a 
small cork to prevent evaporation, and the bottle was put in the 
bath. After an interval of about twenty-two hours the bottle 
was taken out, and, if the salt was found to be dissolved, was 
first well corked, and then rolled, until its contents had been 
thoroughly mixed. It was then replaced in the bath and left 
for another hour, when the height of the free surface of the 
liquid was observed. Not possessing a cathetometer, I required, 
for measuring differences of level, to trust to a steel scale placed 
in contact with the tube. Care was of course taken to avoid 
parallactic errors as much as possible. 
To one of the bottles no salt was added ; and it was kept care- 
fully corked up, so that the quantity of water it, contained might 
be constant. The variation of the height of the water in the 
tube of this bottle was due, of course, to change of temperature 
alone. This variation being observed, and the relative apparent 
thermal expansions of the liquids in the four bottles being 
known from the subsidiary experiments referred to above, the 
variations due to changes of temperature, of the heights of the 
solutions in the tubes of their respective botties could be deter- 
mined and eliminated. The variations of temperature were in 
all cases slight, the bath being large and its daily thermal history 
being very constant. 
The salts used, the acid phosphate of Sodium (H, Na PO, + 
12 H: O) and the basic carbonate of Sodium (Naz CO; + 10 H, O) 
were bought as pure, repuritied by crystallization, and dehy- 
drated by careful heating to the necessarv temperature. 
In all cases, after the solutions had stood awhile, a slight 
fluffy appearance presented itself in the bottles. The mass of 
the precipitated solid was, however, very small—so small that it 
was hardly possible to weigh it. Hence I considered that its 
effect on the result might be neglected. It was probably due to 
the presence of some impurity in the water. 
In both cases I found weak solutions of these salts to have 
