626 
Proceedings of the Royal Society 
4. Comparison of the Volumes of Saline Solutions with the 
Sums of the Volumes of the Constituents. By J. Holms 
Pollok, Chemical Laboratory, Glasgow University. 
Communicated by Sir William Thomson. (Plate XXII.) 
My attention was first drawn to the change of volume which 
takes place when a salt is dissolved in different quantities of water, 
by an experiment shown by Sir William Thomson in his class at the 
Glasgow University. He took a wide glass tube, about 3 feet 
long, sealed at one end, and having a narrow tube attached to the 
other end. The lower half of this tube he filled with a saturated 
solution of common salt, and on the top of that he placed a layer of 
water, which just reached a mark about the middle of the small 
tube. He then corked the open end, and mixed up the water and 
brine by inverting the tube two or three times, when it was seen 
that the solution had contracted. 
After seeing this experiment, I determined to find the amount of 
contraction that takes place ; but, instead of taking a saturated 
solution, I took 1 gramme of the anhydrous salt, and ascertained 
the amount of contraction that takes place on dissolving it in any 
quantity of water from 1 gramme to 20. 
These results I calculated from the tables of specific gravities of 
solutions of salts, given in Storer’s Dictionary of Solubilities. 
Those for the curves at 15° were obtained by Gerlach, with the 
exception of those for alcohol, which were taken by Fownes; the 
specific gravities for the others were obtained by Schiff, with the 
exception of those for lead nitrate, which were by Hassinfratz. The 
contractions were found by means of the formula 
n _ A B A + B 
° a + b r 1 
where C is the contraction in cubic centimetres, A the number of 
grammes of the substance taken, B the number of grammes of water 
added, and a, b , and r the specific gravities of the substance, the 
water, and the resultant solution, all referred to 4° C. 
Thus, I find by calculation in the case of magnesium sulphate at 
15° C., that taking 1 gramme of MgS0 4 , and adding to it 2 ‘961 
