356 Frederick Guthrie on Salt Solutions 



of solution is capable of dissolving. Experimentally, this ap- 

 pears in the results of all my experiments to be described below, 

 that the stronger a solution of any given salt is (being weaker 

 than the cryohydrate), the lower is the temperature below 0° 

 C. to which it must be brought in order to yield ice ; and the 

 stronger a solution of any given salt is (being stronger than 

 the cryohydrate), the less need its temperature be depressed in 

 order to yield the anhydrous salt, or a hydrate richer in salt 

 than the cryohydrate. And, as before stated, the latter kind 

 of separation is quite continuous with the separation of such 

 bodies at temperatures above 0°, such as is exhibited in the 

 ordinary Tables of solubilities referred to in § 123. 



§ 125. Almost all the numerical results are represented 

 graphically in fig. 1. The abscissae are in all cases the per- 

 centages of the anhydrous salt in the water. The ordinates, 

 — or + , are the temperatures Centigrade below or above 0° 

 C. at which solid matter begins to separate on cooling the solu- 

 tion. The point of reflexure (that is, the lowest point of each 

 tracing) shows the temperature at which the particular cryo- 

 hydrate is formed, and also the percentage of anhydrous salt 

 in the cryohydrate. The left-hand branches of the tracings 

 exhibit the temperature at which ice separates from solutions 

 of the respective salts of the corresponding percentage compo- 

 sition. The right-hand branch of each tracing shows the tem- 

 peratures at which the anhydrous salt or a hydrate richer than 

 the cryohydrate is separated. The parts of these branches 

 above the zero-line show the ordinary solubility of the same 

 salts above 0°. The figure is reduced from my drawing, in 

 which the percentages and tenths were centimetres and milli- 

 metres ; and the degrees C. and their tenths were also centi- 

 metres and millimetres. The actual observations, being dotted 

 off, were joined by straight lines. 



§ 126. General remarks on the experiments. — Whenever 

 possible, the solutions of the salts examined were made by dis- 

 solving weighed quantities of the anhydrous pure salt in quan- 

 tities of water measured from a Mohr's burette. The grammes 

 and centimetres were those of the percentages ; that is, to form 

 a 20-per-cent. solution, 20 grms. of the salt and 80 cubic cen- 

 tims. of distilled water were employed. Now and then a con- 

 trol mixture formed by weighing only was examined ; but it 

 appeared that the sources of error introduced by measuring 

 instead of weighing the w T ater were vanishingly small com- 

 pared w r ith the unavoidable errors of observation, notably those 

 mentioned in the next paragraph. 



Super saturation. — With every salt solution from which we 

 wish to get solidification by means of cooling, we have to guard 



