ON WATER AND ITS COMPOUNDS 87 



of liquids in water, 40 just as happens in the combination of substances 

 when evidently new substances are produced. 41 The contraction which 

 takes place in solution is, however, very small, a fact which depends on 

 the small compressibility of solids and liquids, and on the insignificance 

 of the compressing force acting in solution. 42 The change of volume 

 which takes place in the solution of solids and liquids, or the altera- 

 tion in specific gravity 43 corresponding with it, depends on peculiari- 

 ties of the dissolving substances, and of water, and, in the majority 

 of cases, is not proportional to the quantity of the substance dis- 



below and water above. The flask was then shaken in order to accelerate diffusion, and 

 it was observed that the volume became less if the temperature remained constant. 

 This can be proved by calculation, if the specific gravity of the solutions and water be 

 known. Thus at 15 one c.c. of a 20 p.c. solution of common salt weighs 1'1500 grams, 

 hence 100 grams occupy a volume of 86'96 c.c. As the sp.gr. of water at 15 = 0'99916, 

 therefore 100 grains of water occupy a volume of 100'OB c.c. The sum of the volumes is 

 187'04 c.c. On mixing, 200 grams of a 10 p.c. solution are obtained. Its specific gravity is 

 1-0725 (at 15 and referred to water at its maximum density), hence the 200 grams will 

 occupy a volume of 186'48 c.c. The contraction is consequently equal to 0'56 c.c. 



40 The contractions produced in the case of the solution of sulphuric acid in water 

 are shown in the diagram Fig. 17 (page 7.6). Their maximum is 10' 1 c.c. per 100 c.c. of 

 the solution formed. A maximum contraction of 4'15 at 0, 3'78 at 15, and 3'50 at 30, 

 takes place in the solution of 46 parts by weight of anhydrous alcohol in 54 parts of 

 water. This signifies that if, at 0, 46 parts by weight of alcohol be taken per 54 parts by 

 weight of water, then the sum of their separate volumes will be 104'15, and after mixing 

 their total volume will be 100. 



41 This subject will be considered later in this work, and we shall then see that the 

 contraction produced in reactions of combination (of solids or liquids) is very variable 

 in its amount, and that there are, although very rare, reactions of combination in which 

 contraction does not take place, or when an increase of volume is produced. 



4 - The compressibility of solutions of common salt is less, according to Grassi, than 

 that of water. At 18 the compression of water per million volumes =48 vols. for a 

 pressure of one atmosphere ; for a 15 p.c. solution of common salt it is 82, and for a 

 24 p.c. solution 26 vols. Similar determinations were made by Brown (1887) for saturated 

 solutions of sal ammoniac (38 vols.), alum (46 vols.), common salt (27 vols.), and sodium 

 sulphate at + 1, when the compressibility of water =47 per million volumes. This inves- 

 tigator also showed that substances which dissolve with an evolution of heat and with an 

 increase in volume (as, for instance, sal-ammoniac) are partially separated from their 

 saturated solutions by an increase of pressure (this experiment was especially convincing 

 in the case of sal-ammoniac), whilst the solubility of substances which dissolve with an 

 absorption of heat or diminution in volume increases, although very slightly, with an 

 increase of pressure. Sorby observed the same phenomenon with common salt (1863). 



43 The most trustworthy data relating to the variation of the specific gravity of 

 solutions with a change of their composition and temperature, are collected and discussed 

 in my work cited in footnote 19. The practical (for the amount of a substance in 

 solution is determined by the aid of the specific gravities of solutions, both in works and 

 in laboratory practice) and the theoretical (for specific gravity can be more accurately 

 observed than other properties, and because a variation in specific gravity governs the 

 variation of many other properties) interest of this subject, besides the strict rules and laws 

 to which it is liable, make one wish that this province of data concerning solutions 

 may soon be enriched by further observations of as accurate a nature as possible. Their 

 collection does not present any great difficulty, although requiring much time and 

 attention. 



