418 PRINCIPLES OF CHEMISTRY 



composition NaCl,10H,O it solidifies completely at a temperature of 

 23. A solution of table salt saturated at its boiling point boils at 

 about 109, and contains about 42 parts of salt per 100 parts of water. 

 Of all its physical properties the specific gravity of solutions of 

 sodium chloride is the one which has been the most fully investigated. 

 A comparison of the results of Kremers, Gerlach, Schmidt, Marignac, 

 Thomsen, Nicol, and Bender proves 17 that the specific gravity (in 

 vacuum, taking water at 4 as 10000) at 15 in relation to p, or the 

 percentage amount of the salt in solution, is expressed by the equa- 

 tion S 15 = 9991-6 + 71-lTp + 0-2140/A For instance, for a solution 

 200H. 2 O + NaCl, in which case _p=l-6, S 15 = 10106. It is seen 

 from the curve that the addition of water to the solution produces a 



contraction, 18 and that the addition of salt f or the differential -, J 



causes, at 15, a change of specific gravity which is expressed by the 

 straight line 71-17 + 0428^. At and 100 (when the specific 



of ice from a solution containing c grams of salt per 100 grams of water = 0'6c to c = 10 r 

 according to Rosetti = -0'649c to c = 8'7, according to De Coppet (to c = 10)= -0'762<r 

 + 0'0084c 2 , and according to Guthrie a much lower figure. By taking Rosetti's figure and 

 applying the rule given on p. 91 (Chapter I. Note 49) we obtain 



i = 0-649 x ---'-= 2-05. 

 18-5 



The data for strong solutions are not less contradictory. Thus with 20 p.c. of salt, ice 

 is formed at 14'4 according to Karsten, 17 according to Guthrie, 17'6 according 

 to De Coppet. Riidorff says that for strong solutions the temperature of the formation 

 of ice descends in proportion to the contents of the compound, NaCl,2H 2 O (per 100 grams 

 of water) by 0'342 C per 1 gram of salt, and De Coppet shows that there is no proportion- 

 ality, in a strict sense, for either a percentage of NaCl or of NaCl,2H 2 O. The data respect- 

 ing the vapour tension and boiling point of solutions of sodium chloride are as untrust- 

 worthy as the preceding. 



17 A collection of observations on the specific gravity of solutions of sodium chloride 

 and all other aqueous solutions which have been more or less investigated up to the 

 present time is given in my work cited in Chapter I. Note 50. 



Solutions of common salt have also been frequently investigated as regards rate of 

 diffusion (p. 62j, but as yet there are no complete data in this respect. It may be men- 

 tioned that Graham and De Vries demonstrated that diffusion in gelatinous masses (for 

 instance, gelatin jelly or gelatinous silica) proceeds in the same manner as in water, which 

 may probably lead to a convenient and accurate method for the investigation of the 

 phenomena of diffusion. N. Umoff (Odessa, 1888) investigated the diffusion of common 

 salt by means of glass globules of definite density. Having poured water into a cylinder 

 over a layer of a solution of sodium chloride, he observed during a period of several 

 months the position (height) of the globules, which floated up higher and higher as the 

 salt permeated upwards. Umoff found that at a constant temperature the distances of 

 the globules (that is, the length of a column limited by layers of definite concentration)' 

 remain constant ; that at a given moment of time the concentration, q, of different layers 

 situated at a depth z is expressed by the equation B K2 = log. (A q), where A, B, and 

 K are constants ; that at a given moment the velocity of the different layers is propor- 

 tional to their depth, &c. The information respecting diffusion is considerably extended 

 by thede researches, but still this subject, from its importance for the theory of solutions 

 and of liquids in general, yet awaits a perfectly-detailed investigation. 



18 If So be the specific gravity of water, and Sthe specific gravity of a solution containing 



