SODIUM CHLORIDE BEBTHOLLET'S LAWS 429 f 



of solutions of NaCl n at 15 (in vacuo, taking water at 4 as 10,000), 

 with regard to p (the percentage amount of the salt in solution), show 

 that it is expressed by the equation S, 5 = 9991-6 + 7l'I7p + 0'2140j!>*. 

 For instance, for a solution 200H 2 + NaCl, in which case p === 1-6, 

 S 15 = T0106. It is seen from the formula that the addition of water 

 produces a contraction. 18 The specific gravity 19 at certain temperatures 

 and concentrations in vacua referred to water at 4 = 10,000 20 is here 



given for 



15 30 100 



2> = 5 10372 10353 10307 9922 



10 10768 10728 10669 10278 



15 11164 11107 11043 10652 



20 11568 11501 11429 11043 



It should be remarked that Eaum^'s hydrometer is graduated by 

 talcing a JO p.c. solution of sodium chloride as 10 on the scale, and 

 therefore it gives approximately the percentage amount of the salt in a 



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

 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 (Chapter I.), but as yet there are no complete data in this respect. It may be 

 mentioned 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 dis- 

 tances of the globules (that is, the length of a column limited by layers of definite con- 

 centration) 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-Ks = log. (A-g), 

 where A, B, and K are constants; that at a given moment the rate of diffusion of tho 

 different layers is proportional to their depth, &c. 



18 If S be the specific gravity of water, and S the specific gravity of a solution con- 

 taining^? p.c. of salt, then by mixing equal weights of water and the solution, we shall 

 obtain a solution containing ^P of the salt, and if it be formed without contraction, then its 



O } ' 1 



specific gravity x will be determined by the equation = + -, because the volume is 



x S S 



equal to the weight divided by tho density. In reality, the specific gravity is always found 

 to be greater than that calculated on the supposition of an absence of contraction. 



19 Generally the specific gravity is observed by weighing in air and dividing the weight 

 in grams by the volume in cubic centimetres, the latter being found from the weight of 

 water displaced, divided by its density at the temperature at which the experiment is carried 

 out. If we call this specific gravity Sj, then as a cubic centimetre of air under the usual 

 conditions weighs about (V0012 gram, the sp. gr. in a vacuum 5 = 5 1 + 0'0012 (S^-l),. 

 if the density of water = 1. 



80 If the sp. gr. S 2 be found directly by dividing the weight of a solution by the 

 weight of water at the same temperature and in the same volume, then the true sp. gr. 

 S referred to water at 4 is found by multiplying <S 2 by the sp. gr. of water at .the tem- 

 perature of observation. 



