CONDUCTIVITY OF AQUEOUS SOLUTIONS. McKAY. 349 



presence of chlorides, and with potassium ferrocyanide for iron. 

 In none of these tests was any reaction observed. The solu- 

 tions whose conductivities were measured range in the case of 

 potassium sulphate from 1 to 0.1 gr.-eq. per litre and from 0.02 to 

 0.0066, and in the case of magnesium sulphate from 2 to 0.4, and 

 from 0.025 to 0.008 gr.-eq. per litre. All the determinations by 

 Kohlrausch which lie within these limits agree with the writer's 

 within the latter's limits of error, except the value for the solu- 

 tion of magnesium sulphate of concentration 0.5, the conductivity 

 of which, as measured by the writer, is 0.6 percent, less than the 

 value given by Kohlrausch. This difference was found to hold 

 in the case of four specimens of recrystallized salt, both Eimer & 

 Amend's and Merck's. The error of the writer's determination 

 in this case might be 0.4 per cent. 



In the first attempts to prepare the double salt, the K 2 S0 4 

 and MgSO 4 were brought together in equimolecular, or nearly 

 equimolecular proportions. In the case of the salt numbered I 

 below the solution thus obtained was evaporated until crystals 

 began to form. It was then allowed to cool to 75C. and kept at 

 a temperature varying from 60C. to 7oC., until a large part of 

 the salt had crystallized out. The crystals were dried on filter 

 paper, and a portion heated in a platinum crucible to a dull red 

 heat, until the weight was constant. The amount of SO 4 in the 

 dry salt was then determined. The second specimen was crystal- 

 lized out from a solution containing a small excess of MgSO 4 , and 

 w r as treated in a similar manner, except that the drying on filter 

 paper was omitted. The third specimen was recrystallized 

 according to a method given in its essentials in Dittmar's Quan- 

 titative Analysis. 87.1 grammes of powdered recrystallized 

 K 2 SO 4 , together with 153.4 gms. of Eimer & Amend's chemically 

 pure MgSO 4 + 7 H 2 O, which had not, however, been recrystallized 

 by the writer, were dissolved in 350 grammes of hot distilled 

 water. The solution was made up to about 645 grammes, and 

 after it had cooled to 50C, put in a porcelain basin and left 24 

 hours. The crystals thus obtained were washed in pure water, 

 powdered, and then dried on filter paper and by exposure to the 



