OSMOTIC PRESSURES OF AQUEOUS SOLUTIONS OF CALCIUM FERROCYANIDE. 179 



General Scheme of Operations. 



About 500 c.cm. of solution are necessary for a determination of equilibrium 

 pressure and vapour pressure ; as two independent measurements of these quantities 

 are required at some five points in the total range of 40 to 130 atmospheres, it follows 

 that a large amount of salt was necessary. 



To save salt, and in order to have a direct comparison between the two methods ot 

 measuring osmotic pressures, it was decided to use the same " make up " of solution 

 for both these measurements and the densities. The procedure adopted was to make 

 up the solution in the morning (a portion of salt being set apart for analysis) and 

 weigh it, and determine its density in the evening. The vapour-pressure vessels were 

 then tilled and got ready for weighing ; and at the same time the direct osmotic- 

 pressure apparatus was set up for "guard-ring leak" (see 'Phil. Trans.,' A, vol. 206, 

 p. 490). The next morning the vapour-pressure vessels were weighed, placed in the 

 bath, and the air current started ; meanwhile the direct determination of the 

 equilibrium pressure was carried out and the tube set up for "solution leak" (Joe. cit., 

 p. 493). 



Purification of the Salt. 



% 



The calcium ferrocyanide was obtained from Messrs. Kahlbaum as their purest. 

 The first lot of about 2 kgr. was found to contain some potassium, probably in the 

 form of the double salt. This lot, and also all succeeding ones, some 10 kgr. in all, 

 was dissolved to foi"m as nearly a saturated solution at the laboratory temperature as 

 possible (without warming), and then filtered through a Chamberland porcelain filter. 

 The salt was then recovered by recrystallization. Various methods of recrystallization 

 were tried, but the best was to pass, by means of a suction pump, a steady stream 

 of filtered air over crystallizing dishes containing the solution, and at the same time 

 maintain them at from 20-C. to 25 C.* The various crops were freed from mother 

 liquor by means of a platinum ceutrifugalizing machine ; they were then powdered 

 and dried in the air current at the temperature of the laboratory. 



Analyses of samples of the 1st and 2nd recrystallization showed no marked 

 difference in composition, but it should be mentioned that sometimes the salt came 

 out of solution slightly green instead of yellow, and although neither in analysis iior 

 in the experiments (where want of the yellow salt compelled the use of the green) 

 was there found to be any difference in the results, yet it was thought better to avoid 

 the use of these crops as much as possible. It was found that, on prolonged standing, 

 a trace of precipitate came out of solution this we suspected to be CaCO 3 , but tests 

 failed to confirm this the quantity was not enough for analysis. 



The following table gives the analytical results : 



* The solutions seem to decompose at higher temperatures, 

 2 A 2 



