180 THE EARL OF BERKELEY, MR. E. G. J. HARTLEY AND DR. C. V. BURTON: 



It is to be noted that ROSCOE and SCHORLEMMER (2nd ed., ' Treatise on Chemistry, 

 vol. ii., p. 1025), stated that the salt crystallizes with 12Aq; our results point, 

 however, to llAq. 



The foregoing results, considering the imperfection of analytical methods, seemed to 

 justify the assumption that the salt is of unvarying composition as far as the metallic 

 radical is concerned ; hut as it is a highly hydrated salt, with presumably a vapour 

 pressure, it was considered necessary to determine the water content of a sample 

 of the salt used in each " make up " of solution. These determinations are given in 

 the table of the results of the direct osmotic-pressure measurements (p. 191), and 

 were obtained by drying the salt to constant weight in an air oven. 



DENSITIES. 



A 400 c.cm. and a 500 c.cm. flask were obtained, with graduated necks, whose 

 diameter was such that 1 cm. of neck corresponded to 1 c.cm. capacity, and the 

 graduations were in millimetres. The back of the neck was of milk glass, with a blue 

 line running down the middle (Prof. SCHELLBACII'S device for burettes). By this 

 means, readings estimated to O'Ol mm. could be obtained with fair accuracy. The 

 necks were calibrated by adding weighed quantities of water to the flask (already 

 filled with water) when in a constant temperature bath at C. and also at 30 C. 

 The meniscus level was read through a telescope and care was taken to avoid parallax. 

 The capacity of the flask was determined by weighing against a counterpoise on a 

 large Oertling balance. 



A similar process was followed for the determination of the densities. The flask 

 was filled, weighed, and placed in ice, and the level read when the solution had ceased 

 contracting generally in about 5 hours' time. 



Experimental Error. 



It was found that the readings of the meniscus level could be relied on to 

 0'01 mm.; this corresponds to a maximum error of 0'02 c.cm. on 400 c.cm., or 



