RICHARDS AND BONNET. — DISSOLVED CHROMIC SULPHATE. 5 



prevent the occlusion of chromium in the baric sulphate.* For every 

 double gram atom of chromium there were found in two analyses 2.98 

 and 2.99 gram molecules of the sulphuric group, showing that the salt 

 was normal. It was found by difference to contain 42.81 per cent or 

 16.3 molecules of water. 



Dialysis. 



Van Cleef in 1881 used dialysis as a means of partially separating the 

 acid and the basic substances, and his method seemed worthy of a further 

 trial. Accordingly we made a number of experiments in this direction. 

 To our disappointment we found the results difficult to interpret quanti- 

 tatively, partly because dialysis is a slow process, and during its dilatory 

 progress the green solution has time to revert partially into the violet one. 

 The experiments are, however, worthy of brief chronicling, because their 

 qualitative interpretation is clear. 



In the first place violet solutions were subjected to dialysis at tempera- 

 tures from 16° to 20°, the results showing that the diffusing solution is 

 essentially like that which remains behind. Parchment paper, carefully 

 tied and with all accidental apertures sealed with collodion, was used as 

 the diaphragm ; and the level of the liquids was always adjusted in such a 

 way as to prevent filtration. 



For analysis, two precisely similar portions of a given solution were 

 measured by means of a pipette. One of these portions was evaporated 

 and ignited in a platinum crucible, and the other was diluted, and treated 

 with slight excesses of amnionic hydroxide, baric chloride, and hydro- 

 chloric acid in succession. The mixture was heated after the addition 

 of the ammonia, and was digested for some time on the steam bath before 

 filtration. This method, an application of that devised by Kiister and 

 Thiel f for exclusion of iron from baric sulphate, is successfully appli- 

 cable to the case of chromium also. Here are the results : — 



* A gram of the salt yielded in one case 0.2220, and in another 0.2229 grams of 

 chromic oxide ; again in one case 1.017 and in another 1.021 grams of baric 

 sulphate. The baric sulphate was precipitated from a very dilute cold solu- 

 tion containing hydrochloric acid. The mixture was subsequently heated before 

 filtration. The baric sulphate formed in this way is colorless and contains only 

 traces of chromium. This matter will be discussed at length in another paper. 



t Zeit. anorg. Chem., 19, 97 (18 ( J9). This matter will be discussed in detail in 

 another paper. 



