44 



L. F. Nilson, 



manner, he thinks himself authorized to assume the composition of the salt 

 to he Mg.0 2 .SeO f 3H 2 0. It is, however, to be observed, that he seems to 

 have examined it, such as it is obtained, when deposited from boiling water. 

 It may be possible that, thus prepared, it contains 3 mol. of water of crys- 

 tallization, although the behaviour of the salt, described above, seems to 

 indicate another composition. In fact, it is very likely that in the case of 

 MusPRATT's water determination, selenious acid has also been expelled, sele- 

 nite of magnesium, according to my experience, when heated, without diffi- 

 culty parting with its acid. 



2. Diselenite: Mg.0 2 .SeO + H 2 .0 2 .SeO + 3H 2 0. 



Neutral selenite of magnesium easily dissolves in selenious acid. A 

 solution, prepared from 1 mol. of selenite and 1 mol. of acid, having, by 

 spontaneous evaporation, reached such a consistency that it is scarcely 

 liquid, little by little affords the salt in pretty large aggregates in form of 

 six-sided pyramids, composed of innumerable, small, microscopic crystalline 

 individuals, which were but exceptionally observed isolated and then seemed, 

 to possess the form of a six-sided double-pyramid with basal end-faces, trun- 

 cating the summits. The salt easily dissolves in water and is permanent 

 even in dry air. The water of crystallization is parted with at 100° (found 

 1G.47, calc. 16.17 pi*, ct.) 



An aly ses : 



1) 0.5 gr. salt gave 0.234 gi*. selenium = 0.3288 gr. selenious acid and 

 0.1768 gr. sulphate of magnesium = 0.0589 gr. magnesia. 



2) 0.44^)7 gr. salt gave 0.2106 gr. selenium = 0.2959 gr. selenious acid 

 and 0.1605 gr. sulphate of magnesium = 0.0535 gr. magnesia. 



3) 0.2155 gr. salt, kept over sulphuric acid, showed undiminished weight 

 but at 100° lost 0.0355 gr. water. 



Ccntesimally represented : 



found 



1. 2. 3. 



Magnesia 11.79 11.90 — 



Selenious acid . . 65.76 65.81 — 



Water (loss^) . . . . 22 .45 22-29 16.47 



loo. 00 100.00 



calculated 



MgO 40 11.98 

 2Se0 2 222 66.47 

 4H 2 __7_2 



334 100.00 



