160 Progress of Foreign Science. 



be postponed till the other ingredients have been determined, 

 we may calculate with more precision the quantity of iron ne- 

 cessary to precipitate the arsenic acid. For 1 atom of arsenic 

 we take 2 atoms of iron, which makes, according to Berzelius, 

 the relative weight of 3 parts of iron to 2 parts of arsenic. 

 Though an excess of oxide of iron augments the bulk of the 



f)recipitate, it contributes, however, to render the subarseniate 

 ess gelatinous, and more easy to edulcorate. We ought to 

 heat the subarseniate at the fire two successive times, to be 

 sure that it can lose nothing more, for a small quantity of sul- 

 phuric acid often adheres to this precipitate with much energy. 

 As arseniate of barytes is also an insoluble compound, Berze- 

 lius endeavoured to separate the arsenic acid from a neutral 

 solution by muriate of barytes, but he found anomalous results, 

 that render this method of no value *. 



M.Peschier of Geneva, has published lately elaborate analyses 

 of different varieties of mica, the results of which are as follows : 



Silica Alum. Mag. Lime Iron Pr. Titan Mang. Soda Litli. Pot. 



1. Green mica of Vcsuvina 45.7 31.7 0.95 10.75 6.8 0.1 a trace 



2. Black mica of Vesuvius 42.0 8.35 0. 15.7 8.35 15.0 8.5 2.5 



8. Fol. black mica of Sib. 35.5 11.25 — — 10.0 30.0 a trace 1.7 — 0.1 



The latter specimen came from the Uralian mountains, as 

 well as the mica analyzed by Klaproth, who detected no tita- 

 nium in it. His analysis gave, silica, 42.5 ; alumina, 11.5; 

 magnesia, 9; oxide of iron, 22; manganese, 2; potash, 10; 

 and loss, 1. M. Peschier says he has also discovered titanium 

 in the white mica of Siberia, with short rounded plates and 

 metallic lustre, as well as in the mica of Massachusetts. This 

 is the first time that titanium has been recognised in the 

 micas ; Mr. Peschier is prosecuting his researches to see if it be 

 one of their general constituents. It was at the invitation of 

 M. Soret, whose optical researches on mica have been so much 

 admired, that M. Peschier undertook their chemical investiga- 

 tion. Each of the above specimens has only one axis of double 

 refraction f. 



VII. Applicatiofis of Chemistry to Medicine and the Arts. — We 

 perceive nothing of consequence in the foreign journals on the 

 last subject. What belongs to medicine has been already dis- 

 cussed under vegetable chemistry. The quantity of original 

 matter in the present Number of the Journal, precludes the 

 account of optics, promised in our last. 



VIII. Among the many curious papers on Electro-magnetism, 

 with which Gilbert's Annalen have been lately enriched, we 

 have been particularly pleased with that of M. J. J. Prechtl, of 



* Annates de Chiin. ft de Phys. xvii. p. 113. 

 t Jour, de Phys. xciii. p. 2-41. 



