72 CHEMISTRY. 



3. Hydrosulphuric acid, when transmitted through a solution of 

 arsenious acid, precipitates the sesquisulphuret, (orpiment.) 



4. The production of arsienuretted hydrogen. — By adding the 

 matter containing arsenic to the materials for generating hydrogen, 

 on burning a jet of this gas, and holding over it a piece of glass or 

 porcelain, an arsenical ring is formed. None of the above tests 

 can be relied on singly. The best method is to reduce the arsenic 

 by means of heat and charcoal, (or black flux,) in a glass tube ; by 

 which means the arsenical ring may always be produced, and the 

 peculiar odour of burning arsenic be detected. The proper antidote 

 for arsenic is the hydrated peroxide of iron, in a moist state. 



The remaining metals of this order are Chromium, Vanadium, 

 Tungsten, Molybdenum, Columbium, Titanium, Tellurium, and 

 Osmium. None of them are of practical importance except chro- 

 mium, two salts of which are much used in the arts, viz. : chromate 

 of lead, (chrome yellow,) and the bichromate of potash. 



CHAPTER III. 



SALTS. 



The term salt was formerly restricted to a compound of an acid 

 and a salifiable base ; but this definition was necessarily vague, on 

 account of the difficulty of always accurately defining what was an 

 acid and what was a base. Formerly an acid was considered to be 

 an oxidized body which has a sour taste, reddens litmus, and 

 neutralizes alkalies. Subsequent discovery showed the propriety of 

 extending this definition ; for, first, the discovery of the hydracids 

 proved that oxygen is not essential to acidity ; and secondly, some 

 compounds, owing to their insolubility, have neither a sour taste, 

 nor do they redden litmus ; yet they neutralize bases ; thirdly, there 

 are some acknowledged acids, as carbonic, hydrocyanic, &c., which 

 are unable fully to destroy the alkalinity of potassa. 



Chemists of the present day agree to call all such bodies acid 

 which unite with potassa or ammonia, and give rise to bodies similar 

 in constitution and general character, to the salts which sulphuric or 

 some other admitted acid forms. For similar reasons, the present 

 notion of what constitutes an alkaline or salifiable base, is that of a 

 body which unites definitely with admitted acids, to form with them 

 compounds resembling recognised salts. 



There is a very important class of compounds, which, however, 

 would not fall within the range of the above definition of a salt ; and 

 which yet have undoubted claims to be considered as such : they 



