NATURE AND DETECTION OF METALLIC rOISONS. 309 



tarther direction for its preparation, that the oxysulphate from 



which it is obtained, should be made by dissolving iron with 



the aid of heat in nitric acid, afterward precipitating the oxide, 



and redissolving it in sulphuric acid. The salt thus formed 



contains the metal at a maximum of oxidation. 



The whole of the above processes for the detection of arsenic Reduction of 



of course refer to the cases where it has been exhibited only in ^^^ arsenic to 



■' be preferred 

 a fluid state. Whenever it can be accomplished, however, by when practi- 



far the most satisfactory means of arriving at a knowledge of ^*'^'^- 

 the presence of this substance, is to reduce it to a metallic 

 state, which may be readily effected, either by subliming it in a 

 glass tube with the aid of charcoal, or exposed between two 

 plates of copper, according to the plans recommended in che- 

 mical works. 



//. Corrosive Sublimate. 



For the discovery of corrosive sublimate, the methods almost Oxymurlate 



exclusively resorted to until very lately were its precipitation by °^ mercury, 



means of one of the carbonated fixed alkalis, or by lime water, 



which detach it under the form of an orange-coloured, or 



orange yellow, sediment. Dr. Bostock has since recommended Muriate of tin 



muriate of tin : but, to the use of this test there is considera- rei^ornmended 



' by Dr. Bos- 



ble objection, inasmuch as a precipitate, very similar in ap- took. 



pearance to the one obtained from mercury, is always occa- Objection, 

 sioned whenever muriate of tin comes into contact with a solu- 

 tion containing water. This could not fail to render the result 

 of any experiment ambiguous j but should it so happen, that, 

 from a particular circumstance, the employment of the mu- 

 riate might be rendered at all desirable, \<s effect upon the fluid ^^ 

 suspected to contain corrosive sublimate should be collated with degree, 

 the appearance produced from its mixture with an equal quan- ^ou'^ed. 

 tity of water, since the precipitate occasioned in a mercurial 

 solution is remarkably more abundant than in the latter case, 

 and sufficient to dispel all uncertainty arising from this cause. 

 But a test, at once the most easy of application and satisfactory, ^^5^^^;^^^ 

 is furnished by means of galvanism, in which the mercury is supplies a 

 separated in a metallic state. This experiment can be made by much better 

 any person, and almost in any situation. It is merely neces- 

 sary to take a piece of zinc wire, or in its absence a piece of 



us, in some 

 8ur- 



