detection of Poisonous Compounds. 337 



Dr. Christison's method of separating mercury by the pro- 

 tochloride of tin must be regarded as yielding the most 

 conclusive evidence of the existence of its soluble salts in 

 mixed liquids : but as much practice is required in the use of 

 this method ; and as protochloride of tin is a substance which 

 is used in this country only for chemical purposes ; and 

 moreover, is so liable to be decomposed by exposure to air, 

 and converted into a basic salt, (Sn CI + Sn O) the objec- 

 tions to its use, under the circumstances alluded to, are suffi- 

 ciently obvious. 



The beautiful galvanic method invented by Dr. Wollaston, 

 by which the mercury is deposited on gold, improved as 

 it has been, by Mr. Sylvester and Dr. Paris,* and more 

 recently rendered of such delicacy by M. Devergie,t that in- 

 dications of mercury may be obtained where corrosive sub- 

 limate forms but an 80,000th of the solution, still does not 

 recommend itself by such simplicity, nor does it include so 

 many other advantages as the process I am about to describe. 



Process. — Any soft solids existing in the suspected sub- 

 stances are to be broken down, and the whole, if not suffi- 

 ciently fluid, must be well triturated with a little distilled 

 water, and then strained through calico. The liquid is then 

 to be acidulated with hydrochloric acid, raised to the boiling 

 point, and pure copper turnings or wire immersed and boiled 

 in it for about twenty minutes: the copper is then to be 

 removed, washed with distilled water, and dried. If a soluble 

 salt of mercury had been present, the copper will be found 

 covered with a white coating of metallic mercury, varying in 

 brightness with the quantity of mercury present. The coated 

 copper turnings are then to be introduced into a tube of 

 about a quarter of an inch bore, and heated by a spirit lamp, 

 when a ring of metallic mercury, quickly running into glo- 



* Medical Jurisprudence, ii. p. 208. 

 t Annales d'Hyg. Publ. et de Med. Leg. xi. 411. 



