118 DETECTION OF ARSENIC. 



omitted, my purpose being solely to demonstrate the means 

 by which the arsenic they contain may be recognized in com- 

 plex fluids, or mingled with organic matter. Instances have 

 come within my observation where arsenic has been detected 

 in soup, and iii bread. The processes by which the know- 

 ledge of its presence has been ascertained and confirmed in 

 those cases have been elsewhere detailed, and I shall presently 

 proceed to demonstrate that the evidence which accumulated 

 during those investigations, was such as forced upon me the 

 conviction of its presence. It is deemed proper here to advert 

 to the principles on which these experiments are based, and 1 

 shall occupy but a short space in laying down the few general 

 laws involved in this examination. 



The only mode by which any substance may be discovered 

 in a chemical analysis is by combining it with some foreign 

 body called a test or reagent purposely introduced by the ex- 

 perimenter, which by the exercise of affinity, or mutual rela- 

 tion may either combine with the body which we desire to 

 separate, or combining with that, by which the object is alrea- 

 dy united, may by releasing it from its compound, occasion 

 the separation, and consequent precipitation of the object of 

 our search, either in an isolated or compound form. The ex- 

 perienced chemist is generally sufficiently acquainted with the 

 colour, form, and habits of the precipitates which he has occa- 

 sion to produce to determine promptly whether that v/hich 

 is yielded by his reagent is characteristic of the substance 

 sought for, or if no particular object be sought, to judge what 

 substance has been developed by his experiment. On this 

 principle of precipitation by reagents are almost all processes for 

 analysis conducted. Another process, however, and one which 

 is essential to the separation of arsenic, is sublimation, directly 

 the reverse of precipitation, and which is accomplished by the 

 agency of heat, as in precipitation we avail ourselves of solu- 

 tion in water. It can only be employed when the substance to 

 be separated is not liable to be destroyed or decomposed at an 

 elevated temperature. Where sublimation is employed, it may 

 either be accomplished per se, or by the intermediate aid of 

 reagents. This process as well as that of precipitation will be 

 illustrated hereafter. For the ordinary purposes of investigation 

 the production of one well known and characteristic precipi- 



