254 Prof. Giseke's Observations on the Tests for Arsenic. 



not in that which holds the ammonia, however one may 

 neutralize the liquid. The experiment may also be made by 

 putting sal ammoniac or nitrate of ammonia into a liquid con- 

 taining arsenic, and adding lime-water, which then will not 

 give a precipitate, however great the quantity of lime-water 

 that may be added ; or the reverse, by pouring the arsenical 

 solution into lime-water, and even heating the solution in or- 

 der to concentrate it moi'e. Thus it will be seen that in the 

 experiment mentioned by Berzelius, and as he has observed 

 himself^ no precipitate will be obtained by neutralizing the ni- 

 tric solution with ammonia, of which he probably made use. 

 But it is not the ammoniac as caustic alkali, but the nitrate of 

 ammonia which is thereby formed, that prevents the deposit ; 

 and if instead of the ammonia we emj)loy caustic potash or 

 soda for neutralizing the nitric acid, the j^recipitate of arsenite 

 of lime will instantly be formed with lime-water, even with a 

 prevalence of the alkali. 



The results of these experiments lead us also to make a 

 few remarks on tlie method of discovering arsenic in a poi- 

 soned liquid, proposed by M. Pfaft'in his Manual of Analytic 

 Chemistry. He says (vol. ii. p. 392), " If the white arsenic 

 cannot be separated from the contents of the stomach, the whole 

 substance must be boiled in a proportionate quantity of water 

 and nitric acid. The liquid, which in consequence turns 

 yellow, is filtered through a woollen cloth, and the clarified 

 acidulous liquid mixed with acidulated solution of sulphuretted 

 hydroo-en as long as it continues to produce a lemon-coloured 

 turbidity. One may also use lime-water instead of the sul- 

 phuretted hydrogen, and submit the arsenite of lime to subli- 

 mation with 1-4-th of charcoal powder." Nevertheless it is 

 evident, from the experiments above recited, that lime-water 

 will not produce a precipitate of arsenite of lime, owing to the 

 ammoniacal salt prevailing in the arsenical liquid. 



Roloff likewise proposes in these kinds of experiments to 

 neutralize the nitric acid with solution of caustic potash or 

 ammonia*. If he has made a fi-equent use of the latter, it 

 may be easily conceived why, deviating from Rose's plan, he 

 has preferred to recommend sulphuretted hydrogen instead of 

 lime-water, since the latter will give no result. 



As the effect of an ammoniacal salt here observed, which is 

 so similar to that observed by Mr. Gaertnerf in arsenite of 

 copper (Scheele's green), probably depends on double affinity, 



* Vide Journ.f. Ckem. u. Phys. vol. vii. (old series) p. 415. 

 t Ibid. p. 426. 



