1824.] Tests for detecting the Presence of Arsenic. 35 



Instead of using a platina spoon, a glass tube, or the bottom of an 

 oil flask, may be employed ; into either of these, put a little of 

 the suspected solution, and which has exhibited indications of the 

 presence of arsenic by other tests ; then drop in a small crystal 

 of nitre, evaporate the solution to dryness by means of a spirit- 

 lamp, and afterwards heat it strongly in the same way. Add a 

 little distilled water to the residuum, dissolve it, and then add 

 nitrate of silver ; if the solution before heating contained arse- 

 nious acid, it will now contain arseniate of potash, which will 

 o-ive a brick red precipitate with the nitrate of silver, and with- 

 out the intervention of any alkali. From repeated trials, I con- 

 sider the confirmatory evidence afforded by this experiment as 

 amounting almost to demonstration. This experiment is ren- 

 dered shorter, and not less conclusive, by employing the arse- 

 nious acid and nitre both in the state of powder ; but as the 

 former is not always procurable after fatal effects have been 

 produced by it, I have mentioned the solution as affording very 

 satisfactory results. 



I shall now mention the method of confirming the previous 

 experiments by reducing the arsenious acid to its metallic state. 

 If the quantity of arsenious acid procurable be very small, then 

 it is proper to dissolve the whole of it in distilled water, and the 

 precipitates which are obtained by the action of the various 

 reagents should be collected and submitted to the metallizing 

 process ; but if the quantity of arsenious acid be so large that a 

 few grains, or not less than one grain, can be spared for metalli- 

 zation, then the precipitates may be rejected, and much trouble 

 will be spared. 



This process is thus recommended to be performed by Dr. 

 Paris, in his work before alluded to, vol. ii. p. 233 : — 



" Mix a portion of the suspected substance in powder, with 

 three times its weight of black flux ;* put the mixture into a thin 

 glass tube, about eight inches in length, and a quarter of an inch 

 in diameter, and which is hermetically sealed at one end. Should 

 any of the powder adhere to the sides of the tube, it must be 

 carefully brushed off' with a feather, so that the inner surface of 

 its upper part may be perfectly clean and dry. The closed end 

 of the tube, by way of security, may be thinly coated with a mix- 

 ture of pipe-clay and sand ; but this operation is not absolutely 

 necessary. The open extremity of the tube is to be loosely 

 plugged with a piece of paper. The coated end must now be 

 submitted to the action of heat, by placing it in a chaffing dish 

 of red-hot coals, for ten minutes, or a quarter of an hour ; when, 

 if our supposition respecting the nature of the substance has 

 been correct, metallic arsenic will sublime, and be found lining; 

 the upper part of the tube with a brilliant metallic crust. The 



■ This substance may be said to consist of charcoal in a state of cMrcmcly minute 

 division, and the subcarbonatc of potash. It is prepared by deflagrating, in a crucible, 

 two parts of supertartrate of potash with one part of nitrate of potash. 



1)2 



