of Arsenic and Antimony. 407 



so as not to allow one prematurely to assert the absence of 

 the metallic bodies. It is also of importance to be aware that 

 the deposition of metallic stains, both in the case of antimony 

 and arsenic, may go on for a considerable time ; I have col- 

 lected such stains on clean white porcelain for a quarter of an 

 hour or twenty minutes, even when the quantities of metal or 

 its oxide present have been small. This form of apparatus 

 appears therefore to me to have a decided advantage over any 

 apparatus so arranged that a quantity of the gas to be tested 

 is first collected in a receiver and then inflamed and ex- 

 amined ; for, from what has been stated above, it may very well 

 happen that in cases where the quantity of metal present is 

 very small, that the first, second, or even third charge of gas 

 in the receiver may not give any indication by combustion, of 

 metallic stains, because the metal is frequently not present in 

 the first portions of gas liberated. 



The colour of the arsenical stain is generally speaking very 

 distinct from the antimonial. In the former case, it is inva- 

 riably, I believe, of a brown colour, the intensity of which, as 

 well as that of the metallic lustre, increases with the quantity. 

 I suspected at one time that this brown colour might be owing 

 to the presence not of mere metallic arsenic, but to that of a 

 solid hydruret of the metal analogous to that brown coloured 

 solid obtained when water is decomposed by a battery, the 

 negative electrode of which is metallic arsenic. In order to 

 test this opinion by experiment, I collected a sufficient quan- 

 tity of brown arsenical stains upon white porcelain, carefully 

 removed them from the surface of the latter, and exposed them 

 to a temperature between 200° and 300° Fahr. for some time 

 to drive off any adhering moisture. The brown powder was 

 introduced upon a platinum tray into a tube of German glass, 

 and exposed to the flame of a spirit-lamp, whilst a current of 

 perfectly dry oxygen gas was past over it ; a white crystalline 

 sublimate of arsenious acid was soon obtained lining the cool 

 end of the tube, but no ivater "was formed. From this experi- 

 ment I conclude that the brown stains are metallic arsenic 

 mixed perhaps with a little arsenious acid. The antimonial 

 stains, on the other hand, are generally of a very dark colour, 

 almost black in some cases ; whenever, indeed, the stains are 

 of moderate size, they never, as far as I have observed, assume 

 the brown colour so highly characteristic of arsenical stains ; 

 it must, however, be confessed that when the quantity of anti- 

 mony very nearly approaches the minimum, that the stains do 

 begin to assume a brown colour in some cases, closely resem- 

 bling arsenical stains. When the stains are of a compound 

 nature, such as may be obtained from the mixed gaseous hy- 



