34 Mr. R. Phillips on the [Jan. 



and the mixture, after being dried, must be subjected to the 

 metallizing process, to be described presently. 



Nitrate of silver is liable to ambiguity, and on this subject I 

 cannot do better than quote what Dr. Paris has stated in the 

 work already alluded to, vol. ii. p. 241. 



" The alkaline phosphates are found to produce precipitates 

 with silver, analogous in colour and appearance to the arsenite 

 of silver. This constituted one of the principal points in the 

 evidence for the defence, on the trial of Donnall for the murder 

 of Mrs. Downing; and it must be admitted as a valid objection, 

 if the experiment be performed in the manner just stated ; but 

 there are other reagents which will immediately distinguish these 

 bodies, as we shall presently have occasion to state, under the 

 history of the ammoniuret of silver, as a test for arsenic. The 

 author has also shown, that there is a mode of so modifying the 

 application of the present test, that no error or doubt can arise 

 in the use of it, from the presence of any phosphoric salt. This 

 method consists in conducting the trial on writing paper, instead 

 of in glasses ; thus — drop the suspected fluid on apiece of white 

 paper, making with it a broad line ; along this line a stick of 

 lunar caustic is to be slowly drawn several times successively, 

 when a streak is produced of a colour resembling that known by 

 the name of Indian yellow ; and this is equally produced by the 

 presence of arsenic, and that of an alkaline phosphate, but the 

 one from the former is rough, curdy, and flocculent, as if effect- 

 ed by a crayon, that from the latter is homogeneous and uniform, 

 resembling a water-colour laid smoothly on with a brush ; but a 

 more important and distinctive peculiarity soon succeeds, for in 

 less than two minutes the phosphoric yellow fades into a sad 

 green, and becomes gradually darker, and ultimately quite 

 black ; while, on the other hand, the arsenical yellow remains 



fermanent, or nearly so, for some time, when it becomes brown, 

 n performing this experiment, the sunshine should be avoided, 

 or the transitions of colour will take place too rapidly. It would 

 be also prudent for the inexperienced operator to perform a simi- 

 lar experiment on a fluid known to contain arsenic, and on 

 another with a phosphoric salt, as a standard of comparison." 



The ambiguity arising from the use of nitrate of silver has 

 also been most satisfactorily obviated by Mr. Smithson (Annals 

 of Philosophy, Aug. 1822). This method consists in converting 

 the arsenious into arsenic acid, or rather into arseniate of 

 potash; and Mr. S. observes, " that a drop of a solution of 

 oxide of arsenic in water, which at a heat of54 , 5° of Fahr. con- 

 tains not above l-80th of oxide of arsenic, put to nitrate of 

 potash in the platina spoon and fused, affords a considerable 

 quantity of arseniate of silver. Hence when no solid particle of 

 oxide of arsenic can be obtained, the presence of it may be 

 established by infusing in water the matters contained in it." 



