Mr, Humes' s Remarks on his Test for Arsenic. 107 



three letters, only one of which has been quoted by Dr. R. 

 Many of my expressions and even words have been warped 

 from their proper authority without a fair acknowledgement 

 of the source; I need mention but one instance here, and 

 that is, where I have pointed out the difference between 

 the arsenite and arseniafe of silver 3 the first, as a beautiful 

 yellow ; the second, as a precipitate of a red-brick colour. 



These gentlemen seem to have taken the same quantity 

 of ammonia as well as of the solution of silver in every 

 case ; for they used apparently the same glass-rods to de- 

 tect the te7ith as for ihe Jj/ty thousandth part, of a grain of 

 arsenic. The diameters of these rods were, we must be- 

 lieve, always alike, as no caution on this score was ob- 

 served by Dr. Marcet, to whom the chemical part is chiefly 

 ascribed ; therefore, unless the two solutions be propor- 

 tioned to each other in point of strength and dilution, the 

 application of the test in the way Dr. R. has recommended, 

 must be fortuitous if not erroneous, otherwise the caution 

 given, (p. 137,) that "the quantity of either ammonia or 

 nitrate of silver employed, can scarcely be too small," is 

 quite insignificant. 



Why two glass-rods were employed when one must ne- 

 cessarily have two ends, I cannot explain ; nor can I con- 

 ceive why a glass-rod should be used for the ammonia, and 

 a c/ea/i glass-rod for the solution of silver: — such niceties 

 are surely unnecessary to the success of the experiment; 

 I would rather have inculcated more attention to be paid 

 to the solution of silver, which should always be prepared 

 with ihe Jiised nitrate, so that there may be no excess of acid. 



It is in all cases material to know in what quantity of 

 water the fractional part of a grain of white arsenic is dis- 

 solved ; for this circumstance is of great moment in making 

 comparative estimates of the powers of cheiTiical tests, 

 especially one of such uncommoit delicacy and security as 

 that now under consideration, — the arresting of a mere in- 

 visible atom of white oxide of arsenic by means of silver, so 

 as to make it obvious to our sense. 



Having brought my test to such perfection, it is proper 

 that I should give you some account of tlie methods I pur- 

 su'.d in the investigation, that other praciitioners may be 

 enabled to apply its powers wiih lie same success. 



The fir>t riohition, with which I inaiie the experiments, 

 was prepared in this wav. One grain by weight of white 

 oxide of arsenic in powder, together with 4999 grains of 

 distilled water, were introduced into- a glass matrass. By 



the 



