Dr. Rainy on the Detection of Arsenic. 79 



2. The eflfect of dilution was next examined. When the fluid was to 

 the arsenic as 07ie miUion to one, the deposit was distinctly foraied in 

 fifteen to twenty minutes. Thus, one thousandth of a grain of arsenious 

 acid in one thousand grains of fluid, and consequently constituting one 

 millionth part, gave a distinct coating to one square inch of copper surface 

 in twenty minutes. The same quantity of arsenious acid in two thousand 

 grains of fluid, also gave a deposit on the copper ; but it was less distinct, 

 and required a longer time. It appears then, that with a dilution of one 

 million times, the effect is distinct and prompt, and when the dilution is 

 earned to two million times, it is indistinct and tedious. A dilution of 

 two million times appears to constitute the practical limit in Reinsch's 

 process. By continued boiling it is easy, of course, to concentrate the 

 fluid, so as to bring the dilution within these limits, if arsenic be present in 

 any proportion, bowever small ; for there appears to be no loss of arsenic 

 by evaporation during the boiling. 



3. The proportion of muriatic acid in the solution has a considerable 

 influence on the rapidity of the deposition and even on its production, 

 when the arsenic is in very minute quantity. Thus, if the arsenic is less 

 than one millionth, the process is very slow in a fluid containing one tenth 

 muriatic acid of the ordinary strength ; but when it amounts to one 

 seventh or one sixth, the deposition is much accelerated. And in solu- 

 tions in which the quantity of arsenic is so small, that with the ordinary 

 proportion of acid no deposit is obtained, the copper becomes distinctly 

 coated if the proportion of muriatic acid is doubled. 



4. From these observations it would follow that the rapidity witb which 

 copper acquires a distinct arsenical coating, is directly as the proportion of 

 arsenious acid and also of muriatic acid in the solution, and inversely as 

 the extent of the copper surface. 



5. As copper receives a coating of a similar colour from other metals, 

 from sulphur and sulphuretted compounds, the mere formation of such a 

 deposit cannot be considered a conclusive proof of the presence of arsenic. 

 It is merely a convenient method of se2')arating the suspected substance, 

 in order that it may be subjected to the appropriate tests. The most 

 satisfactory of these tests are, the formation of a white crystalline sublimate 

 by heating the coated copper — the solution of this sublimate in water, 

 and its conversion, by the appropriate reagents, into arsenite of silver — 

 oqjiment — and arseniate of silver, all of which are very easily recognised 

 by the peculiarities of their colour and other properties. In estimating 

 the value of Reinsch's process, it is therefore necessary to ascertain, not 

 only the smallest quantity, and the utmost dilution under which it can be 

 separated and distinctly exhibited on copper, but also the smallest quantity 

 which, when so separated, can be satisfactorily subjected to the conclusive 

 tests. 



In repeated experiments, I found that one thousandth of a grain of 

 arsenious acid in one million times its weight of fluid, could be separated 

 as a distinct deposit on copjier. The copper thus coated, when heated 



