The acid vapour may be easily recognised by its colour and smell, or by placing a piece of moistened litmus paper within the tube. The _arseniate_ may be identified by the _brick-red_ precipitate, produced in its solution, by _Nitrate_ of Silver. So small is the quantity of arsenic required for this latter mode of trial, that _Mr. Smithson_, in a late paper, observes “that a drop of a solution of arsenious acid in water, which at the height of 54·5 _Fah._ contains not more than 1/80th of the acid, put to nitrate of potass in a 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 which contain it.”[250] B. _By the application of certain re-agents, or tests, to its solutions._ _a._ _Fused Nitrate of Silver, or Lunar Caustic._ For this test we are indebted to _Mr. Hume_, who first suggested its application in the Philosophical Magazine for May 1809, (vol. xxxiii). His method of using it is as follows: into a clean Florence flask introduce two or three grains of the suspected substance, in the state of powder, to which add about eight ounces of rain or distilled water, and heat the solution until it begins to boil; then while it boils frequently shake the flask, and add to the hot solution a grain or two of sub-carbonate of potass, agitating the whole to make the mixture uniform. Pour into a wine glass about two table spoonsful of the solution, and touch the surface of the fluid with a stick of lunar caustic. If arsenic be present, a beautiful yellow precipitate will instantly proceed from the point of contact, and settle towards the bottom of the glass as a flocculent and copious precipitate. By this test the 60th part of a grain may be satisfactorily recognised in two ounces of water. The presence of some alkali is essential to the success of the experiment, since arsenious acid is incapable, by the operation of simple affinity, to decompose the _nitrate of silver_.[251] The validity of this test has been questioned on several distinct grounds, and which the author has endeavoured to answer in another work[252]; such, however, is the importance of the question in its judicial consequences, that we shall re-consider it on the present occasion.