(_b_) _By precipitating metallic mercury from its solution, by the contact of a single metal._ It should be generally known that, by virtue of superior affinity, certain metals will decompose the solution of corrosive sublimate, with different phenomena; in those cases where the precipitating metal is capable of forming a direct union with mercury, we shall find the precipitates to consist of an amalgam of the metal employed; where no such combination takes place, the mercury may be frequently seen standing on the surface as a metallic dew. This is particularly striking when iron or steel has been employed. In the evidence given on the trial of _Mary Bateman_[283], better known by the name of the “Yorkshire Witch,” _Mr. Thomas Chorley_, surgeon at Leeds, stated that he had received from his assistant, _Mr. Hammerton_, a jar which he had carefully preserved in his possession, and of the contents of which he gave the following account. “Upon tasting a portion, it was very acrid, styptic, and permanent upon the tongue; I then took a small quantity of it upon a clean knife, and rubbed it with my finger; a change of colour immediately appeared; _further rubbing produced numerous globules of quicksilver_, and the knife was, at the same time, blackened by it; this change of colour led me to suspect that it must be a mercurial composition, and having made a solution of it, and subjected it to a series of tests and experiments, it is my opinion, that the mixture in the pot did contain _honey_, and _corrosive sublimate of mercury_. In order, however, more fully to satisfy myself upon this point, a mixture was made of these ingredients, when it was found to yield the same results.” In the above experiment, the steel knife decomposed the sublimate, forming a _chloride of iron_, while the mercury, thus disengaged in its metallic form, being unable to amalgamate with the iron, appeared in globules[284] upon its surface. At the same time the knife _became blackened_ owing to the precipitation of carbonaceous matter from the steel. (_c_) _Carbonate of Potass._ A saturated solution of this salt, added to that of corrosive sublimate, will produce a _deep brick coloured_ sediment, which is stated to consist of per-carbonate of mercury[285]; while a muriate of potass will be found to remain in solution. The _sub-carbonate of potass_ will occasion a somewhat different precipitate, of a _clear brick_ colour, and consisting of a mixture of the carbonate, and oxide of the metal.[286] (_d_) _Ammonia._ A solution of the volatile alkali produces a _white precipitate_, which is an insoluble triple salt, composed of muriatic acid, ammonia, and oxide of mercury; being heated it grows yellow; it passes afterwards to red, and according to _Orfila_ gives out ammoniacal gas, nitrogen, calomel, and metallic mercury. In this operation the oxide of mercury is supposed to be deoxidized by the hydrogen which results from a portion of the decomposed ammonia.