There are three[330] alkalies—_potass_, _soda_, and _ammonia_. To the two former the epithet _fixed_ has been applied, since they require a very high temperature for their sublimation; while to the third, that of _volatile_ has been assigned, because, when uncombined, it exists in a state of gas. _Potass_, as it was considered the product of vegetation, has received the name of the _vegetable_ alkali, while _soda_, as the base of rock salt, has been distinguished by that of _mineral_ alkali. The distinctions, however, originally established by _Avicenna_, must now be abandoned, for they have not the slightest foundation in truth; _potass_, so far from being the exclusive product of vegetation, exists as a constituent part of the _Granite_, which forms the foundation of our globe; it has also been discovered in the _Pumice stone_; in some minerals of the _Zeolite_ family; in the _Leucite_; in the aluminous ores of _La Tolfa_, &c. and, although potass is undoubtedly procured by lixiviation from the ashes of burnt wood, and other vegetable substances, yet there is ample grounds for supposing that the living plant receives it from the soil in which it vegetates. POTASS, or _Potash_— LIQUOR POTASSÆ—POTASSA FUSA, or _Kali Causticum_—_Lapis Infernalis_—_Causticum commune acerrimum_. POTASSA CUM CALCE—POTASSÆ SUB-CARBONAS, or _Salt of Tartar_—_Potash_—_Pearl ash_. Potass is rarely met with in a pure form, except in the laboratory of the philosophical chemist, and is therefore not likely to become an object of judicial enquiry; but in various states of mixture, as presented in the different preparations above enumerated, it may become the accidental, as well as criminal means of poisoning; we shall therefore consider the chemical history of these different preparations separately, and then describe the symptoms which they generally occasion. _Liquor Potassæ._ This may be considered as a nearly pure solution of potass, although, as it is usually prepared, it contains small portions of _muriate_ and _sulphate of potass_, _silica_, and _lime_. It is a limpid, dense, colourless solution; when rubbed between the fingers it feels soapy, in consequence of a partial solution of the cuticle. As it constitutes a medicine in common use, and, moreover, forms the basis of many _quack medicines_, as well as of those preparations which are sold under the name of _Depilatories_, it may readily become the accidental instrument of mischief. _Chemical Tests for its detection._ There cannot exist any difficulty in this investigation; its highly alkaline characters will be immediately announced by its effects on the vegetable test papers, and by its power of saturating acids; while the particular species of alkali may be readily identified by the following reagents.