ALKALIES. (FR., ; Gnu., Alhali.) The name alkali, in its widest sense, is given to a large class of compounds which possess certain distinctive properties. In its most restricted sense it is applied to four substances only, viz. potash, soda, lithia, and ammonia ; and these four sub stances are usually called the alkalies proper. But under the same title are included the hydrates of the metals barium, strontium, and calcium, which possess alkaline properties to a greater or less degree ; these are commonly known as the alkaline earths. A large number of organic bodies, both natural and artificial, are also classed as alkalies under the generic title of organic alkalies or alkaloids. Each of these classes will be treated of in detail in the following artie]ea, and it will be necessary here only to state the characteristic properties which are common to the three. They are (1) solubility in water ; (2) they neutralize the most powerful acids, and with the weaker acids from salts having alkaline properties ; (3) they exert a caustic or corrosive action upon animal and vegetable matters ; and (4) they alter the colours of many vegetable colouring matters, such Mt litmus, turmeric, and others.
Organic Alkalies, or Alkaloids.—Numerous vegetable, and some animal substances, have been classed as alkalies, on account of the very striking analogy which they present, in consti tution with the volatile alkali, ammonia. From the processes by which some alkaloids have been prepared artificially, they are regarded by modern chemists as ammonia in which all or part of the hydrogen is replaced by a compound organic radical, composed usually of carbon and hydro gen. All the alkaloids possess alkaline properties in some degree, and combine directly with acids, forming salts of more or less stability ; some have a strongly alkaline reaction with vegetable colouring matters, but in others this reaction is much feebler. By far the larger number of organic alkalies are obtained from the vegetable kingdom, some few from the animal kingdom, while a considerable number have of late years been prepared on a small scale by various chemical processes. Among the vegetable alkaloids are found some of the most valuable medicines, such
as aconite, brucine, cinchonine, morphia, quinine, strychnine, ; among these are also numbered many of the most virulent poisons known.
The method employed for the extraction of vegetable alkaloids from the plants which contain them is in most cases the same. If the alkaloid he soluble in water, as these compounds usually are, a strong infusion of the plant in pure water is made; but if the alkaloid be insoluble, a little mineral acid is added to the water. This solution is filtered and concentrated, after which the alkaloid is precipitated with carbonate of lime, ammonia, or carbonate of soda. The precipitate is collected on a filter, washed, and dried. When thoroughly dry, it is treated with alcohol to dissolve the alkaloid, and the solution is filtered and evaporated. The alcoholic solution usually requires deeolorizing and purifying, which are effected in a variety of ways. Some of the most important alkaloids employed in medicine will be considered below.
Aconitine.—This alkaloid is obtained from the leaves of the Aconitam nvellus. The leaves are infused in alcohol, and the solution is treated with milk of lime, which liberates the alkaloid iu solution. To the filtered liquid a little sulphuric acid is added, and the precipitated sulphate of lime is filtered off. The filtrate is evaporated until free from alcohol, when the aconitine is precipitated by an alkaline carbonate. This precipitate is re-dissolved in alcohol, and the solution is decolorized by animal black, and evaporated to dryness. The residue is re-dissolved in sulphuric acid, and precipitated anew with an alkaline carbonate ; the precipitate thus obtained yields pure aeonitine on treatment with ether. It is deposited from this solution in a white powder, or sometimes in the form of a compact, transparent, vitreous mass. It is inodorous, intensely bitter, and dissolves in fifty parts of boiling water ; its solution is powerfully alkaline. A very small quantity causes death with violent tetauic oonvulsious.