University of Leipsic. Salicylic acid exists ready-formed in the flowers of Spiræa ulmaria (meadow-sweet), and as methyl-salicylic acid in oil of wintergreen. It is also prepared from indigo and from salicin, a substance found in the bark of several species of willow and poplar. But the best mode of preparing it is that proposed by Kolbe and Lautemann, which is thus described by Watts:
Dry carbonic anhydride is passed into warm phenol, with addition of small pieces of sodium. The metal then dissolves, with evolution of hydrogen, and a stiff paste is formed, containing the isomeric salts, salicylate and phenyl-carbonate of sodium, together with unaltered phenol. On acidulating with hydrochloric acid, the phenyl-carbonic acid is decomposed, with evolution of carbonic anhydride, and the salicylic acid which is set free may be separated from the phenol by solution in strong aqueous carbonate of ammonium. The solution, boiled down till it acquires a slight acid reaction, filtered from separated resin, and mixed with hydrochloric acid, yields salicylic acid, to be purified by recrystallization with the aid of animal charcoal.
Reasoning from the fact that salicylic acid can thus be prepared from phenol and carbonic acid, and from the further fact that on the application of heat it again splits up into those two acids, Kolbe was led to inquire whether it possessed the antiseptic properties of phenol. The value of the latter substance as an anti-ferment is well known, but its poisonous properties, as well as its disagreeable smell and acrid taste, render it unsuitable as a means of preserving articles of food, or as a medicinal agent. It was evident, therefore, that if salicylic acid, which is odorless, almost tasteless, and, when taken in small quantities, innocuous, possessed antiseptic properties equal to those of phenol, one of the most urgent wants of modern life would be at once supplied. Experiment proved the conjecture to be correct, and thus many years of theoretical investigation were crowned by practical results of the highest value.
Among the experiments made with this substance we may mention the following: Brewer's yeast (quantity not stated), which causes alcoholic fermentation of sugar, was found to have no effect upon a solution of glucose containing the one-thousandth part of salicylic acid. Half a gramme of the acid suffices to check the fermentation produced by five grammes of beer-yeast, acting on 120 grammes of sugar dissolved in a litre of water. Experiments made by Neubauer show that 100 grammes of salicylic acid suffices to absolutely prevent fermentation in 1,000 litres of must, or fresh-pressed juice of grapes. Flour of mustard, which, when mixed with lukewarm water, gives out the pungent oil of mustard, is perfectly odorless when a small quantity of salicylic acid is added to it. So, too, this acid prevents the action of emulsin (the ferment contained in almonds) upon amygdalin, and the conversion of the latter into oil of bitter-almonds. Milk treated with 0.04 per cent, of salicylic acid remained uncoagulated for thirty-six
