The most injurious ingredient in distilled spirits is commonly known as “fusel oil,” which term comprises several products of alcoholic fermentation, possessing a higher boiling point than ethylic alcohol, and consisting chiefly of amylic alcohol, accompanied by small proportions of butylic and propylic alcohols. Several varieties of fusel oil exhibiting distinctive properties are met with, but that obtained from potato-spirit is the most common. As a rule, the spirits prepared from malted grain contain the smallest proportion. In the manufacture of whisky, a danger of promoting the formation of fusel oil is incurred by carrying on the distillation to the furthest point, in order to obtain the greatest possible quantity of alcohol. In Great Britain, the fermented mash is removed from the remaining grain before its introduction into the still; but in this country the entire mash is occasionally taken, by which means a larger yield of alcohol is supposed to be effected. This practice is evidently open to the objection that the solid matters of the wort are liable to suffer destructive distillation, and engenders the formation of fusel oil. Another result, sometimes experienced, is the imparting of a smoky flavour to the product, which was originally intentionally communicated to the famous “poteen” whisky of Ireland, by using malt dried by means of burning turf. This quality is said to be still artificially obtained by the use of creosote. Genuine whisky, when recently made, is nearly colourless; but, if preserved in casks, it gradually acquires a brownish colour. It contains minute quantities of tannic acid, and ethylic and amylic acetates and valerianates. The specific gravity generally ranges between 0·9220 and 0·9040, corresponding to 48 and 56 per cent. of alcohol. The solid extract in whisky is usually below 1 per cent., and the total volatile acids under 0·1 per cent. In regard to the average composition of whisky, chemical literature furnishes but very meagre data. The examination of a large number of samples of ordinary American whisky in 1881, for the New York State Board of Health, gave the following results:--Specific gravity ranged from 0·9018 to 0·9645; alcohol (by weight) from 23·75 to 52·58; solid residue, from 0·100 to 0·752; ash, from 0·0020 to 0·0280. Several samples of rye whisky, examined by Mr. Green,[113] showed alcohol (by weight) from 32·50 to 51·20; tannic acid, 0·0003; acetic acid, 0·0012 to 0·002; sugar, 0·002 to 0·005; solid residue, 0·160 to 0·734.