Various official definitions of whisky have been given by different authorities. These definitions are not wholly concordant, some of them differing in many essential and basic points.

The object of the revenue laws being to raise money, the descriptions of the products are not important, and hence, as a rule, are not given. The revenue tax is laid on proof spirit; that is, on the total volume of spirit produced, calculated to a volume in which the quantity of ethyl alcohol is exactly one-half by volume that of the whole liquid.

Various kinds of spirits are made in distilleries, under the direction of the revenue law, but most of them have exactly the same rate of tax.

Before the enactment of the revenue law there was no supervision whatever of the manufacture of whisky, except that which was imposed by the conditions of trade. Often the whisky was used almost in the fresh state; at other times, through exigencies of the trade, it was kept for a greater or less length of time in wood. It was soon found that if freshly distilled whisky was placed in wood it began to acquire a better flavor, and became more palatable, more aromatic, and more desirable in every way. These changes were supposed to be due to the elimination of the more harsh and disagreeable components of whisky, by absorption, by oxidation or otherwise. Especially, was it supposed that the harsh taste and bad odor of the newly distilled whisky was due to the substance or substances known as "fusel oil." As this harsh taste and bad odor disappeared, it was reasonable to assume that the cause of it, namely, fusel oil, disappeared also. Modern chemical investigations show that substances which were supposed to be fusel oil, namely, the higher alcohols, do not disappear on ageing, except in the proportion as the contents of the cask may be evaporated. Even then, it has been discovered that the so-called fusel oils evaporate less rapidly than the ethyl alcohol, so that at the end of the ageing period the relative content of the "fusel oil" may be actually increased. The improvement in the character of the liquor on ageing must be due to some other cause. The chemist has discovered that there is a change taking place in the constituents of the distilled liquor, by which various forms of combination are secured, certain oxidations accomplished, certain ethers, esters, and acids developed, and as the result of the action of all these factors, the flavor, character and aroma of the distillate are vastly improved. There is, therefore, a sharp line to be drawn between new whisky, freshly distilled, and old whisky that has been properly matured and mellowed.