By far the greater quantity of ordinary alcohol, however, is distilled in highly-effective apparatus which produces a nearly pure "silent" or "neutral" spirit of from 94 to 96 per cent, strength in one continuous operation. The "Coffey" or "patent" still, which is largely used in this country, may be taken as example; though other stills used abroad and differing in detail claim to be of at least equal efficacy, especially for dealing with thick washes.

In this still the alcohol is expelled by blowing steam through the wash, which is spread in layers over a considerable surface. The wash enters at the top of the still, and passes downwards over a series of perforated diaphragms of copper plate, trickling over small orifices which serve as so many traps where the liquid is met by the uprising steam. The latter, bubbling through the perforations, heats the wash, and carries off the alcohol vapour, so that by the time the wash reaches the lowest plate it has been deprived of all its alcohol, and passes out with its dissolved and suspended solids, whilst the alcohol, steam, and volatile impurities pass to the top of the apparatus. The higher these vapours get, the richer they become in alcohol; their boiling point becomes lower; and more and more of the steam is condensed. The part of the still in which this operation takes place is termed the "analyser."

The escaping vapours are now led into the bottom of a similar column of perforated plates, termed the ' rectifier." From top to bottom of this column there passes a zig-zag tube full of cold liquid to serve as a condenser. To economise heat this liquid is, in fact, the cold wash on its way to the analyser. The alcoholic vapours passing upwards through the plates into one cool chamber after another become cooled, and deposit more and more of their water as they near the top, finally condensing as strong alcohol on an unperforated copper sheet (the " spirit plate "), and passing out. The strength of the spirit thus continuously obtained is about 94 to 96 per cent, by volume, whereas with a simple still the strongest alcohol obtainable, even by repeated distillations, is only about 90 to 92 per cent., and at this strength is produced only in small quantities from each operation.

A mixture of weak, impure alcohol and oils (" hot feints ") collects in the bottom of the "rectifier"; this is pumped into the this is condensed and collected as "feints" in a separate receiver.

"analyser" again to recover the alcohol. But towards the end of a distilling process, instead of completing the purification of all the alcohol, it is found more economical to raise the temperature of the apparatus and distil off the whole residue of impure spirit:

Fig. 25. - diagram of coffey's still.

A, analyser; B, rectifier; a, perforated copper diaphragms; m. pump, forcing wash through zig-zag tube n to top of A: g, pipe leading spirit-vapour to bottom of rectifier.

Fig. 26. - double column continuous still. A, Supply tank; B, liquor regulator; C, preheater; D, boiling column; E, rectifying column; F, rectifying column rectifier; G, rectifying column condenser; H, rectifying column cooler , K, steam regulator; L, oils cooling tank; M, run-off tester (Blair, Campbell, and McLean, Ltd., Glasgow).

Here the fusel oil which has accumulated during the process separates to a large extent from the weak spirit and is skimmed off, the remaining "feints" being redistilled with the wash of the succeeding operation to recover the ethyl alcohol from them.

The "Coffey" still was patented in 1832. Various other "continuous" alcohol stills are employed, especially on the Continent and in America: their essential principle is similar to that of Coffey's apparatus. Barbet's stills have a considerable reputation in France and abroad, as have also those of Messrs. Egrot and Grange. Guillaume's system includes a sloping, instead of vertical, distilling column, to obviate risk of obstruction when thick washes are being distilled. Of the others, it will suffice to mention the Ilges "automatic" still, which is said to yield a pure spirit of 96 to 96.5 per cent, strength, and to recover all the fusel oil and low wines at strengths of 80 and 97 per cent. respectively.

The alcohol of high strength distilled in apparatus of the Coffey or allied types is a nearly pure product, and is known as "silent," "neutral," or "patent-still" spirit. Large quantities are employed without further purification, as, for instance, in making acetic acid, explosives, and varnishes; in the production of methylated spirit or other forms of industrial alcohol; in extracting drugs, preparing spirituous medicines, and so on.

Rectification

When, however, special purity is required, or when the alcohol obtained is of cruder character, it is further purified by special rectification. This is usually accomplished by diluting the spirit with water to a strength of about 45-50 per cent., and re-distilling it. Various kinds of stills are used for the purpose. Some are "intermittent" stills which fractionate the distillate, the impurities being eliminated in the first and last runnings; others are "continuous action' rectifying stills. Purification by treat-men: with wood charcoal is also employed, the alcohol, suitably diluted, being passed through a battery of cylindrical filters containing the charcoal. The purifying effect is mainly a chemical one, due to oxidation. According to circumstances, the filtration through charcoal may either precede or follow a rectification by distillation.