_Obs._ During the mutual action of the platinum-black and the vapour of alcohol, the temperature increases, and continues to do so until all the oxygen contained in the air enclosed in the case is consumed, when the acetification stops. On opening the case for a short time, to admit of a fresh supply of air, the operation recommences, thus showing its dependence on the oxygen of the atmosphere. For this transmutation, 100 grains of alcohol require 71 grains (equal to 200 cubic inches) of oxygen, or about 1000 cubic inches of atmospheric air. To render the process continuous and rapid, a fresh supply of air must, therefore, be constantly provided. This may be effected by either having a loosely covered opening at the top of the case, and several much smaller ones near its lower part; or (and preferably) by means of two small glass tubes passing through the lid or cover, one of which terminates just below the point of insertion, whilst the other divides into branches which reach to within a short distance from the bottom, as shown in the _engraving_. In this way a very slow current of fresh air will always be kept up in the apparatus. [Illustration] In practice, we find, that by loosely spreading the platinum-black on pieces of platinum-gauze, and supporting these on small tripods or bars of glass or porcelain (or even wood), the watch-glasses and their troublesome suspension may be dispensed with; as also may be the strip of porous paper, provided a temperature of not less than 90° Fahr. be maintained in the case or acetifier, which may easily be done by the application of artificial heat in the absence of sunshine. On the large scale, a case of wood with a glass roof, or even a well-seasoned cask or vat may be employed, in which case the temperature of the apparatus must be kept up either by means of steam-pipes or flues, or by the supply of warm air. On the small scale, a hand bell-glass placed on a dish, with a single watch-glass or piece of platinum-gauze, and a single capsule containing alcohol, may be used, provided the bell-glass be supported on three very small wedges, to admit of a supply of air. A modification of this is sometimes employed, in which the alcohol is supplied, in drops, to the platinum-black, by means of a long, tubular funnel passing through the mouth of the bell-glass, and having its lower extremity drawn to a very fine point, as shown in the _engr._ To ensure success, the platinum-black should be either fresh-prepared, or recently washed and very gently heated, before placing it in the acetifier. Spongy platinum, though ordered by many chemical compilers, does not answer well for this process.