[Illustration: Fig. 80.—Apparatus for freezing carafes of water.] An experiment is often performed by which water is frozen in a vacuum. By putting a saucer full of water under the receiver of an air-pump it will first boil, and then become a solid mass of ice. It is not difficult to understand the cause of this. The water boils as soon as the air is removed; but in order to pass from the liquid to the gaseous state without the assistance of exterior heat, it gives out heat to the surroundings, and in so doing becomes ice itself. This fact Mr. Carré has made use of in the apparatus shown above (fig. 80). A small pump creates a vacuum in the water bottles, and ice is formed in them. This apparatus might easily be adopted in country houses, and in places where ice is difficult to procure in summer. The only inconvenience attending it is the employment of sulphuric acid, of which a considerable quantity is used to absorb the vapour from the water, as already referred to. If proper precautions are taken, however, there will be no danger in using the apparatus. The mode of proceeding is as follows:—The bottle full of water is joined to the air-pump by a tube, and after a few strokes the water is seen in ebullition. The vapour thus disengaged traverses an intermediate reservoir filled with sulphuric acid, which absorbs it, and immediately condenses it, producing intense cold. In the centre of the liquid remaining in the carafe some needles of ice will be seen, which grow rapidly, and after a few more strokes of the pump the water will be found transformed into a mass of ice. This is very easy of accomplishment, and in less than a minute the carafe full of water will be found frozen. The problem for the truly economical formation of ice by artificial means is one of those which have occupied chemists for a long time, but hitherto, notwithstanding all their efforts, no satisfactory conclusion has been arrived at. Nearly every arrangement possesses some drawback to its complete success, which greatly increases the cost of the ice, and causes inconvenience in its production. The usual mode in large towns is to collect the ice, in houses constructed for the purpose, during the winter, and this simple method is also the best, so far as at present has been ascertained. [Illustration: Fig. 81.—Retort and Receiver.] In connection with vaporization we may now mention two processes referred to just now (page 83); viz., sublimation and distillation. The former is the means whereby we change solid bodies into vapour and condense the vapour into proper vessels. The condensed substances when deposited are called _sublimates_, and when we go into Chemistry we shall hear more of them. The mode of proceeding is to place the substance in a glass tube, and apply heat to it. Vapour will be formed, and will condense at the cool end of the tube. The sublimate of sulphur is called “Flowers of Sulphur,” and that of perchloride of mercury “Corrosive Sublimate.”