86 SCIENTIFIC RECREATIONS. 



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 

 rilled 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 



Fig. 8 1. Retort and Receiver. 



houses constructed for the purpose, during the winter, and this simple 

 method is also the best, so far as at present has been ascertained. 



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 con- 

 dense 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 Sublim ate." 



Distillation is a more useful process, or, at any rate, one more frequently 

 employed, and is used to separate a volatile body from substances not 

 volatile. A distilling apparatus (distillo, to drop) converts a liquid to 

 vapour by means of heat, and then condenses it by cold in a separate vessel. 



The distilling apparatus consists of three parts, the vessel in which the 

 liquid is heated (the still, or retort), the condenser, and the receiver. The 

 simple retort and receiver are shown in fig. 81. But when very volatile 

 vapours are dealt with, the arrangement shown on next page is used (fig. 82). 



