FREEZING. 29 



centigrade, is obtained when the crystallised solid known as 

 Glauber's salt, or sulphate of sodium, is powdered and stirred up 

 with liquid hydrochloric acid (a corrosive mineral acid which 

 must be carefully handled to avoid spilling and consequent 

 damage). A thin glass cylinder of water immersed in a wooden 

 pail or stoneware jar (not a metal vessel, as this would be attacked 

 by the acid) containing a quantity of the powdered salt and acid 

 will soon become frozen hard just as it would in snow and salt. 



Many other freezing mixtures can be prepared in similar 

 fashion, using different kinds of saline matters and other acids ; for 

 the artificial manufacture of ice these substances have long been 

 superseded by far cheaper processes, mostly dependent on the 

 production of cold by the quick evaporation of volatile fluids. 



Expt. 25. To Freeze Water by the Evaporation of Ether. 

 Obtain a test-tube, about half an inch in diameter and 5 or 6 

 inches long. Wrap muslin or lampwick round the lower third, 

 and pour into it two or three teaspoonfuls of clean water as cold 

 as possible. Drop some ether on to the muslin so as to wet it, 

 and blow vigorously on the wetted muslin with a pair of bellows. 

 If a thermometer be placed in the water it will be immediately 

 seen that this treatment cools the water; and by continuing to 

 drop on ether and make it evaporate quickly by the bellows the 

 water may in time be frozen to ice. The clamp-stand represented 

 in fig. 12 may be used for holding the test-tube; but a simpler 

 holder will answer perfectly well, consisting of a piece of wire 

 twisted round the tube, somewhat after the fashion of the paper 

 in fig. 11, with the ends thrust into a cork projecting out of a 

 bottle full of water, so that the arrangement cannot be easily upset. 



Very strong acetic acid (the acid of vinegar) may be thus frozen 

 to an ice-like mass much more readily than water; the acid is 

 often termed glacial acetic acid from this property, and its strength 

 is judged of by the ease with which it freezes. Several other 

 substances possess the same property; that is when tolerably 

 pure they readily freeze or solidify on chilling to some extent ; 

 but if impure and admixed with other substances they do not 

 become solid until they are much more chilled ; so that the extra 

 extent to which they have to be cooled before they solidify serves 

 as a sort of measure of the amount of impurity present. 



Expt. 26. To Freeze out Stearine from Olive Oil. Many 

 natural oils are not simple substances, but are mixtures of two or 

 more fatty matters, one of which when separate is a fluid at 

 ordinary temperatures and the other a solid. Genuine olive oil 

 (not adulterated with other substances, as is often the case) is such 

 a mixture, and when chilled for a long time (as in winter, or 



