()N WATKi; AND ITS CoMrol'NDS 



57 



or under the receiver of an air-pump and over substances which attract 

 water chemically. By weighing a substance before and after drying, it 

 is easy to determine the amount of hygroscopic water from the loss in 

 weight. 13 Only in this case the amount of water must be judged with 



13 In order t dry any substance at about 100 that is, at the boiling point of water 

 (hygroscopic- water passes off at this temperature) an apparatus called a ' drying-oven ' 

 is employed. It consists of a double copper box ; water is poured into the space 

 between the internal and external boxes, and the oven is then heated over a stove or by 

 any other means, or else steam from a boiler is passed between the walls of the two 

 boxes. When the water boils, the temperature inside the inner box will be approximately 

 100 C. The substance to be dried is placed inside the oven, and the door is closed. 

 Several holes are cut in the door to allow the free passage of air, which carries off the 

 aqueous vapour by the chimney on the top of the oven. Often, however, desiccation is 

 carried on in copper ovens heated directly over a lamp fig. 13). In this case any desired 



FIG. 13. Drying oven, composed of brazc-d copper. It is heated by a lamp. The object to be dried 

 is placed on the gauze inside the oven. The thermometer indicates the temperature. 



temperature may be obtained, which is determined by a thermometer fixed in a special 

 orifice. There are substances which only part with their water at a much higher 

 temperature than 100, and then such air baths are very useful. In order to directly 

 determine the amount of water in a substance which does not part with anything except 

 water at a red heat, the substance is placed in a bulb tube. By first weighing the tube 

 empty and then with the substance to be dried in it, the weight of the substance taken may 

 be found. The tube is then connected on one side with a gas-holder full of air, which, on 

 opening a stop-cock, passes first through a flask containing sulphuric acid, and then into 

 a vessel containing lumps of pumice stone moistened with sulphuric acid. In passing 

 through these vessels the air is thoroughly dried, having given up all its moisture to the 

 sulphuric m-id. Thus dry air will pass into the bulb tube, and as hygroscopic water is 

 entirely given up from a substance in dry air at even the ordinary temperature, and still 



