296 



Domestic Science 



when no more acid is left on the zinc. A further 

 supply of. the gas may be at once obtained by simply 

 opening the tap. 



To determine the composition of water by weight, 

 the delivery tube of the hydrogen generator is connected 

 to the tube marked with an arrow in Fig. 99. The first 

 drying-tube contains a small quantity of the strongest 

 sulphuric acid, while the second is nearly filled with 

 granulated calcium chloride. The bulbed tube and 

 its content of copper oxide is weighed carefully before 

 the commencement of the experiment. Both tube 

 and copper oxide should be dried in a hot air oven 



Fig. 99. 



for 15 minutes at a temperature of about 110 C. 

 before being weighed. The smaller bulb and the 

 attached calcium chloride tube are for the collection 

 and absorption of the water formed, and should be 

 weighed before starting. When the apparatus has 

 been set up, it should be tested to see whether it is 

 gas-tight. Attach a glass tube to the end of the last 

 absorbent tube, and let the end of this tube dip beneath 

 the surface of water in a beaker to a depth of three 

 inches. Allow a little hydrogen to pass through the 

 apparatus. This should cause immediate bubbling 

 in the water and, when the stream of hydrogen is 



