302 Domestic Science 



consider the composition of water and of chalk. Pure 

 water, from whatever source obtained, however pre- 

 pared, and by whomsoever examined, is always found 

 to contain 7-94 parts by weight of oxygen combined 

 with 1 part of hydrogen, provided the examination 

 be carried out with sufficient care. Small variations 

 from these figures may be noted, but none greater 

 than can be accounted as errors of experiment. Again, 

 the percentage composition of pure chalk is always found 

 to be : calcium, 40 % ; carbon, 12 % ; oxygen, 48 % ; 

 or, put in another way, 1 part by weight of carbon is 

 always found in combination with 4 parts of oxygen 

 and 3-33 parts of calcium in every sample of calcium 

 carbonate, of which compound chalk is one of the 

 naturally occurring forms. 



207. The composition of water by volume was 

 shown to be of a simple character, two volumes of 

 oxygen combining with one volume of hydrogen to 

 form two volumes of steam. Similar regularity exists 

 between the volumes of other reacting gases. Thus, 

 two volumes of acetylene combine with five volumes of 

 oxygen to form four volumes of carbon dioxide and two 

 volumes of steam. In the form of a general statement, 

 this is known as Gay-Lussac's " Law of Gaseous 

 Combination", and may be put thus : 



When gases combine, the volumes of the reacting 

 gases and those of any gaseous products bear simple 

 ratios to one another. 



Further reference will be made to both the above 

 laws in later work. 



208. In concluding this chapter, the question of 

 the differences between a mixture of substances and 

 a chemical compound will be considered, with special 



