COMBININa WEIGHTS. 77 



combine together. It is found that one grain of hy- 

 drogen (for hydrogen, thaugh a gas, can be readily 

 weighed) will combine with eight grains of oxygen, to 

 form nine grains of steam, and this relative propor- 

 tion between the two elements of water is perfectly 

 invariable. One grain of hydrogen can combine with 

 exactly sixteen grains of sulphur, to form seventeen 

 grains of sulphuretted hydrogen (182), a compound 

 shortly to be described, or, with six grains of carbon, 

 to form seven grains of carburetted hydrogen (131). 



140. These numbers, then, express the quantity of 

 each of these substances which can combine with one 

 another, for, of course, it is perfectly the same whether 

 we take a grain, an ounce, a pound, or any other 

 weight. But this is not all ; the number thus found 

 for carbon, namely six, is not merely the quantity of 

 that substance which could combine with one of hy- 

 drogen, but represents the quantity of carbon which 

 can combine with eight parts of oxygen, to form car- 

 bonic oxide (127), or twice eight, 16, parts of oxygen 

 to form carbonic acid ; and, again, eight parts of 

 oxygen is not merely the quantity which can combine 

 with one part of hydrogen, or with six parts of carbon, 

 but is exactly the quantity which is able to combine 

 with a definite proportion or equivalent, of any other 

 substance. 



141. The numbers which are in this manner ob- 

 tained, are called combining weights, proportionals, 

 equivalents, &c, ; they express the relative proportions 

 in which substances combine together. Some com- 



7* 



