CHAP. x. THE GREAT PROBLEMS OF CHEMISTRY. 87 



and oxygen, combine to form a variety of compounds, 

 such as nitrous oxide or " laughing gas," nitric oxide, 

 and several others. Citrous oxide, or in chemical lan- 

 guage, nitrogen monoxide, consists of 28 parts by weight 

 of nitrogen to 16 of oxygen, and all the other compounds 

 of the same gases consist of two, three, four, or five times 

 as much oxygen to the same quantity of nitrogen. 

 Water consists of 16 parts of oxygen to 2 of hydrogen, 

 and there is another compound in which 32 parts of oxy- 

 gen combine with the same weight of hydrogen, forming 

 hydrogen-dioxide or oxygenated water. This law ap- 

 plies to every chemical compound yet discovered, and as 

 every element has a minimum proportionate weight, 

 which can combine with any other element, these are 

 called the atomic or combining weights of the elements. 

 As the weight of the hydrogen in all its combinations is 

 much less than the weight of the element it combines 

 with, this gas is taken as the unit of measurement of 

 atomic weights. Nitrogen is thus found to have an 

 atomic weight of 14, oxygen 16, and chlorine 35. 

 These are all gases; but many solids have much lower 

 atomic weights, carbon being 12, and the rare metal 

 beryllium only 9. Of other metals, that of aluminium 

 is 27, copper 63, iron 56, silver 107, tin 117, and gold 

 196. There is thus no constant relation between atomic 

 weights and specific gravities. Tin is a little lighter 

 than iron, but has nearly double its atomic weight; gold 

 has a high atomic weight, but bismuth has a higher still, 

 although only half its specific gravity. 



These facts are elucidated, and to some extent ex- 

 plained, by the atomic theory of Dalton. He supposed 

 each element to consist of atoms, an atom being the 



