34 EQUIVALENT rfUMBERS — ISOMERIC BODIES 



eight times heavier than two gallons of the latter, so that by weight these 

 two gases, when thus burned, unite together nearly in the proportion 

 of 1 to 8, — one pound of hydrogen forming nine pounds of water. 



Again, when pure carbon is burned in the air, it unites with a fixed 

 and constant weight of oxygen to form carbonic acid ; it never unites 

 with more, and it does not form carbonic acid when it unites with less. 



Now this law of fixed and definite proportions is found to hold in re- 

 gard to all bodies, and in all cases of chemical combination. Thus we 

 have seen that — 

 By weight. By weight. 



1 of hydrogen combines with 8 of oxygen to form water. 



So 6 of carbon combine . . . 8 . j» . . . carbonic oxide, 

 and 14 of nitrogen 8 nitrous oxide. 



Hence 1 of hydrogen, 6 of carbon, and 14 of nitrogen unite respec- 

 tively with the weight (8)' of oxygen. These several numbers, there- 

 fore, are said to be equivalent to each other (they are equivalent numbers). 

 Or they represent the fixed and definite proportions in which these seve- 

 ral substances combine together (t)iey are definite proportionals). Some 

 chemists consider these numbers to represent the relative weights of the 

 atoms or smallest particles of which the several substances are made up, 

 and hence not unfrequently speakof them as the atomic weights of these 

 substances, or more shortly their atoms. 



For the sake of brevity, it is often useful to represent the simple or 

 elementary bodies shortly by the initial letter of their names. Thus 

 hydrogen is represented by H, carbon by C, and nitrogen by N, and 

 these letters are used to denote not only the substances themselvgs, but 

 that quantity which is recognised as its equivalent^ proportional^ or 

 atomic weight. Thus : 



Equivalent 

 Symbol. or atomic Name, 



weights. 



H denotes 1 by weight, of hydrogen. 



C . . . 6 carbon. 



O. . . 8 oxygen. 



N . . . 14* nitrogen. 



Chemical combination is expressed shortly by placing these letters in 

 juxta- position, or sometimes in brackets, with the sign plus (+) between 

 them. Thus HO or (H -{- O) denotes the combination of one atom or 

 equivalent of hydrogen with one of oxygen, that is, water ; and at the 

 same time a weight of water (9), equal to the sum of the atomic weights 

 (1 + 8) of hydrogen and nitrogen. 



A number prefixed or appended to a symbol, denotes that so many 

 equivalents of the substance represented by the symbol are meant, as 

 that number expresses. Thus 2 HO, 3 H O, or 3 (H -f O), mean two 

 or three equivalents of water, 3 H, or H3 three equivalents of hydrogen, 

 and 4C or C4, 2 N or Ng, four of carbon and two of nitrogen respec- 

 tively. 



n. Not only are the quantities of the substances which unite together 

 definite and constant, but the 'properties or qualities of the substances 

 formed are in general equally so. The properties of pure water or o*" 



• More correctly 1, 613, 8013, a»i 1419. 



