208 Dr. MacneveNs Exposition of [Sept 



expected ; and they must be admitted until some exception to 

 them be discovered. 



It is a circumstance much in favour of the atomic theory, that 

 it assigns a mechanical and very satisfactory cause, why elemen- 

 tary atoms unite only in proportions which are multiples of each 

 other. The compound molecules which contain oxygen combine 

 likewise in a multiplex ratio, if we attend only to the oxygen 

 they contain. This must be owing, in all probability, to a cause 

 similar to that which occasions the like proportions between 

 elementary atoms themselves. 



The proportion of oxygen, in the oxides that unite, is a pro- 

 blem highly important to be verified. The investigation has 

 accordingly occupied and still continues to occupy the attention 

 of the ablest chemists with a result the most satisfactory. 



Indeed but for the law which shows that the oxygen in an acid 

 is always a multiple of the oxygen in the base by a whole num- 

 ber, no combination composed of several oxides could be 

 calculated, nor any analysis verified in a decisive manner for the 

 theory. 



35. The employment of numbers facilitates the expression of 

 chemical proportions, and, by determining the weight of the 

 elementary atoms, figures exhibit the numeric result of an ana- 

 lysis in a manner at once simple and easily remembered. But^ 

 in oi'der to draw up a table of the relative weights of the atoms 

 of bodies, some one must be selected for comparison whose atom 

 shall be denoted by unity. 



There are only two elementary bodies possessed of the requi- 

 site qualities to serve as our unit. These are oxygen and 

 hydrogen. Mr. Dalton has made choice of hydrogen, because 

 it is the lightest of all known bodies. Sir H. Davy embraced it 

 from him, but changed its value ; and Mr. Brande, in his Manual 

 of Chemistry, lately published, hashkewise adopted the unit and 

 computation of Dalton, but designates the relation of elementary 

 parts by the term proportionals. This choice was not the hap- 

 piest, for hydrogen has disadvantages from which oxygen is free. 

 The weight of an atom of hydrogen is so small, that if we 

 employ it as our unit, the number representing an atom of some 

 of the metals becomes inconveniently great. 



Besides, hydrogen enters much less frequently into compounds 

 than oxygen, and, of course, the unit of comparison, when 

 applied to hydrogen, does not nearly so much facihtate calcula- 

 tion as when it is applied to oxygen. Add, that oxygen consti- 

 tutes among elementary bodies a particular class ; and is, as it 

 were, the centre round which chemistry turns. It exists in the 

 greater number of inorganic bodies, and, without exception, in 

 all the products of organic nature. For these reasons Berzehus 

 preferred this unit, as most convenient and most agreeable to the 

 scientific views of chemistry. He represents it by 100. (Thom- 

 son's Annals, ii. 451.) 



