4 HISTORICAL INTRODUCTION. 



many curious discoveries, respecting the me- 

 talic combinations which he treated of in that 

 volume. 

 He made Mr. Dalton made choice of the atom of hy- 



the atom of . _ , . . , . . . 



hydrogen drogen for his unity. And in this he has been 

 followed by Dr.* Henry of Manchester, and by 

 one or two chemical gentlemen in London. 

 But this method has been rejected by almost all 

 the British chemists, and by all the chemists, 

 without exception, in Europe and America. 

 The choice was unhappy, for very obvious 

 reasons. 1. Because the atom of hydrogen is 

 the most difficult of all to determine ; and 

 chemists are not yet all agreed about its weight. 

 According to Professor Berzelius, it is only 

 O06&5; while the greater number of British 

 chemists consider it as 0-125, or twice as much 

 as Berzelius's estimate. Now, if we reckon the 

 atomic weight of hydrogen by unity, and commit 

 an error respecting its relation to that of other 

 bodies ; this error will affect the atomic weights 

 of all other bodies, and will make them all either 

 too heavy or too light ; whereas, if we make 

 choice of oxygen for our unity, any error re- 

 specting the atom of hydrogen will be confined 

 to that atom, and will not affect the accuracy of 

 the atomic weights of other bodies. 2. Hydro- 

 gen, so far as we know at present, combines 

 with but few of the other simple bodies ; while 

 oxygen unites with them all, and often in vari- 



