THE ELEMENTS OF THE CHEMIST 123 



which for a time seems to provide a satisfactory explanation of 

 all the facts under consideration. But before long some more 

 accurate measurement or the observation of some neglected and 

 apparently trivial circumstance requires a revision of the accepted 

 doctrine or its displacement by a new one. 



These remarks apply specially to the case of the chemical 

 elements. The word element has received many applications, 

 and even at the present day in ordinary speech it is used some- 

 times in a poetical sense, with general allusion to air or water, or 

 it simply means a constituent or ingredient in a mixture of 

 things. Passing over any further reference to popular or ancient 

 usage the word element received for the first time a definition 

 with a scientific character from Robert Boyle in the seventeenth 

 century. And his definition has been current among chemists 

 since his day. An element, according to Boyle, is a substance 

 which resists analysis. It consists of one kind of matter, and 

 by no known process is it possible to extract from it more than 

 one kind of stuff. It is only in the most recent times, namely, 

 since the discovery of radium in 1902 and the related substances, 

 that this definition, accepted as it has been for upwards of two 

 hundred years, can no longer be applied without qualification to 

 many substances which previously would have been included 

 without hesitation under it. On the other hand, we have long 

 since learned that several substances which formerly answered 

 to the definition, having resisted the then known methods of 

 analysis, such as lime, baryta, and the alkalis, are really com- 

 pound bodies consisting of oxides of metals. But no real 

 advance beyond a position of mere speculation could be accom- 

 plished until the phenomena of chemical combination were 

 studied quantitatively and the laws of chemical combination 

 were established. The law of definite proportions, the law of 

 multiple proportions, and the law of reciprocal proportions were 

 enunciated more or less clearly more than a hundred years ago, 

 and all subsequent experiment has only served to establish 

 them the more firmly. Then came in 1808 the Atomic Theory of 

 John Dalton, which at once supplied an explanation of the 

 observed facts. This theory assumes that each element consists 

 of minute separate particles, all alike in size, weight, and 

 chemical properties, and that when chemical combination takes 

 place between any two or more elements to form a compound 

 a definite and limited number of the particles of one kind are 



