602 



SCIENCE. 



[N. S. Vol. xr. No. 277. 



terpreting chemical combination as a joining 

 together of elementary corpuscles. Boyle's 

 view had little influence on chemistry, 

 and I think naturally. In the absence 

 of more definite chemical knowledge than 

 then existed, I should consider the concep- 

 tion that water and alcohol, for example, 

 are perfectly continuous, and that their 

 mixing is a perfect blending, quite as plaus- 

 ible as, and less contradictory to the evi- 

 dence of the senses, than the view that thej' 

 consist of discrete particles which mix, but 

 do not penetrate each other. Up to the be- 

 ginning of the present century those who 

 adopted the .atomic hypothesis did so from 

 supposed physical or metaphysical necessitj', 

 rather than on the basis of any satisfactory' 

 chemical evidence. 



The general acceptance of the atomic 

 hypothesis by chemists was due to the ex- 

 perimental establishment of two laws, which 

 are among the most fundamental principles 

 of chemistry — the Law of Constant or Defi- 

 nite Proportions, and the Law of Multiple 

 Proportions. For the benefit of those who 

 have forgotten their chemistry I may state 

 briefly in what these laws consist. 



The Law of Constant Proportions. — Every 

 chemical compound has an invariable com- 

 position, that is, the relative weights of 

 the elements entering into it are invari- 

 able. On comparing the composition of 

 numerous substances, it was soon found 

 that to each element might be attributed a 

 certain number, the combining number, which 

 represents the proportion by weight in 

 which that element enters into combination. 

 If we compare, for instance, the compounds 

 which the akali metals lithium, sodium and 

 potassium form with the halogens chlorine, 

 bromine and iodine, we find the following 

 relations : 7 parts lithium unite with 35.4 

 parts chloi'ine ; 80 parts bromine, or 127 

 parts iodine; similarly 23 parts sodium 

 unite with 35.4 parts chlorine ; 80 parts 

 bromine, or 127 parts iodine, and so on. 



Lithium. 



7. 



Chlorine. 



35.4. 



Sodium. 



23. 

 Bromine. 



80. 



Potassium. 



39. 

 Iodine. 



127. 



These figures, which merely express the 

 results of analysis, are the combining num- 

 bers, and to each of the seventy-five or 

 more elements belongs its own proper com- 

 bining number. 



The Law of Multiple Proportions. — Ele- 

 ments often combine in more than one pro- 

 portion. When this is the case, the different 

 weights of the one, which unite with a given 

 weight of the other, bear a simple relation 

 to each other. A good example of this is 

 found in the chlorides of the metal molyb- 

 denum, of which four are known. The 

 combining number of molybdenum is 96, 

 and 96 grams molybdenum combine with re- 

 spectively 2, 3, 4 and 5 times 35.4 grams 

 chlorine, 35.4 being the combining num- 

 ber of the latter, the resulting compounds 

 being represented by the formulas MoCIj, 

 Mod,, MoCl^ and MoCl^. A somewhat 

 more complicated case is found in the par- 

 af&ne series of hydrocarbons, CH^, C^Hj, 

 CjHg, C^Hj„, C^Hjj, etc. Both laws apply as 

 well when the compounds contain more 

 than two elements. 



These two laws hold without exception 

 through the many thousands of known 

 chemical compounds. They involve nothing 

 hypothetical, being simply the expression 

 of analytical results in a particular form. 

 They were'established mainly through the 

 labors of Eichter, Proust, Dalton and Ber- 

 zelius, but to Dalton belongs the credit of 

 having employed the atomic theory in ex- 

 plaining them. 



If matter be absolutely continuous and 

 capable of anj^ degreee of subdivision, it is 

 diflicult to see why each element should 

 have a definite combining number, which 

 holds without exception, and why there 

 should be distinct compounds well marked 

 off from others, instead of different sub- 



