148 CORRELATION OF PHYSICAL FORCES. 



by a large majority of chemists, presents great difficulties 

 when extended to all chemical combinations. 



The equivalent ratios in which a great number of substances 

 chemically combine, hold good in so many instances, that the 

 atomic doctrine is believed by many to be universally appli- 

 cable, and called a law ; and yet, when followed in the com- 

 binations of substances whose mutual chemical attractions are 

 very feeble, the relation fades away, and is sought to be 

 recovered by applying a separate and arbitrary multiplier to 

 the different constituents. 



Thus, when it was found that a vast number of substances 

 combined in definite volumes and weights, and in definite 

 volumes and weights only, it was argued that their ultimate 

 molecules or atoms had a definite size and weight, as otherwise 

 there was no apparent reason why this equivalent ratio should 

 hold good ? why, for instance, water should only be formed of 

 two volumes or one unit by weight of hydrogen, and of one 

 volume or eight units by weight of oxygen ? why, unless there 

 were some ultimate limits to the divisibility of its molecules, 

 should not water, or a fluid substance approximating to water 

 in character, be formed by a half, a third, or a tenth part of 

 hydrogen, with eight parts of oxygen ? 



It was perfectly consistent with the atomic view that a 

 substance might be formed with one part combined with eight 

 parts, or with sixteen, or with twenty-four, for in such a sub- 

 stance there would be no subdivision of the (supposed indi- 

 visible) molecule ; and this held good with many compounds : 

 thus, fourteen parts by weight, say grains of nitrogen, will 

 combine respectively with eight, sixteen, twenty-four, thirty- 

 two, and forty parts by weight, or grains, of oxygen. 



So, again, twenty-seven grains of iron will combine with 

 eight grains of oxygen or with twenty-four grains, i.e. three 

 proportionals of oxygen. No compound is known in which 

 twenty-seven grains of iron will combine with two propor- 

 tionals or sixteen grains of oxygen ; but this does not much 

 affect the theory, as such a compound may be yet discovered, 

 or there may be reasons at present unknown why it cannot be 

 formed. 



But now comes a difficulty : twenty-seven parts by weight 



