SCIENCE IN THE LAST HALF CENTURY. 71 



and heat. The pbeuomemi of electrolytic decomposition showed that 

 there was a like close relation between these units and electricity. The 

 quantity of electricity generated by the combination of any two units 

 is sufficient to separate any other two which are susceptible of such 

 decomposition. The phenomena of isomorphism showed a relation 

 between the units and crystalline forms ; certain units are thus able to 

 replace others in a crystalline body without altering its form, and others 

 are not. 



Again, the laws of the effect of pressure antl heat on gaseous bodies, the 

 fact that they combine in definite proportions by volume, and that such 

 proportion bears a simple relation to their combining weights, all har- 

 monized with the Daltonian hypothesis, and led to the bold speculation 

 known as the law of Avogadro — that all gaseous bodies, under the same 

 physical conditions, contain the same number of units. In the form in 

 which it was first enunciated this hypothesis was incorrect — perhaps it 

 is not exactly true in any form ; but it is hardly too much to say that 

 chemistry and molecular physics would never have advanced to their 

 present condition unless it had been assumed to be true. Another 

 immense service rendered by Daltou, as a corrollary of the new atomic 

 doctrine, was the creation of a system of symbolic notation, which not 

 only made the nature of chemical compounds and processes easily in- 

 telligible and easy of recollection, but, by its very form, suggested new 

 lines of inquiry, The atomic notation was as serviceable to chemistry 

 as the binomial nomenclature and the classiflcatory schematism of 

 Liuna'us were to zoology and botany. 



Side by side with these advances arose another, which also has a close 

 parallel in the history of biological science. If the unit of a compound 

 is made up by the aggregation of elementary units, the notion that 

 these must have some sort of definite arrangement inevitably suggests 

 itself; and such phenomena as double decomposition i)ointed, not only 

 to the existence of a molecular architecture, but to the possibility of 

 modifying a molecular fabric without destroying it, by taking out some 

 of the component units and replacing them by others. The class of 

 neutral salts, for example, includes a great number of bodies in many 

 ways similar, in which the basic molecules, or the acid molecules, mny 

 be replaced by other basic and other acid molecules without altering 

 the neutrality of the salt; just as a cube of bricks remains a cube so 

 long as any brick that is taken out is replaced by another of the same 

 shape and dimensions, whatever its weight or other properties may be. 

 Facts of this kind gave rise to the conception of " types" of molecular 

 structure, just as the recognition of the unity in diversity of the struct- 

 ure of the species of plants and animals gave rise to the notion of bio- 

 logical "types." The notation of chemistry enabled these ideas to be 

 represented with precision ; and the^' acquired an immense importance 

 when the improvement of methods of analysis, which took place about 



