THE CHEMICAL ELEMENTS. 747 



salt in which all the qualities of sweetness or saltness are pre- 

 served. These molecules, however, although the elements of sub- 

 stances, are not the ultimate elements of matter, but on the con- 

 trary are themselves aggregates — often very complex aggregates 

 — of still smaller units which are considered to be the elemental 

 atoms. Of such atoms we must admit as many different kinds as 

 we have distinguished chemical elements, and the atoms are for 

 the present the ultimate limit of our analysis of matter. These 

 atoms are now the ideal chemical elements. Starting from the 

 atoms, the orders of combination are, first, the union of the atoms 

 to form the molecules which are the nuclei of definite substances, 

 then the aggregation of these molecules to form material masses. 



Obviously we may conceive of the union of either similar or 

 of dissimilar atoms ; and while the union of unlike atoms results 

 in the production of molecules of compound substances, the union 

 of like atoms (all of oxygen or all of hydrogen, for example) 

 yields molecules of elementary substances. So far as the primary 

 structure is concerned, there is no distinction between an ele- 

 mentary substance like oxygen gas and a compound substance 

 like water. In each case the material is an aggregate of similar 

 molecules, and owes its physical qualities to the external relations 

 of its peculiar units ; but, while the molecules of oxygen gas are 

 each composed of two atoms of oxygen, the molecules of water 

 consist each of two atoms of hydrogen and one of oxygen. . 



In admitting the possibility of the union of similar atoms to 

 form the molecules of elementary substances, the new philosophy 

 of chemistry differs most markedly from the old. The system of 

 Lavoisier was based on a conception of dualism originally sug- 

 gested by sexual relations; and the term elective affinity, which 

 was so constantly used to explain chemical changes, was a phase 

 of this conception. The elements of two kinds paired together to 

 form acids or bases, and the acids and bases paired to form salts, 

 and chemical changes were the consequence of the superior afi&nity 

 of another acid or another base for the temporary mate of a fel- 

 low-companion. At the time of Lavoisier, the grosser features of 

 these dualistic conceptions, which so disgust us in the earlier 

 writers on chemistry, had disappeared ; and, still later, Berzelius 

 attempted to place the system on a scientific basis by referring 

 the dualism to electrical relations. But there was an entire con- 

 tinuity of thought from first to last, and in this was involved the 

 prevailing idea that strength of chemical union depended on op- 

 position of qualities. But this idea, which I have no doubt many 

 scholars who studied chemistry under the old system still retain, 

 was an entire misconception. 



One of the strongest combining forces known to chemistry is 

 that which holds together the dissimilar atoms of oxygen and 



