258 



SCIENCE 



[N. S. Vol. XXXIV. No. 870 



Tluis modern geology was preceded by 

 a period of explanation by ' ' catastrophes, ' ' 

 and modern biology by a period in which 

 the theory of separate creation of each of 

 the Linueau species was elevated almost 

 to the rank of a religious dogma; in both 

 cases the main difficulties the new view 

 (of continuity, as I am calling it) had to 

 encounter, were due to the wrench it cost 

 to break with the old faitiiliar extra-ex- 

 perimental theories or explanations. This 

 again was to be expected; the theories 

 were invented by men whose minds were 

 deeply impressed by certain observations 

 or relations, their object was to "explain" 

 these relations and keep them before the 

 mind, so that the theories can hardlj' be 

 blamed if in addition they kept out of the 

 mind more recently discovered facts whose 

 existence their originators never suspected. 



Our own science has passed through a 

 similar evolution. Modern chemistrj^ be- 

 gan with the study of the most striking of 

 all chemical phenomena, the phenomena of 

 combiostion; and just when these were 

 robbed of much of their mystery by the 

 discovery that like other chemical reac- 

 tions thej^ obej'ed the law of conservation 

 of weight, a new interest was awakened by 

 the discovery of phases of invariable com- 

 position, the typical chemical compounds, 

 whose study laid the foundations of quan- 

 titative analysis, and led to the remarkable 

 so-called laws of chemical combination 

 which Dalton's atomic theory was in- 

 vented to explain. 



The existence of such compounds is fa- 

 miliar enough to us, but a century ago one 

 of the most celebrated chemists of his day 

 strove for eleven years to show that no 

 such thing's could be. This may serve as a 

 measure of the interest and attention at- 

 tracted by these substances at the time of 

 their discoverj' ; no wonder they alone were 

 designated "chemical" compounds, small 



blame to those who thought that substances 

 so striking in some respects must prove 

 unique in all ; how natural that the idea of 

 coutinuitj' advocated by the defeated Ber- 

 thollet should be forgotteu, most natural that 

 the extra-experimental theories invented at 

 the beginning of the nineteenth century 

 should make a sharp distinction between 

 these chemical compounds and all else. 



They did so ; and as all good theories of 

 that type do, they kept men's minds on the 

 facts they were invented to explain ; while 

 the slowly accumulating "exceptions"— 

 facts out of harmony with the tendencj^ of 

 the theory — being unexplained, and thus 

 lacking a powerful aid to publicity, failed 

 of their due influence on opinion. The dis- 

 covery of the dissociation of chemical com- 

 pounds by Deville and Debray — that mar- 

 ble, for instance, could be formed or broken 

 up by the action of an air-pump — had sur- 

 prisingly little effect at the time; but the 

 work of Horstmann, Gibbs and van't Hoff, 

 not to mention lesser lights by name, has at 

 last made it abundantly clear that the 

 "affinity of the atoms" which binds to- 

 gether the constituents of chemical com- 

 pounds is subject to the same laws and may 

 be measured in the same way as the forces 

 — hitherto deemed distinct — which are re- 

 sponsible for the formation of solutions and 

 adsorpta. 



Thus, hand in hand with the study of 

 chemical equilibrium, the idea of continuity 

 entered chemistry, and has transformed it. 



The high-school text books, however, as 

 a class, in their tendency deny this contin- 

 uity in toto. 



In them the chemical compound and 

 the element ("chemical individuals" for 

 short) retain their former place as "the 

 only two distinct kinds of matter"; and 

 mechanical mixtures are distinguished by 

 being separable into their ingredients "by 

 mechanical means," thus ignoring the fact 



