72 SCIENCE IX THE LAST HALF CENTURY. 



Tbns a definite relation was established between the hypothetical units 

 the beginning of our period, enabled the coiupositiou of the so-called 

 "organic" bodies to be determined with rapidity and precision.* A 

 large proportion of these compounds contain not more than three or 

 four elements, of which carbon is the chief; but their number is very 

 great, and the diversity of their physical and chemical properties is as- 

 tonishing. The ascertainment of the proportion of each element in 

 these compounds aifords little or no help towards accounting for their 

 diversities ; widely different bodies being often very similar, or even 

 identical, in that respect. And, in the last case, that of isomeric com- 

 pounds, the appeal to diversity of arrangement of the identical compo- 

 nent units was the only obvious way out of the difficulty. Here again 

 hypothesis proved to be of great value; not only was the search for 

 evidence of diversity of molecular structure successful, but the study 

 of tr.e process of taking to pieces led to the discovery of the way to put 

 together, and vast numbers of compounds, some of them previously 

 known only as products of the living economy, have thus been artifi- 

 cially constructed. Chemical work at the present day is, to a large 

 extent, synthetic or creative; that is to say, the chemist determines, 

 theoretically, that certain non-existent compounds ought to be pro- 

 ducil)le, and he i)roceeds to produce them. 



It is largely because the chemical theory and practice of our epoch 

 have passed into this deductive and synthetic stage, that they are en- 

 titled to the name of the " New Chemistry," which they commonly re- 

 ceive. But this new chemistry has grown up by the help of hypotheses, 

 such as these of Dalton, and of Avogadro, and that singular conception 

 of " bonds" invented to colligate the tacts of " valency " or " atomicity," 

 the first of which took some time to make its way ; while the second 

 fell into oblivion for many years after it was propounded for lack of em- 

 pirical justification. As for the third, it may be doubted if any one 

 regards it as more than a temporary contrivance. 



But some of these hypotheses have done yet further service. Com- 

 bining them with the mechanical theory of heat and the doctrine of the 

 conservation of energy, which are also pi-oducts of our time, physicists 

 have arrived at an entirely new conception of the nature of gaseous 

 bodies and of the relation of the physico chemical units of matter to 

 the different forms of energy. The conduct of gases under varying 

 pressure and temperature, their ditfusibilitj', their relation to radiant 

 heat and to light, the evolution of heat when bodies combine, the ab- 

 sorption of heat when they are dissociated, and a host of other molecu- 

 lar phenomena, have been shown to be deducible from the dynamical 

 and statical principles which apply to molar motion and rest; and the 

 tendency of the physico-chemical science is clearly towards the reduc- 

 tion of the problems of the world of the infiniltely little, as it already 



* "At present more orgauic analyses are made in a single day than were accom- 

 plished before Liebig's time iu a whole year" — Hofmanu, Faraday Lecture, p. 46. 



