94 PROGRESS OF SCIENCE IN THE CENTURY. 



Extension of the Atomic Conception. — Here it 

 must be recalled that while physical enquiries into 

 the constitution of matter [or attempts to form a 

 conception of molecular motion] were mainly con- 

 cerned with gases, the solid and liquid states were 

 also studied. The solid state, where the mass has a 

 proper volume and a proper form, more or less dif- 

 ficult to change, began gradually to be conceived of 

 as one in which the relations of the molecules are 

 such that mutual displacement is not easy. En- 

 quiries into crystallisation begun by Steno (1669), 

 re-stimulated by the genius of Hauy (1781), con- 

 tinued by many workers (Weiss, Von Lang, etc.), 

 also proved suggestive, notably, for instance, when 

 Mitscherlich (1820) elaborated what Klaproth 

 (1798) had observed that the same substance might 

 have different crystalline forms (e.g., calc spar and 

 arragonite). 



Gradually, too, the atomic conception was extended 

 to liquids which differ from gases in occupying a 

 definite volume and from solids in having no proper 

 form and much less internal friction. Especially 

 through enquiries into the phenomena of osmosis and 

 of solution, the theoretical conception of gases was 

 applied to liquids. But this was hardly realised 

 till towards the end of the century; indeed it may 

 be associated with the work of Van't Hoff (1887). 



Instead of trying to follow the multitudinous lines 

 of research, we propose to take a single illustration 

 — the liquefaction of gases — which may serve to sug- 

 gest the unity of the different states of matter. 



Liquefaction of Gases. — Erom the time of Fara- 

 day's researches in 1823 to the recent work of Dewar, 

 popular imagination has been impressed by the re- 

 peated announcement, that such and such a gas had 



