320 HISTORY OF CHEMISTRY. [LECTURE XV. 



Although I cannot enter into further details here, I may add 

 that Senarmont attacked and, partially at least, solved the pro- 

 blem of determining, and of realising, the conditions under 

 which those naturally occurring minerals are formed which 

 are met with crystallised in veins. In particular, he employed 

 water for this purpose, and he caused it to act under pressure 

 at about 350. 135 It may further be stated that Sainte Claire 

 Deville 136 and his pupils discovered and made use of the 

 favourable effect of hydrofluoric acid and of other fluorine com- 

 pounds in promoting crystallisation ; and that Hautefeuille 13 ~ 

 was the first who artificially prepared potash and soda felspars. 

 Mention must also be made here of the numerous syntheses of 

 minerals that were carried out by Friedel and his pupils. 



The introduction of the idea of the critical temperature or 

 of the absolute boiling-point signalised a great advance in our 

 knowledge of the connection between the different states of 

 aggregation of substances. 



Cagniard de la Tour 138 observed, as long ago as 1822, that 

 on heating liquids in sealed tubes which they almost fill, a tem- 

 perature can be attained at which the menis.cus disappears, and 

 the whole presents a perfectly homogeneous appearance. From 

 this he concluded that. at this temperature the liquid is con- 

 verted into gas notwithstanding the pressure. Although these 

 experiments were highly noteworthy, still they did not attract 

 any considerable attention ; and it was only thirty-three years 

 later that Wolf 139 and Drion 14 tried to determine, in the cases 

 of a few liquids, the temperatures at which they pass into the 

 state observed by Cagniard de la Tour. Mendelejeff, in 1861, 

 introduced the very appropriate name " absolute boiling-point " 

 to designate this temperature ; and he defined it as the 

 temperature at which both the cohesion of the liquid and 

 its heat of evaporation vanish, and the liquid itself is con- 

 verted into vapour irrespective of pressure and of volume. 141 



135 Annalen. 80, 212. 136 Caron and Deville, ibid. 108, 55 ; 109, 

 242, etc. 137 Comptes Rendus. 90, 830. 138 Ann. Chim. [2] 21, 127, 

 178 ; 22, 410. 139 Ibid. [3] 49, 265. ]4 Ibid. [3] 56, 33. 141 Anna- 

 len. lip, I. 



