304 GREAT MEN OF SCIENCE 



liquefied, and its dependence upon the temperature; he 

 concludes in the case of carbonic acid that its 'Cagniard de 

 la Tour state' (critical point) lies in the neighbourhood of 

 90° F. (32° C) at which the gas and the liquid are equal in 

 density, and that it therefore does not become liquid at 

 higher temperatures, whereupon he points out that the other 

 gases which he could not liquefy must require the use of 

 still lower temperatures than he was able to produce. This 

 result was confirmed fully twenty-six years later (1873) by 

 Thomas Andrews,^ who followed with still greater exactitude 

 and much improved apparatus, the connection between 

 pressure and volume of carbon dioxide at different tem- 

 peratures; he found, in surprisingly good agreement 

 with Faraday, 3i°C to be the critical temperature of 

 carbonic acid, and gave detailed curves showing its com- 

 plete behaviour. 



Upon this basis, Van der Waals (1837-1923; lived in 

 Leyden and Amsterdam) immediately put forward the ad- 

 mirable equation known under his name, which summarised 

 the relationship above stated in a simple manner holding for 

 all gases, and also giving the long known departures from 

 the gas laws of Boyle and Mariotte and also of Gay-Lussac 

 and Dalton; it also summarised all questions of the lique- 

 faction of gases (1873). This achievement represented the 

 interconnection of a large number of single facts, which 

 could thereby also support one another. This equation 

 further connects in the best possible way with the results 

 of the kinetic theory of gases, which likewise leads to per- 

 ceptible departures from the simple gas laws, as soon as the 

 actual volume occupied by the molecules becomes appre- 

 ciable in relation to the total volume of the gas, or when the 

 forces begin to come into play, with which the gas mole- 

 cules attract one another. Van der Waals' equation gives 



^ 1813-1885; began as a practising physician, and then became pro- 

 fessor of chemistry at Belfast. 



