3o6 GREAT MEN OF SCIENCE 



was successfully accomplished in 1877. The higher degree 

 of cooling, already recognised by Faraday as necessary, was 

 attained in these very excellently conducted experiments by 

 very simple means, already known since Dalton's time, of 

 allowing the strongly compressed gas, cooled as far as pos- 

 sible, to expand suddenly.^ 



Although liquefaction only existed for a short time in the 

 form of a mist or a few drops, it was actually seen, and the 

 certainty that liquid oxygen exists was obtained. It was 

 now settled that the opinion generally held by the unin- 

 structed public after the failures of so many years, that air is 

 a 'permanent,' or 'incoercible' gas, was incorrect. After 

 the existence of molecular forces, however small, had been 

 proved, this possibility could have been regarded as a cer- 

 tainty; for molecules which have any perceptible attraction 

 for one another, must, at a sufficiently low temperature, and 

 hence at a sufficiently small distance apart, cohere to form a 

 liquid, and finally a solid, as a result of the correspondingly 

 increased molecular attraction. 



The method just described, and also that used in ordinary 

 refrigerating machines, was not suitable for the preparation 

 of liquid air on a large scale; here the experiment of Joule 

 and Thomson, carried out in a suitable manner, leads to suc- 

 cess. This experiment, in which the gas is pumped round in 

 a circle, only produces a very small fall in temperature, and 

 it might therefore have been regarded as entirely unfitted 

 for the purpose; but Joule and Thomson had shown that the 

 small fall in temperature at the constriction increases con- 

 siderably when the gas is cooled; this is readily understood, 

 since at the lower temperature the molecules are closer to- 

 gether, and hence their attractive force is greater. Ac- 

 cordingly, in the present liquid air machines, the circuit of 



* L. Cailletet in Paris; he had already carried out, before Andrews, 

 measurements on gases at high pressure, which Van der Waals was able 

 to make use of in support of his equation. 



