732 Professor Dewar [Jan. 18, 



space." Later investigators, while altering the details of Davy's 

 theoretical explanations of the gaseous state, in substance acknow- 

 ledged the legitimacy of his ideas by extending and perfecting 

 his hypothesis. A short time after the publication of Dalton's 'New 

 System of Chemical Philosophy,' Leslie and Wollaston made their 

 memorable experiments on the freezing of water by inducing rapid 

 evaporation of the isolated liquid in a receiver by chemically absorb- 

 ing or condensing the vapour as quickly as it is produced in a 

 separate part of the same vessel. Thus originated the process of 

 reaching lower temperatures and maintaining them by the continuous 

 distillation of a fluid, or what is now called distillation in vacuo 

 or under reduced pressure. Gay Lussac succeeded in showing that 

 by such means a temperature as low as the freezing-point of mercury 

 could be reached. Cagniard de la Tour in 1822 made his most 

 startling experiments, proving that the dilatation of liquids has a 

 limit beyond which, in spite of compression, they become gases. He 

 succeeded in vaporising ether in a space twice the volume of the 

 liquid, and noticed that before disappearing the liquid seemed to 

 occupy the whole space ; the tube finally looking empty, until on 

 cooling a thick mist appeared for a minute, and then suddenly the 

 liquid in the old state. This change in the case of ether from the 

 liquid to the gas he showed took place at 160°, the pressure then 

 being 37 to 38 atmospheres. He made similar experiments with 

 alcohol, bisulphide of carbon and water, showing that the same state 

 could be brought about in each case, only at different temperatures 

 and pressures for each substance. This beautiful investigation, in- 

 volving the use of sealed tubes together with manometric observations 

 — the temperatures and pressure being very considerable — was indeed 

 of first-rate importance, but its real value was not appreciated until 

 many years later. 



In the experiments of Faraday and Davy high pressures were 

 obtained by generating the gases in sealed tubes bent into the shape 

 of an inverted U, so that one leg containing the re-acting chemicals 

 might be heated if necessary, and the other cooled to form a con- 

 denser for the liquefied gas. Davy, who was always alive to the 

 possibility of practical utility resulting from scientific discovery, 

 had the idea of using liquid gases as agents for the production of 

 motive power and as the means by which great reductions of tempe- 

 rature could be effected, owing to the rapidity with which they 

 could be rendered aeriform. Although Davy's mechanical sugges- 

 tion has not been generally adopted, there can be no doubt about 

 the successful application of such liquids to command steady low 

 temperatures by their evaporation. 



The next great advance was the production of large quantities of 

 liquid carbonic acid by Thilorier in 1835, and the remarkable dis- 

 covery that when ejected into the air its rapid evaporation reduced 

 the temperature to such an extent as to cause its own solidification 

 into the well-known snow. He further observed that the coefficient 

 of expansion of the liquid gas is greater than that of any aeriform 



