Effect of Pressure in Lowering the Freezing- Point of Water. 123 



figures that it may almost be taken for granted that, as in this 

 instance, when there is an exception in this particular, it is due 

 to some subsequent change having taken place in one or other 

 of the colours. In the case before us our first tracing was co- 

 loured with the verdigris-green : it was unsatisfactory ; but on 

 making a new tracing and colouring it according to our amended 

 observations, it at once became harmonious in colour, and as- 

 sumed an intelligible form, though all our colouring will not 

 enable us to convey the idea of ruby-gemmed flowers like the 

 substance used, the transparency of glass contributing much to 

 the general effect. 



XII. The Effect of Pressure in Lowering the Freezing- Point 

 of Water experimentally demonstrated,. By Professor W. 

 Thomson, Glasgow*. 



ON the 2nd of January 1849, a communication entitled 

 " Theoretical Considerations on the Effect of Pressure 

 in Lowering the Freezing- Point of Water, by James Thomson, 

 Esq., of Glasgow," was laid before the Royal Society, arid it 

 has since been published in the Transactions, vol. xvi. part 5 f. 

 In that paper it was demonstrated that, if the fundamental 

 axiom of Carnot's Theory of the Motive Power of Heat be 

 admitted, it follows, as a rigorous consequence, that the tem- 

 perature at which ice melts will be lowered by the application 

 of pressure ; and the extent of this effect due to a given amount 

 of pressure was deduced by a reasoning analogous to that of 

 Carnot from Regnault's experimental determination of the 

 latent heat, and the pressure of saturated aqueous vapour at 

 various temperatures differing very little from the ordinary 

 freezing-point of water. Reducing to Fahrenheit's scale the 

 final result of the paper, we find 



t = nx 0-0135; 



where t denotes the depression in the temperature of melting 

 ice produced by the addition of n "atmospheres" (or n times 

 the pressure due to 29*922 inches of mercury), to the ordinary 

 pressure experienced from the atmosphere. 



In this very remarkable speculation, an entirely novel phy- 

 sical phaenomenon was predicted in anticipation of any direct 

 experiments on the subject ; and the actual observation of the 

 phaenomenon was pointed out as a highly interesting object 

 for experimental research. 



* From the Proceedings of the Royal Society of Edinburgh, February 

 1850. 



\ It will appear also, with some slight alterations made by the author, 

 in the Cambridge and Dublin Mathematical Journal, Nov. 1850. — W. T. 



