THE BEGINNINGS OF IIFE. 43 



dissociating agency of heat, which tends to increase 

 the distance between the ultimate atoms and molecules 

 of bodies^. The chemical affinities holding together 

 the constituent atoms or molecules of certain com- 

 pounds are, however, too feeble to withstand the 

 dissociating influence of an intense amount of heat. 

 As the temperature rises, the chemical affinities which 

 bind together the dissimilar atoms into compound 

 molecules become more and more weakened, and may 

 be at last overcome before liquefaction takes place. 

 Still larger is the number of compounds which are 

 unable to endure the disruptive agency of the higher 

 temperatures necessary to reduce them to the state of 

 gas or vapour. In the case of those substances, more- 

 over, which are capable of being reduced to either 

 physical condition by the aid of heat, innumerable 



* * Bunsen and Hopkins have shown that substances which expand 

 when fused have their point of fusion raised by mechanical pressure, 

 that is to say, since mechanical force must be overcome in melt- 

 ing, the tendency to melt must be overcome by heat before that 

 opposition can be overcome ; and the pressure required to keep them 

 solid at any temperature above their natural point of fusion may be 

 looked upon as the mechanical representative of the force with which 

 they tend to fuse at that temperature. Prof. W. Thomson has shown 

 that, on the contrary, water, which expands in freezing, has its point 

 of fusion lowered by pressure; that is to say, since mechanical force 

 must be overcome by crystallizing, crystallization will not take place 

 under increased pressure, unless the force of crystalline polarity be 

 increased by reducing the temperature. . . . Similar principles hold true 

 with respect to the solubility of salts in water.' — Bakerian Lecture, ' On 

 the Direct Correlation of Mechanical and Chemical Forces,' by H. 

 C. Sorby (' Proceed, of Royal Soc' vol. xii. 1863, p. 542). 



