270 Dr. van der Kolk on the Mechanical 



separated to be, at a lower temperature, capable either of com- 

 bining spontaneously (that is, by mere cooling) or of remaining 

 separate. From this point of view, therefore, compounds may 

 be divided into two groups, — the first containing those which, 

 when decomposed by heat, are recomposed again when cooled ; 

 and the second, those in which this does not take place. 



There seems to me to be a connexion between this property 

 and the following. It is known from Favre and Silbermarm's 

 experiments that in many cases of chemical combination heat is 

 evolved, while in a few cases heat is absorbed; accordingly, 

 chemical compounds may be divided into two series from this 

 point of view also. The way in which this property is connected 

 with that previously referred to, will become clear from the fol- 

 lowing considerations. 



If we figure to our minds a body, at first in some definite con- 

 dition, at 0° C. for example, and then suppose heat to be ap- 

 plied to it, it will at a given temperature have taken up a given 

 quantity of heat. This heat goes partly to raise its temperature, 

 partly to cause molecular changes (so-called internal work), and 

 partly to perform external work. This whole quantity of heat 

 increases continually as the temperature rises. Elevation of 

 temperature, as well as change of state of aggregation from the 

 solid to the liquid, and from the liquid to the gaseous state, 

 always requires absorption of heat. In order to find the quantity 

 of heat existing in the body after the process, we must deduct 

 from the total quantity the portion which is converted into ex- 

 ternal work. The quantity of heat then remaining is called by 

 Thomson* the mechanical energy of the body in the given state. 

 According to the suppositions we have made, this is not an ab- 

 solute measure of the energy, but indicates how much more 

 energy is accumulated in the body than was in it in a given 

 condition at 0° C. Kirchhofff has called the influence which 

 the body exerts upon external matter, while passing from the 

 first condition to the second, the effective function ( Wirkungsf unc- 

 tion) of the body for this change. This is therefore, with the 

 contrary sign, the exact equivalent of what Thomson calls its 

 energy. 



According to this, every body possesses, in a given condition, 

 a given quantity of energy. Let us now suppose two bodies, 

 such as oxygen and hydrogen, which can combine by an electric 

 spark. Before conibination, each contains a certain quantity of 

 energy ; by the combination heat is generated ; and if the vapour 

 of water which is formed is cooled down to the temperature ex- 

 isting before the combination, the quantity of energy which the 



* Phil. Mag. S. 4. vol. ix. p. 523. 



f Poggendorff's Annalen, vol. ciii. p. 177- 



