v.] CHEMICAL ENERGIES. 53 



the mutual gravitation of two bodies is proportionate, 

 supposing the distance given, to the product of their 

 masses. 



What greatly increases the dijBQculty of any such 

 inquiry is, that in the case of many thermo-negative 

 substances, the potential energy of their elements is 

 not all parted "with in the . act of combination ; some 

 of it remains in the compounds, and is converted 

 into heat when they again enter into combination. To 

 take an instance almost at random, hydrochloric acid 

 and potash are both thermo-negative ; and yet when an 

 equivalent of the one combines with an equivalent of the 

 other, they give out no less than 15,656 heat-units. 

 Neutral salts are perhaps the only perfectly thermo-nega- 

 tive substances : and, containing no potential energy, they 

 do not seek to combine with others. They may be com- 

 pared to a body which has no potential energy due to its 

 position, and is in stable equilibrium because it can fall 

 no lower. 



In order to discover these dynamic equivalents (if they 

 exist, which appears highly probable), it will of course be 

 necessary in the case of each element to observe the quan- 

 tities of heat it gives out in combining with various other 

 elements, and the quantities its compounds again give out 

 in their combinations. The problem is no doubt a difficult 

 one, but it ought not to be very much more difficult to us 

 than was the determination of the combining equivalents 

 to Dalton. 



Some simple substances appear to be capable of entering 

 into a thermo-positive state. Phosphorus is the most re- 

 markable instance of this yet known : it is capable of 

 assuming the allotropic state called red or amorphous Red or 

 phosphorus, in which it has taken up a considerable quan- XoJ? °"^ 

 tity of heat. If red phosphorus is heated up to a certain phorus. 

 point, it suddenly passes back into the ordinary colourless 

 state, and the heat which it had taken up is given out in 

 such quantity as to vaporize a part of the phosphorus.^ 

 The number of heat-units has not been determined, so far 



1 Miller's Chemistry, vol. ii. p. 201. 



