IV.] PRIMARY FORCES, 43 



the substances combine, forming a compound with new- 

 properties. 



It is a very important generalization, that all primary All pri- 

 forces are attractive ; there is no such thing in nature as a JP^'"^ 



•^ ' _ ° _ forces are 



primary repulsive force. For this, as for every other ulti- attractive, 

 mate physical law, no cause can be assigned except the 

 Divine will. But its purpose is obvious. The universe is Purpose oi' 

 held together by attractive forces ; and if, as 1 believe, the 

 nebular, or, as I prefer to call it, the condensation theory 

 of world-formation is true, the universe has been formed 

 by the action of attractive forces. Eepulsive forces, on the 

 contrary, it is obvious, could neither form a world nor hold 

 it together. 



It needs no proof that gravitation and capillarity are 

 attractive forces. lu the case of affinity this is not quite 

 so evident. But we know that the action of chemical 

 forces tends to union, and not to separation ; and we know 

 also that it tends to condensation. The proof of this latter 

 assertion is, that (among gases, in which alone it is pos- 

 sible to study chemical forces purely and simply) the 

 volume of a compound is often less than the joint volume 

 of its constituents when uncombined, and never greater.'^ 



From the truth that all primary forces are attractive, it Potential 

 follows that potential energy is never in a hody, but always |^"ffunc* 

 hetween tvjo hodies ; being due to their mutual attraction, tio^ of two 

 for attraction must be between at least two things. The attracting 

 energy due to the mutual attraction of two bodies is not a ^oi^ies. 

 function of either body separately, but a joint function of 

 the two. The potential energy due to a weight that is 

 ready to fall is a function neither of the weight nor of the 

 earth that attracts it separately, but of both jointly. The 

 potential energy due to the capillarity of two rain-drops 

 that are ready to run together is a function of neither 



' The immediate effect of the combination of oxygen and hydrogen is an 

 iricrease of their volume so great as to be capable of causing an explosion. 

 But this is due to the production of heat ; and if the water, which is the 

 result of their combination, is in the form of vapour, or steam, at the same 

 temperature and pressure as the oxygen and hydiogen before combination, 

 it occupies only two-thirds of their joint volume. 



