82 PROGRESS : ITS LAW AKD CAUSE. 



Thus much premised, we pass at once to the statement 

 of the law, which is this -.-:— Every active force 2^'f'oduces 

 more than one change — every cause produces more than one 

 effect. 



Before this law can be duly comprehended, a few exam- 

 ples must be looked at. When one body is struck against 

 another, that which we usually regard as the effect, is a 

 change of position or motion in one or both bodies. But 

 a moment's thought shows us that this is a careless and 

 very Lncomplete view of the matter. Besides the visible 

 mechanical result, sound is produced ; or, to sj)eak accurate- 

 ly, a vibration in one or both bodies, and in the surround- 

 ing air : and under some circumstances we call this the ef- 

 fect. Moreover, the air has not only been made to vibrate, 

 but has had sundry currents caused in it by the transit of 

 the bodies. Further, there is a disarrangement of the par- 

 ticles of the two bodies in the neighbourhood of their point 

 of collision ; amounting in some cases to a visible conden- 

 sation. Yet more, this condensation is accompanied by the 

 disengagement of heat. In some cases a spark — that is, 

 light — results, from the incandescence of a portion struck 

 off; and sometimes this incandescence is associated "with 

 chemical combination. 



Thus, by the original mechanical force expended in the 

 collision, at least five, and often more, different kinds of 

 changes have been produced. Take, again, the lighting of 

 a candle. Primarily this is a chemical change consequent 

 on a rise of temperature. The process of combination 

 having once been set going by extraneous heat, there is a 

 continued formation of carbonic acid, water, &c. — in itself 

 a result more complex than the extraneous heat that first 

 caused it. But accompanying this process of combination 

 iherf; is a production of heat ; there is a production of light ; 

 there is an ascending column of hot gases generated ; there 

 are currents established in the surrounding air. Moreover 



