A HISTORY OF SCIENCE 



to itself, it has in that very act ceased to be c has be- 

 come e. If, after the production of e, c still remained 

 in the whole or in part, there must be still further 

 effects corresponding to this remaining cause : the total 

 effect of c would thus be > e, which would be contrary 

 to the supposition c = e. Accordingly, since c be- 

 comes e, and e becomes /, etc., we must regard these 

 various magnitudes as different forms under which 

 one and the same object makes its appearance. This 

 capability of assuming various forms is the second 

 essential property of all causes. Taking both prop- 

 erties together, we may say, causes an indestructible 

 quantitatively, and quantitatively convertible objects. 



" There occur in nature two causes which apparently 

 never pass one into the other," said Mayer. "The 

 first class consists of such causes as possess the proper- 

 ties of weight and impenetrability. These are kinds 

 of matter. The other class is composed of causes 

 which are wanting in the properties just mentioned 

 namely, forces, called also imponderables, from the 

 negative property that hd,s been indicated. Forces are 

 therefore indestructible, convertible, imponderable objects. 



"As an example of causes and effects, take matter: 

 explosive gas, H + O, and water, HO, are related 

 to each other as cause and effect ; therefore H + O = 

 HO. But if H + O becomes HO, heat, cal., makes its 

 appearance as well as water; this heat must likewise 

 have a cause, x, and we have therefore H+O+X 

 HO -\-cal. It might be asked, however, whether H + O 

 is really = HO, and x = cal. t and not perhaps H +O = 

 cal., and x = HO, whence the above equation could 

 equally be deduced ; and so in many other cases. The 



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