386 Sir. W. J. ]\Iacquorn Rankine on the Science of Energetics. 



physics relate ; if, then, there be general laws respecting energy, such 

 laws must be applicable, mutatis mutandis, to every branch of physics, 

 and must express a body of principles as to physical phenomena in 

 general. 



In a paper read to the Philosophical Society of Glasgow on the 5th 

 of January 1853, a first attempt was made to investigate such principles, 

 by defining actual energrj and 'potential energy, and by demonstrating a 

 general law of the mutual transformations of those kinds of energy, of 

 which one particular case is a previously known law of the mechanical 

 action of heat in elastic bodies, and another, a subsequently demonstrated 

 law which forms the basis of Professor William Thomson's Theory of 

 thermo-electricity. 



The object of the present paper is, to present in a more systematic 

 form, both these and some other principles, forming part of a science 

 whose subjects are, material bodies and physical phenomena in general, 

 and which it is proposed to call the Science of Energetics. 



VIII. Definitions oe certain Teems. 



The peculiar terms which will be used in treating of the Science of 

 Energetics are purely abstract ; that is to say, they are not the names of 

 any particular object, nor of any particular phenomena, nor of any par- 

 ticular notions of the mind, but are names of very comprehensive classes 

 of objects and phenomena. About such classes it is impossible to think 

 or to reason, except by the aid of examples or of symbols. General terms 

 are symbols employed for this purpose. 



Substance. 

 The term "substance" will be applied to all bodies, parts of bodies, 

 and systems of bodies. The parts of a substance may be spoken of as 

 distinct substances, and a system of substances related to each other may 

 be spoken of as one complex substance. Strictly speaking, the term 

 should be " material substance,'" but it is easily borne in mind, that in 

 this essay none but material substances are referred to. 



Property. 



The term "property'' will be restricted to invariable properties; 

 whether such as always belong to all material substances, or such as con- 

 stitute the invariable distinctions between one kind of substance and 

 another. 



Mass. 



Mass means " quantity of substance.'' Masses of one kind of substance 

 may be compared together by ascertaining the numbers of equal parts 



