168 CORRELATION OF PHYSICAL FORCES. 



abstract or generalised meaning of the term cause, and its 

 concrete or special sense; the word itself being indiscriminately 

 used in both these senses. 



Another confusion of terms has arisen, and has, indeed, 

 much embarrassed me in enunciating the propositions put 

 forth in these pages, on account of the imperfection of scien- 

 tific language ; an imperfection in great measure unavoid- 

 able, it is true, but not the less embarrassing. Thus, the 

 words, light, heat, electricity, and magnetism, are constantly 

 used in two senses viz. that of the force producing, or the 

 subjective idea of force or power, and of the effect produced, 

 or the objective phenomenon. The word motion, indeed, is 

 only applied to the effect, and not to the force, and the term 

 chemical affinity is generally applied to the force, and not to 

 the effect ; but the other four terms are, for want of a dis- 

 tinct terminology, applied indiscriminately to both. 



I may have occasionally used the same word at one 

 time in a subjective, at another in an objective sense ; all I 

 can say is, that this cannot be avoided without a neology, 

 which I have not the presumption to introduce, or the autho- 

 rity to enforce. Again, the use of the term forces in the 

 plural might be objected to by those who do not attach to 

 the term force the notion of a specific agency, but of one 

 universal power associated with matter, of which its various 

 phenomena are but diversely modified effects. 



Whether the imponderable agents, viewed as force, and 

 not as matter, ought to be regarded as distinct forces or as 

 distinct modes of force, is probably not very material, for, as 

 far as I am aware, the same result would follow either view ; 

 I have therefore used the terms indiscriminately, as either 

 happened to be the more expressive for the occasion. 



Throughout this Essay I have placed motion in the same 

 category as the other affections of matter. The course of 

 reasoning adopted in it, however, appears to me to lead inevi- 

 tably to the conclusion that these affections of matter are 

 themselves modes of motion ; that, as in the case of friction, 

 the gross or palpable motion, which is arrested by the contact 

 of another body, is subdivided into molecular motions or 

 vibrations, which vibrations are heat or electricity as the case 



