234 Transactions of the American Institute. 



ask wliat proof science can furnish us that mechanical motion, heat, 

 light, and electricity are thus mutually convertible. As we have 

 already hinted, the time was when these forces were believed to be 

 various kinds of imponderable matter, and chemists and physicists 

 talked of the union of iron with caloric as they talked of its union 

 with sulphur, regarding the caloric as muc^i a distinct and incon- 

 Tertible entity as the iron and sulphur themselves. Gradually, how- 

 ever, the idea of the indestructibility of matter extended itself to 

 force. And as it was believed tliat no material particle could ever 

 be lost, so, it was argued, no portion of the force existing in nature 

 can disappear. Hence arose the idea of the indestructibility of force. 

 JBut, of course, it was quite impossible to stop here. If force cannot 

 be lost, the question at once arises, what becomes of it when it passes 

 beyond our recognition? This question led to experiment, and out 

 of experiment came the great fact of force-correlation ; a fact which 

 distinguished authority has pronounced the most important discovery 

 of the present century.* These experiments distinctly proved that 

 when any one of these forces disappeared, another took its place ; 

 that when motion was arrested, for example, heat, light or electricity 

 was developed. In short, that these forces were so intimately related 

 or correlated, to use the word then proposed by Mr. Grove,t that 

 when one of them vanished, it did so only to reappear in terms of 

 another. But one step more was necessary to complete this magnifi- 

 cent theory. What can produce motion but motion itself? Into 

 what can motion be converted, but motion? May not these forces, 

 thus mutually convertible, be simply different modes of motion of 

 the molecules of matter, precisely as meclianical motion is a motion 

 of its mass ? Thus was born the dynamic theory of force, first brought 

 out in any completeness by Mr. Grove, in 1842, in a lecture on the 

 "Progress of Physical Science," delivered at the London Institution. 

 In that lecture lie said : *' Light, heat, electricity, magnetism, motion, 

 are all convertible material affections. Assuming either as the cause, 

 one of the others will be the effect. Thus heat may be said to pro- 

 duce electricity, electricity to produce heat; magnetism to produce 

 electricity, electricity magnetism ; and so of the rest.":j: 



♦Armstrong, Sir Wm. In his address a? President of tlic British Association for the Advance- 

 ment of Science. Rep. Brit. Assoc, 18().3, li. 



+ Grove, W. R., in 1842. Compare " Nature " i, 335, Jan. 27, 1870. Also Appleton's Journal, iii, 

 324. March 19. 1870. 



X Id., in Preface to the Correlation of Physical Forces, 4th cd. Reprinted in the Correlation and 

 Conservation of Forces, edited by E. L. Yoiimans, p. 7. New York, 1805. D. Appleton & Co. 



