468 POPULAR SCIENCE MONTHLY. 



ter is through the action of its attribute forces upon our senses; it is 

 indeed possible that matter only exists through that quality which 

 makes it the residence of the physical forces; it is extremely probable 

 that all natural forces affect all matter and originate in matter. 



There are just three corollaries to the general 'Law of Substance/ 

 the Law of Persistence of all Existences; these are: 



1. The Law of the Persistence of Matter per se. 



2. The Law of Persistence of Force as an Attribute of Matter. 



3. The Law of Persistence of Energy, whether as affecting a mass of 

 matter or in process of transfer or of transformation; affecting varying 

 quantities and kinds of matter; passing from one quantity of matter to 

 another; changing, in inverse direction, the quantity of matter affected 

 and the velocity-component of the energy; the product of mass and 

 mean velocity-square remaining constant for the whole universe. 



The distinction between force and energy was not, in earlier times, 

 very exactly observed; but it is easy to perceive in the context to the 

 enunciation of either corollary to the fundamental law the fact that 

 writers usually well understood the principle which they sought to state. 

 It had, by Faraday's time, come to be well understood by many scientific 

 men that matter is persistent, that its characteristic forces cling to it 

 persistently and that energy is the product of forces and motion, and 

 is consequent upon inertia. 



The writer took occasion, in a paper read before the American So- 

 ciety of Civil Engineers (December 9, 1873), criticizing Professor Tait's 

 'Sketch of Thermodynamics/ his assignment to Sir Humphry Davy of 

 a prior place and his depreciation of the work of Mayer, to show that 

 Eumford is entitled to a larger credit than is ordinarily assigned him 

 even by those who admit his first appearance in this line of investigation 

 at the close of the eighteenth century. It is easy to show that, not only 

 was Eumford the first to exhibit by experimental research the fact of the 

 equivalence of thermal and dynamic energy, but that he was the first to 

 establish with some degree of approximation their quanti valence. In 

 fact, he secured data giving a much closer determination of the 'me- 

 chanical equivalent of heat' than did Joule, or any other investigator of 

 later years up to the middle of the century; at which date, while an ap- 

 proximate value had been hit upon, so great was the variety of constants 

 published that the real value was still exceedingly uncertain. Professor 

 Tait, however, was the first to call attention to the fact that Eumford 

 actually gave data sufficient to afford a basis for computation of the 

 equivalent, but he made the resultant figure 940 foot-pounds, assuming 

 the horse-power at 33,000 foot-pounds per minute, and failing to note 

 the fact that the engineer's 'horse-power' is considerably larger than the 

 power of the average horse. 



Taking the generally accepted and fair mean value for the power of 



