14 THE PROGRESS AND SPIRIT OF 



in proving that the specific heats of certain substances, com- 

 pound as well as simple, when multiplied by their chemical 

 equivalents, give a constant quantity as the product. And 

 again, the experiments recently made by Mr. Joule and 

 Professor W. Thomson, on the mutual convertibility of heat 

 and dynamical force, demonstrate the remarkable fact that, 

 in whatever way mechanical force is employed to produce 

 heat, the same amount of heat is produced by the same 

 amount of force. We doubt not that the progress of science 

 will so multiply the number of these instances of quantitative 

 relation and equivalents of power, as ultimately to submit 

 them to some general law, as well as to practical application 

 in various forms. The very phrases, if duly weighed, show 

 that approach to mathematical proof, which is the surest 

 test of truth. Where we can bring numbers to verify results 

 obtained, and to predict results sought for, we may be 

 certain that we are treading on safe ground. 



It will be noticed that we have not hitherto spoken of 

 Gravitation as a physical force ; though it is the one with 

 which we are most familiar in every incident of life, and to 

 which we look as the most universal agent upon matter, 

 as well on the globe we inhabit, as in the innumerable 

 worlds surrounding us in space. We place it apart from 

 other physical forces, because, while thus familiar to our 

 senses in its effects, it is to our deeper meditation the most 

 mysterious as well as vast and sublime of the powers which 

 act in the universe. Human genius has discovered and 

 mathematically defined its laws. By knowledge of these 

 laws, human science has been carried, and is ever penetrating 

 farther, beyond our own planetary system ; while within this 

 system, they have enabled us to predict events in time and 

 space, and to define physical conditions of the planets and 

 their satellites, seemingly unapproachable by man. With 

 all this knowledge and perpetual application of the power, 



