I 9 4 SPECULATIVE SCIENCE 



free-will made during a great many centuries, we fear the path 

 of observation will lead us no turther than we have already come. 

 We beg pardon for this little digression, which was really 

 necessary to the understanding of our author's physical theory. 

 Lucretius proceeds to state that atoms have always moved and 

 always will move with the same velocity, or, as translated by 

 Mr. Munro, ' The bodies of the first beginnings in time gone by 

 moved in the same way in which they now move, and will ever 

 hereafter be borne along in like manner, and the things which 

 have been wont to be begotten, will be begotten after the same 

 law,' for there is nothing ' extra,' nothing outside and beyond 

 the atoms which can either add to or take away from what we 

 should call the energy of the universe. This proposition fore- 

 shadows the doctrine of conservation of energy. It is coupled 

 with the assertion that the sum of matter was never denser or 

 rarer than it now is, a proposition which we may admit, in the 

 sense that the mean density of the universe is constant, but 

 the connection of this proposition with what may be called the 

 constancy of the total amount of motion in the universe escapes 

 us. But it is clear, in all his work, that Lucretius conceived 

 two things as quite constant : atoms were neither created nor 

 destroyed, and their motion could neither be created nor 

 destroyed. He believed that each atom kept its velocity un- 

 altered. The modern doctrine is that the total energy of the 

 'universe is constant, but may be variously distributed, and is 

 possibly due to motion alone ultimately, though this last point 

 has not been yet proved. Many a fierce battle has been waged 

 over the question, whether what was called the i quantity of 

 motion ' in the universe was constant. Newton, with perfect 

 accuracy, declared that it was not, defining the quantity of 

 motion in a body as the product of mass and velocity. Leibnitz 

 declared that it was constant, defining the quantity of motion 

 as the product of the mass and the square of its velocity, but 

 observing that when apparently the quantity of motion dimi- 

 nished, it was simply transferred to the molecules of the body, so 

 as to escape our observation as motion. Davy and Joule have 

 proved him right in some cases, and shown that our senses still 

 detect the motion as heat. It is conceivable, but not yet proved, 

 that Leibnitz may be right in all cases, and that \vha.t we call 



