SCIENCE IN THE LAST HALF CENTURY. 77 



larities and apparent dysteleologies in nature to the disobedience, or 

 sluggish yielding, of matter to the shaping and guiding influence of 

 those reasons and causes which were hypostatized in his ideal " Forms." 

 In modern science, the conception of the inertia, or resistance to change, 

 of matter is complex. In part, it contains a corollary from the law of 

 causation : A body can not change its state in respect of rest or motion 

 without a sufficient cause. But, in part, it contains generalizations from 

 experience. One of these is that there is no such cause resident in any 

 body, and that therefore it will rest or continue in motion so long as no 

 external cause of change acts upon it. The other is that the effect which 

 the impact of a body in motion produces upon the body on which it im- 

 pinges depends, other things being alike, on the relation of a certain 

 quality of each which is called "mass." Given a cause of motion of a 

 certain value, the amount oi' motio^i, measured by distance travelled in 

 a certain time, which it will produce in a given quantity of matter, say 

 a cubic inch, is not always the same, but dei)ends on what that matter 

 is ; — a cubic inch of iron will go faster than a cubic inch of gold. Hence, 

 it appears, that since equal amounts of motion have, ex hypothesis been 

 produced, the amount of motion in a body does not depend on its speed 

 alone, but on some property of the body. To this the name of " mass " 

 has been given. And since it seems reasonable to suppose that a large 

 quantity of matter, moving slowly, possesses as much motion as a small 

 quantity moving faster, " mass" has been held to express '• quantity of 

 matter." It is further demonstrable that, at any given tinie and place, 

 the relative mass of any two bodies is expressed by the ratio of their 

 weights. 



When all these great truths respecting molar motion, ur the move- 

 ments of visible and tangible masses, had been shown to hold good not 

 only of terrestrial bodies, but of all those which coiistitute the visible 

 universe, and the movements of the macrocosm had thus been expressed 

 by a general mechanical theory, there remained a vast number of phe- 

 nomena, such as those of light, heat, electricity, magnetism, and those 

 of the physical and chemical changes, which do not involve molar mo- 

 tion. iSTewton's corpuscular theory of light was an attempt to deal with 

 one great series of these phenomena on mechanical principles, and it 

 maintained its ground until, at the beginning of the nineteenth century, 

 the undulatory theory proved itself to bo a much better working hy- 

 pothesis. Heat, up to that time, and indeed much later, was regarded 

 as an imponderable substance, caloric ; as a thing which was absorbed 

 by bodies when they were warmed, and was given out as they cooled 5 

 and which, moreover, was cai)able of entering into a sort of chemical com- 

 bination with them, and so becoming latent. Rumford and Davy had 

 given a great blow to this view of heat by proving that the quantity of 

 heat which two portions of the same body could be made to give out, by 

 rubbing them together, was practically illimitable. This result brought 

 philosophers face to face with the contradiction of supposing that a 



