SCIENCE AND REVELATION. 49 



posed not to rest in the elasticity of gases as an ultimate property 

 beyond which we can not go, but to regard it as itself a consequence 

 of the molecular constitution of bodies, and of the motions and mutual 

 collisions of the ultimate molecules of a gas. Respecting tbe attraction 

 of gravitation we have not at present made a similar advance. Specu- 

 lations, indeed, have not been wanting on the part of those who have 

 endeavored to account for it. But none of these so fits into the 

 known phenomena of Nature as to carry with it a conviction of its 

 truth. Yet there is one indication that though we can not at present 

 explain the cause of gravitation, yet it may be explicable by what are 

 called second causes. The mass of a body is measured by its inertia ; 

 and, though we commonly think of a body of large mass as being 

 heavy, and though we compare the masses of two bodies most easily 

 and accurately through the intervention of weight, yet the idea of 

 mass may be acquired, and means might easily be suggested by which 

 the ratio of the masses of two bodies might be experimentally deter- 

 mined, w^ithout having recourse to gravitation at all. Now, according 

 to the law of gravitation, the force with which a given body attracts 

 another at a given distance is strictly proportional to the mass of the 

 latter. If we suppose the attracting body to be the earth, and the 

 attracted bodies to be in one case a brass weight, and in the other a 

 piece of marble, it follows that if they make equilibrium when placed in 

 the pans of a true balance — I make abstraction of the effect of the buoy- 

 ancy of the air — their masses are strictly equal, and, accordingly, that 

 weight is a true measure of mass. But there is no reason a priori, so 

 far as with our present knowledge we can see, why this should be so. 

 We know that if the bodies in the scale-pans were formed, one of brass 

 and the other of iron, and there were a magnet concealed under the 

 table on which the operator placed his balance, the masses would not 

 be equal when there was equilibrium. But that the law is true, and 

 that, accordingly, weight is a true measure of mass, follows with the 

 highest probability from the third of Kepler's laws, and was proved 

 experimentally by Newton, by experiments with pendulums. New- 

 ton's experiment has since been repeated by Bessel, with all the refine- 

 ments of modern appliances, with the result that, so far as the most 

 exact experiments enable us to decide, the law is strictly true. This 

 is perhaps the only instance, as Sir William Thomson remarked to me 

 in conversation, in which there is an exact agreement between two 

 quantities, and yet we are unable to give any reason why they should 

 agree. That such is the case, holds out some prospect of scientific 

 men being able some day to explain gravitation itself — that is, to 

 explain it as the result of some still higher law. 



Such is the nature of our progress in scientific investigation. We 

 collect facts ; we endeavor to co-ordinate them and ascertain the laws 

 w^hich bind them together ; we endeavor to refer these laws to their 

 proximate causes, and to proceed step by step upward in the chain of 



VOL. XXXII. — i 



