xxvii.] GENERALISATION. 605 



ivory, glass, marble, quartz, meteoric stones, &c., but was 

 unable to detect the least difference. This conclusion is 

 also confirmed by the ultimate agreement of all the calcu- 

 lations of physical astronomy based upon it Whether 

 the mass of Jupiter be calculated from the motion of its 

 own satellites, from the effect upon the small planets, 

 Vesta, Juno, &c., or from the perturbation of Encke's 

 Comet, the results are closely accordant, showing that 

 precisely the same law of gravity applies to the most 

 different bodies which we can observe. The gravity of 

 a body, again, appears to be entirely independent of its 

 other physical conditions, being totally unaffected by 

 any alteration in the temperature, density, electric or 

 magnetic condition, or other physical properties of the 

 substance. 



One paradoxical result of the law of equal gravitation 

 is the theorem of Torricelli, to the effect that all liquids 

 of whatever density fall or flow with equal rapidity. If 

 there be two equal cisterns respectively filled with mer- 

 cury and water, the mercury, though thirteen times as 

 heavy, would flow from an aperture neither more rapidly 

 nor more slowly than the water, and the same would be 

 true of ether, alcohol, and other liquids, allowance being 

 made, however, for the resistance of the air, and the 

 differing viscosities of the liquids. 



In its exact equality and its perfect independence of 

 all circumstances, except mass and distance, the force of 

 gravity stands apart from all the other forces and pheno- 

 mena of nature, and has not yet been brought into any 

 relation with them except through the general principle 

 of the conservation of energy. Magnetic attraction, as 

 remarked by Newton, follows very different laws, de- 

 pending upon the chemical quality and molecular struc- 

 ture of each particular substance. 



We must remember that in saying " all matter gravi- 

 tates," we exclude from the term matter the basis of light- 

 undulations, which is immensely more extensive in amount, 

 and obeys in many respects the laws of mechanics. This 

 adamantine substance appears, so far as can be ascertained, 

 to be perfectly uniform in its properties when existing in 

 space unoccupied by matter. Light and heat are conveyed 

 by it with equal velocity in all directions, and in all parts 



