25G THE PRINCIPLES OF SCIENCE. 



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 

 or weight of a body, again, appears to be entirely inde- 

 pendent of its other physical conditions, being totally 

 unaffected by any alteration in the temperature, density, 

 electric or magnetic condition, or other physical proper- 

 ties of the substance. 



One almost 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 mercury 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, or any other liquids, 

 allowance being made for the resistance of the air, and the 

 differing viscosities of the liquids. 



In its exact equality and its perfect independence of 

 every circumstance, 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 a wholly different law as 

 depending 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 almost infinitely more extensive 

 in amount, and obeys in many other respects the laws of 

 mechanics. This adamantine basis of undulations appears, 

 so far as can be ascertained, to be perfectly uniform in its 

 properties when existing in space unoccupied by matter. 



