5C2 THE POPULAR SCIENCE MONTHLY. 



cal resistance of metals would disappear ; cooled to the tempera- 

 ture of liquid oxygen, the red oxide of mercury becomes yellow, 

 and both sulphur and bichromate of potash turn white. 



Surprising as are the figures which denote the molecular mo- 

 tion due to the temperature of water, more surprising still are the 

 computations which declare the chemical energy in the gases 

 which unite to form water. Measuring the heat liberated in their 

 union, it is found that the molecules of hydrogen and oxygen 

 possess as chemic motion energy equal to lifting the masses in- 

 volved some eleven hundred miles from the surface of the earth. 

 It is imagined that the molecular motions representing tempera- 

 ture, chemical affinity, electrification, or other energy, coexist 

 without confusion, just as air sustains, in perfect order, the super- 

 posed harmonies of an orchestra and chorus. The extremely 

 rapid motion of molecules, acting at their comparatively vast sur- 

 faces, must immensely exalt forces which, between masses, are 

 but feeble. A simple model may help to make this clear. Let 

 two cylindrical wheels, similar in all respects and covered with 

 rubber, be brought into contact on a floor they will in a slight 

 degree adhere ; let the wheels be swiftly turned in a direction 

 toward each other, and they will press each other with consider- 

 able force force proportionate to their speed, which force at 

 high speed will exalt their weak adhesion to somewhat of the in- 

 tensity of cohesion as manifested between molecules. The model 

 can illustrate something more : as a unit it does not change its 

 place, albeit that its halves are in rapid motion ; were its dimen- 

 sions too small for microscopic view, the motion of its parts would 

 be undetected, and, because the motions would balance each other, 

 a mass built up of such pairs of molecules would be in seeming 

 rest. 



While the chemists are busy disentangling the orbits in which 

 swim the atoms and molecules of the laboratory, the physicists 

 are equally active in endeavoring to reduce to mechanics the 

 various phases of energy. Here the first and decisive step was 

 taken when the revelations of form and color to the eye were ex- 

 plained as borne by ethereal waves, which follow one another at 

 a rate so prodigious as to yield the impression of rest ; which ex- 

 planation, indeed, had long been suggested in the phenomena of 

 sound where air-waves are palpably concerned. The notable 

 points of agreement in both spheres of action are that a medium 

 can transfer motion as perfectly as if the two bodies connected by 

 it were in immediate contact ; moreover, that the efficiency of the 

 medium increases as its density diminishes ; and that the me- 

 dium itself exacts no toll whatever, relapsing when its work is 

 done into the seeming rest from which its task awakened it. 

 With apparatus acoustic in model, the late Dr. Hertz, of Bonn, 



