500 THE POPULAR SCIENCE MONTHLY. 



similarly explained as kinetic. If we swiftly turn a gyroscopic 

 wheel we can only change its plane of rotation by expending force, 

 which force is repaid when the metal is allowed to resume its origi- 

 nal plane of motion. It is imagined that in like manner the par- 

 ticles in an elastic spring move swiftly in a definite plane ; if de- 

 flected from that plane they oppose resistance and stand ready to 

 do work in returning thereto. Of kindred to the kinetic theory of 

 elasticity is the modern explanation that heat consists in a dis- 

 tinct and ceaseless molecular motion, on which motion, indeed, 

 depend the dimensions of masses. Take a cube of lead or iron 

 from summer air into an ice-house and at once the proportions of 

 the mass begin to shrink. And the molecules themselves, whether 

 of lead, iron, or other element, are imagined by Helmholtz as vor- 

 texes born of the ether in which without resistance they forever 

 whirl. As observation proves in the case of a rapidly rotated 

 chain, substantial rigidity can be conferred by motion sufficiently 

 swift. Nor are molecules without something of individuality. 

 We are wont to think of masses of solid iron as precisely similar, 

 but experience proves that one bar or shaft of iron varies from 

 another by all that has differenced the past history of the two. A 

 careful workman uses his die of strongest steel for only a short 

 term of service, well assured that the metal, despite its seeming 

 wholeness, suft'ers serious internal shocks at every blow shocks 

 which, were no caution exercised, would soon reveal themselves 

 in fracture and ruined work. In phenomena of this type, which 

 every day confront the electrician and the engineer as well as the 

 mechanic, there seems a prophecy of the sensibility and memory 

 which dawn with organic life. 



In the broad field of wave energy the mechanical analogies 

 point to the sway of a single law of motion. If a pendulum is to 

 be maintained in its vibrations, it must receive impulses sympa- 

 thetically timed impulses related to its own period of ^wing as 

 decided by its length. In like manner a piano string vibrates 

 responsively to the note which, when struck, it sends forth ; and 

 a gas, such as sodium, when comparatively cool, intercepts in the 

 spectroscopic field precisely those waves which a glowing body of 

 similar gas in the flame of sun or star has radiated. A pane of 

 glass which transmits only the red rays of sunlight, when molten 

 emits the rays complementary to red, and glows as greenish blue. 

 Uncolored glass, which transmits light perfectly, conducts heat 

 badly, because' the vibrations are unlike; for the same reason 

 conductors of electricity the metals, for example are opaque. 

 The late Dr. Hertz, in generating electric waves intermediate in 

 amplitude between those of sound and light, discovered that 

 opaqueness is a relative term arm a wave with appropriate di- 

 mensions and it has a passport through any substance whatever ; 



