PHYSICAL FORCES. 



G7Z 



the same thing ; light is resolved simply into a 

 vibration, or an undulation of the particles of a 

 medium. Of course the rejection of an ether 

 implies the assumption of universal matter, 

 which is maintained by Prof. Groves, who de- 

 nies that there is any support for the notion of 

 a universal vacuum in the celestial spaces. 



But if heat and light are thus <?/<?, 



and science has undertaken to state what kinds 

 of motion they are, it becomes obvious that all 

 the allied forces are of a kindred nature ; and 

 the question arises, What are the several kinds 

 of motion which constitute these various 

 energies ? This is the grand dynamical prob- 

 lem of the future, which it will require the 

 cooperating scientific intellect of the world to 

 solve. Xevertheless year by year prog- 

 being made, and we may here call attention to 

 an able and ingenious attempt in this direc- 

 tion which has been recently put forth. Her- 

 bert Spencer has put the question, " What is the 

 particular mode of motion which constitutes 

 electricity ? " and favored us with his answer, 

 which is so important as to have commanded 

 extensive scientific approval. We condense his 

 views. 



That electricity is .some kind of molecular 

 vibration, different from the molecular vibra- 

 tions which luminous bodies give off, must be gen- 

 erally admitted. Beyond those oscillations of 

 molecules from which light and heat result, many 

 would suspect that there will, in some 

 ari?e compound oscillations. Let us consider 

 whether the conditions under which electricity 

 arises are not such as to generate compouiid 

 oscillations, and whether the phenomena of 

 k-ity are not such as to result from com- 

 pound oscillations. 



The universal antecedent to the production 

 of electricity is the contact of heterogeneous 

 substances. If, then, electricity is some mode 

 of molecular motion, and if, whenever it is pro- 

 duced, the contact of substances having unlike 

 molecules is the antecedent, there seems thrust 

 upon us the conclusion, that electricity results 

 from some mutual action of molecules, whose 

 motions are unlike. What must this mutua. 

 actioa be ? The answer to this question does 

 not seem difficult to reach, if we take the sim- 

 plest case the case of contact electricity. When 

 t\vo pieces of metal of the same kind and of the 

 same temperature are applied to one another, 

 there is no electrical excitation ; but if the met- 

 als applied to one another be of different kinds, 

 there is a genesis of electricity. This, which 

 has been regarded as an anomalous fact, a fact 

 so anomalous that it has been much disputed, 

 because apparently at variance with every hy- 

 pothesis, is a fact to which an interpretation 

 is at once supplied by the hypothesis, that elec- 

 tricity results from the mutual disturbance of 

 unlike molecular motions. For if, on the one 

 Land, we have homogeneous metals in contact, 

 their respective molecules oscillating simulta- 

 neously, will give and take any forces which 

 '.mpress on one another without producing 



an oscillation of a new order. But if, on the other 

 hand, the molecules of the one muss have periods 

 of oscillation different from those of the other 

 mass, then their mutual impacts will not agree 

 with the period of oscillation of either, but will 

 generate a new system, differing from and much 

 slower than that of either. The production of 

 what are called beats in acoustics will best illus- 

 trate this. It is a familiar fact that two strings, 

 vibrating at different rates, for a time concur ha 

 sending off aerial waves in the same direction 

 at the same instant ; that their vibrations get- 

 ting more and more out of correspondence, 

 they send off then* aerial waves hi the same di- 

 rection at exactly intermediate instants; and 

 presently, coming once more into correspond- 

 ence, they again generate coinciding wav 

 that, when their periods of vibration differ but 

 little, and when, consequently, it takes an ap- 

 preciable tune to complete their alternations of 

 agreement and disagreement, there results an 

 audible alternation in the sound a succession 

 of pulses of louder and feebler sound. In other 

 words, besides the primary simple and rapid 

 series of waves constituting the two sounds 

 themselves, there is a series of compound 

 waves resulting from their repeated conflicts 

 and concurrent 



Xow. if instead of the two strings communi- 

 cating their vibrations to the air, each commu- 

 nicated its vibrations to the other, we should 

 have just the same alternation of concurrent 

 and conflicting pulses. And if each of the two 

 strings was combined with an aggregate of 

 others like itself, in such a way that it com- 

 municated to its neighbors both its normal and 

 its abnormal vibrations, it is clear that through 

 each aggregate of strings there would be prop- 

 agated one of these compound masses of oscil- 

 lations, in addition to this simple rapid oscilla- 

 tion. 



This illustration will make it manifest, that 

 where a mass of molecules, which have a cer- 

 tain period of vibration, is placed in contact 

 with a mass of molecules which have another 

 period of vibration, there must result an alter- 

 nation of coincidences and antagonisms in the 

 molecular motions, such as will make the mole- 

 cules alternately increase and decrease one 

 another's motions. There will be instants at 

 which they are moving in the same direction, and 

 intervening instants at which they are moving in 

 opposite directions ; whence will arise periods 

 of greatest and least deviations from their or- 

 dinary motions. And these greatest and least 

 deviations being communicated to neighboring 

 molecules, and passed on by them to the next, 

 will result hi waves of perturbation propagated 

 throughout each mass. 



Let us now ask what will be the mutual re- 

 lations of three waves? Action and reaction 

 being equal and opposite, it must happen that 

 whatever effect a molecule of the mass A pro- 

 duces upon an adjacent molecule of the mass B, 

 must be accompanied by an equivalent reverse 

 effect u.;>on itself. If a molecule of the mass A 



