32 



SCIENTIFIC THOUGHT. 



engaged. The solution of the latter problem involved 

 experiment as well as calculation. The different states 

 and properties of matter had to be studied from quite 

 novel points of view : they had to be defined in terms 

 of the different kinds of motion and of inertia, i.e., 

 resistance to motion or capacity for motion. The popular 

 conceptions of solidity, rigidity, fluidity, expansion, pres- 

 sure, weight, required to be translated into the language 

 of ordinary dynamics, that it might appear to what 



vocabulary of physical optics. He 

 has, however, whilst working inde- 

 pendently, been careful to point 

 out to what extent his views 

 agree with or are anticipated by 

 the important writings of Cauchy 

 and Poisson in France. Up to his 

 time the ether was universally 

 spoken of as a fluid. Stokes led 

 up to the " elastic solid " and the 

 " jelly " theory of the ether. " Un- 

 doubtedly," he says, "it does vio- 

 lence to the ideas that we should 

 have been likely to form a priori 

 of the nature of the ether to assert 

 that it must be regarded as an 

 elastic solid in treating of the 

 vibrations of light. When, how- 

 ever, we consider . . . the diffi- 

 culty of explaining these phenomena 

 by any vibrations due to the con- 

 densation and rarefaction of an 

 elastic fluid such as air, it seems 

 reasonable to suspend our judg- 

 ment and be content to learn from 

 phenomena the existence of forces 

 which we should not beforehand 

 have expected. . . . The following 

 illustration is advanced, not so 

 much as explaining the real nature 

 of the ether, as for the sake of 

 offering a plausible mode of con- 

 ceiving how the apparently opposite 

 properties of solidity and fluidity 

 which we must attribute to the 

 ether may be reconciled. Suppose 

 a small quantity of glue dissolved 



in a little water so as to form a 

 stiff jelly. This jelly forms, in fact, 

 an elastic solid : it may be con- 

 strained . . . and return to its 

 original form when the constraining 

 force is removed, by virtue of its 

 elasticity ; but if we constrain it 

 too far it will break. Suppose now 

 the quantity of water to be 'in- 

 creased' . . . till we have a pint 

 or a quart of glue-water. The jelly 

 will then become thinner. ... At 

 last it will become so far fluid as to 

 mend itself again as soon as it is 

 dislocated. Yet there seems hardly 

 sufficient reason for supposing that 

 at a certain stage of the dilution the 

 tangential force whereby it resists 

 constraint ceases all of a sudden. 

 In order that the medium . . . 

 should have to be treated as an 

 elastic solid, it is only necessary that 

 the amount of constraint should 

 be very small. The medium would, 

 however, be what we should call a 

 fluid as regards the motion of solid 

 bodies through it. ... Conceive 

 now a medium having similar pro- 

 perties, but incomparably rarer 

 than air, and we have a medium 

 such as we may conceive the ether 

 to be, a fluid as regards the motion 

 of the earth and planets through it, 

 an elastic solid as regards the small 

 vibrations which constitute light " 

 ( ' Papers,' vol. ii. p. 11 sqq. ) 



