the Ltiminiferons jEther. 347 



ment, and be content to learn from phaenomena the existence 

 of forces which we should not beforehand have expected. The 

 explanations whicli I had in view are those which belong to 

 the geometrical part of the theory; but the deduction^ from 

 dynamical calculations, of the laws which in the geometrical 

 theory take the place of observed facts must not be overlooked, 

 althougli here the evidence is of a much more complicated 

 character. 



The following illustration is advanced, not so much as ex- 

 plaining the real nature of the asther, as for the sake of offering 

 a plausible mode of conceiving how the apparently opposite 

 properties of solidity and fluidity which we must attribute to 

 the aether may be reconciled. 



Suppose a smali quantity of glue dissolved in a little water, 

 so as to form a stifFjelly. This jelly forms in fact an elastic 

 solid : it may be constrained, and it will resist constraint, and 

 return to its oriijinal form when the constraining force is re- 

 moved, by virtue of its elasticity ; but if we constrain it too 

 far it will break. Suppose now the quantity of water in which 

 the glue is dissolved to be doubled, trebled, and so on, till at 

 last we have a pint or a quart of glue water. The jelly will 

 thus become thinner and thinner, and the amount of con- 

 straining force which it can bear without being dislocated will 

 become less and less. 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 not 

 be dislocated, and therefore 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. The velocity of propagation of normal vibrations 

 in our medium would be nearly the same as that of sound in 

 water ; the velocity of propagation of transversal vibrations, 

 depending as it does on the tangential elasticity, would become 

 very small. Conceive now a medium liaving similar proper- 

 lies, but incomparably rarer than air, and we have a medium 

 such as we may conceive the asther to be, a fluiil as regards 

 the motion of the earth and planets through it, an elastic solid 

 as regards the small vibrations which constitute light. Per- 

 haps we should get nearer to the true nature of the aether by 

 conceiving a medium bearing the same relation to air that thin 

 jelly or glue water bears to pure water. The sluggish trans- 

 versal vibrations of our thin jelly are, in the case ot the aether, 

 replaced by vibrations projiagated with a velocity of nearly 



