318 Mr. stokes, ON THE FRICTION OF FLUIDS IN MOTION, 



elastic substances, as iron, copper, &c., are yet to a very sensible degree plastic. The plasticity of 

 lead is greater than that of iron or copper, and, as appears from experiment, its elasticity less. On 

 the whole it is probable that the greater the plasticity of a substance the less its elasticity, and 

 ince versa, although this rule is probably far from being without exception. When the plasticity 

 of the substance is still further increased, and its elasticity diminished, it passes into a viscous 

 fluid. There seems no line of demarcation between a solid and a viscous fluid. In fact, the 

 practical distinction between these two classes of bodies seems to depend on the intensity of 

 the extraneous force of gravity, compared with the intensity of the forces by which the parts 

 of the substance are held together. Thus, what on the Earth is a soft solid might, if carried 

 to the Sun, and retained at the same temperature, be a viscous fluid, the force of gravity at 

 the surface of the Sun being sufficient to make the substance spread out and become level at 

 the top : while what on the Earth is a viscous fluid might on the surface of Pallas be a soft solid. 

 The gradation of viscous, into what are called perfect fluids seems to present as little abruptness as 

 that of solids into viscous fluids ; and some experiments which have been made on the sudden 

 conversion of water and ether into vapour, when enclosed in strong vessels and exposed to high 

 temperatures, go towards breaking down the distinction between liquids and gases. 



According to the law of continuity, then, we should expect the property of elasticity to run 

 through the whole series, only, it may become insensible, or else may be masked by some other 

 more conspicuous property. It must be remembered that the elasticity here spoken of is that 

 which consists in the tangential force called into action by a displacement of continuous sliding: 

 the displacements also which will be spoken of in this paragraph must be understood of such 

 displacements as are independent of condensation or rarefaction. Now the distinguishing property 

 of fluids is the extreme mobility of their parts. According to the views explained in this article, 

 this mobility is merely an extremely great plasticity, so that a fluid admits of a finite, but 

 exceedingly small amount of constraint before it will be relieved from its state of tension by its 

 molecules assuming new positions of equilibrium. Consequently the same oblique pressures can be 

 called into action in a fluid as in a solid, provided the amount of relative displacement of the 

 parts be exceedingly small. All we know for certain is that the eff'ect of elasticity in fluids, 

 (elasticity of the second kind be it remembered,) is quite insensible in cases of equilibrium, and 

 it is probably inseiisible in all ordinary cases of fluid motion. Should it be otherwise, equations (8) 

 and (12) will not be true, or only approximately true. But a little consideration will show that 

 the property of elasticity may be quite insensible in ordinary cases of fluid motion, and tiiay yet 

 be that on which the phenomena of light entirely depend. When we find a vibrating string, 

 the small extent of vibration of which can be actually seen, filling a whole room with sound, 

 and remember how rapidly the intensity of the vibrations of the air must diminish as the distance 

 from the string increases, we may easily conceive how small in general must be the amount 

 of the relative motion of adjacent particles of air in the case of sound. Now the extent of 

 the vibration of the ether, in the case of light, may be as small compared with the length of a 

 wave of light as that of the air is compared with the length of a wave of sound : we have no 

 reason to suppose it otherwise. When we remember then that the length of a wave of sound in air 

 varies from some inches to several feet, while the greatest length of a wave of light is about . 00003 

 of an inch, it is easy to imagine that the relative displacement of the particles of ether may be so 

 small as not to reach, nor even come near to the greatest relative displacement which could exist 

 without the molecules of the medium assuming new positions of equilibrium, or, to keep clear of the 

 idea of molecules, without the medium assuming a new arrangement which might be permanent. 



It has been supposed by some that air, like the luminiferous ether, ought to admit of 

 transversal vibrations. According to the views of this article such would, mathematically speaking, 

 be the case ; but the extent of such vibrations would be necessarily so very small as to render 

 them utterly insensible, unless we had organs with a delicacy equal to that of the retina adapted 

 to ^receive them. 



