J. W. Gibbs— Electric Theory of Light 129 



that the limits within which this elementary viscosity (say) 

 varies, will increase with the degree of molecular agitation of 

 the liquid. On the basis of this postulate I then endeavored 

 to explain sedimentation kinetically, both in its relations to 

 temperature and the effect of precipitants. 



One point which antagonizes this hypothesis must not be 

 lost sight of : Two or more particles sufficiently near together 

 will tend to screen each other ; and receiving impact mainly on 

 their outer surfaces will be brought to permanent coherence so 

 long as the conditions of pronounced molecular agitation last. 

 This is actually observed in water suspensions at 100°, in solu- 

 tion-suspensions, in ether suspensions, etc. In these instances 

 there seems to be difficulty in preserving .the granular state 

 (Bull. 36, p. 38). 



To pass judgment on the validity of such explanation, it is 

 necessary to have in hand better statistics of the size of the 

 particles relatively to the water molecule, than are now avail- 

 able. Inasmuch as the particles in pure water are individ- 

 ualized and granular, it is apparantly at once permissible to 

 infer the size of the particles from the observed rates of subsi- 

 dence. But my observations show that the said rate decreases 

 in marked degree with the turbidity of the mixture. Hence 

 the known formulae for single particles are not rigorously 

 applicable, though it cannot be asserted whether the cause of 

 discrepancy is physical or mathematical in kind. It follows 

 that special deductions must be made for the subsidence of 

 stated groups of particles before an estimate of their mean size 

 can fairly be obtained. 



Phys. Lab., U. S. G. S., Washington, D. C. 



Art. XYI. — A Comparison of the Electric Theory of Light 

 and Sir William Thomson's Theory of a Quasi-labile Ether; 



by J. WlLLARD GlBBS. 



A remarkable paper by Sir William Thomson, in the No- 

 vember number of the Philosophical Magazine, has opened a 

 new vista in the possibilities of the theory of an elastic ether. 

 Since the general theory of elasticity gives three waves char- 

 acterized by different directions of displacement for a single 

 wave-plane, while the phenomena of optics show but two, the 

 first point in accomodating any theory to observation, is to 

 get rid (absolutely or sensibly) of the third wave. For this 

 end, it has been common to make the ether incompressible, or, 

 as it is sometimes expressed, to make the velocity of the third 

 wave infinite. The velocity of the wave of compression be- 

 comes in fact infinite as the compressibility vanishes. Of 



Am. Jour. Sci— Third Seribs, Vol. XXXVII, No. 218.— Feb., 1889. 

 9 



