ON OPTICAL THEORIES, 261 
electro-magnetic effects become rapidly periodic they travel with the 
velocity of light, and the direction to which the change of property is 
related is in the wave front, at least for isotropic media. 
The rigidity or quasi-rigidity through which the medium has the 
power of propagating these transverse waves of small displacement may 
be given to it through other motions which are going on independently 
of the light. The free passage of the planets through space proves that 
it can have little if any viscosity or rigidity, though, according to the 
views of Professor Stokes, while behaving as a perfect fluid for all 
appreciable motions, it might conceivably be rigid for the very small 
displacements in a light-wave. Taking Sir W. Thomson’s estimate of 
_ the density of the ether as about 10-?? grammes per cubic centimetre, the 
rigidity required for the propagation of light would be about 10-1. The 
| rigidity of glass is about 2°5 x 10!!. While it might, then, be conceivable 
that the ether should have this very small rigidity and yet offer no 
appreciable resistance to the earth’s motion, it is difficult to reconcile 
_ this with its power of standing electric stress, and we are forced to con- 
@lude that the change implied in electric displacement is much more 
than a mechanical displacement of the molecules of a perfect fluid. A 
- guasi-rigidity might be conferred on the fluid by filling it with vortices, 
and it might thus become capable of conveying transverse waves and of 
standing electric stress. Electric and magnetic polarisation would then 
consist in definite arrangements of the vortex rings or filaments. Changes 
in these arrangements, or in some of the properties connected with them, 
on constitute electric and magnetic displacements, and possibly also 
ight. 
We should then have a complete electro-magnetic theory of optics, or 
‘Yather a complete theory of the ether embracing electro-magnetism and 
optics, but towards this theory our present knowledge has made only a 
small advance. 
Report of the Committee, consisting of Professors RAMSAY, TILDEN, 
_ MarsHati, and W. L. Goopwin (Secretary), appointed for the 
_ purpose of investigating certain Physical Constants of Solution, 
_ especially the Expansion of Saline Solutions. 
Your Committee have to report as follows : 
They have obtained apparatus for determining the rates of expansion 
of saline solutions from —20° C. to +60° C. 
They have devised experiments for determining the distribution of a 
weighed quantity of water between molecular weights of two salts, the 
three substances being placed in separate vessels in the same enclosed 
Space kept at a constant temperature. 
But further progress in either of these directions was interrupted by 
the continued illness of one of the Committee. 
Your Committee respectfully ask for reappointment. 
