1899-1900.] Lord Kelvin on the Motion in an Elastic Solid. 235 
a larger value than 100 in (11), we could readily fit the formula 
to give, in an assemblage in which ’6 x 10~ 3 of the whole space 
is occupied by the atom, exactly the refractive index of oxygen, 
nitrogen, or argon, or any other gas. It is remarkable that 
according to the particular assumptions specified in § 5, a density 
of ether in the centre of the atom considerably greater than 100 
times the density of undisturbed ether is required to make the 
refractivity as great as that of oxygen. There is, however, no 
difficulty in admitting so great a condensation of ether by the 
atom, if we are to regard our present problem as the basis of a 
physical hypothesis worthy of consideration. 
§ 18. There is, however, one serious, perhaps insuperable, 
difficulty to which I must refer in conclusion : the reconciliation 
of our hypothesis with the result that ether in the earth’s 
atmosphere is motionless relatively to the earth, seemingly proved 
by an admirable experiment designed by Michelson, and carried 
out with most searching care to secure a trustworthy result, by 
himself and Morley.* I cannot see any flaw either in the idea or 
in the execution of this experiment. But a possibility of escaping 
from the conclusion which it seemed to prove may be found in a 
brilliant suggestion made independently by Titzerald f and by 
Lorentz,^ of Leyden, to the effect that the motion of ether 
through matter may slightly alter its linear dimensions ; according 
to which, if the stone slab constituting the sole plate of Michelson 
and Morley’s apparatus has, in virtue of its motion through space 
occupied by ether, its lineal dimensions shortened one one-hundred- 
millionth § in the direction of motion, the result of the experiment 
would not disprove the free motion of ether through space occupied 
by the earth. 
* Phil. Mag., December 1887. 
t Public Lectures in Trinity College, Dublin. 
X Versuch einer Theorie der eledrischen und optischen Erscheinungen in 
bewegten Korpen. Leiden, 1895. 
§ This being the square of the ratio of the earth’s velocity round the sun 
(30 kilometres per sec.) to the velocity of light (300,000 kilometres per sec.). 
