thus arrived at what may be called the historical problem, as to the cause of gravitation.
From Kepler's discoveries of the relation between the distances and periodic times of the planets, Newton was able to show that the law of gravitation is as the inverse square of the distance. But this in no way explained the cause of this law. And in spite of all attempts in the mean time, up to the year 1885 no clue had been found.
In that year, however, as the result of an attempt to discover what properties a medium possesses, that it might fulfil the functions of the ether, including gravitation, the property of all granular matter in close piling to expand under strain was recognized, I believe for the first time.
This property was at once recognized as an obvious clue to the cause of gravitation, however intractable it might prove.
The experiments made in 1885 will serve to illustrate the cause of gravitation, besides affording me the gratification, I cannot but feel, in first exposing the verification of my conviction in this house and before this audience.
Hitherto in this lecture it has been found necessary to refer to granular medium as con-
