ABSTRACT
The investigation of curious anomalies thrown up during his exacting determination of the atomic weight of thallium between about 1862 and 1870 led Crookes and his apprentice Charles Gimingham to the construction of the eye-catching and puzzling light-mill or radiometer. As shown in Chapter 7, he first demonstrated this to the scientific public, namely the Royal Society, in April 1875. Although he adopted an empirical tone, refusing to speculate on the mechanism of the repulsive effect he was investigating, by March 1875 it was clear to contemporaries that Crookes believed that incipient radiation had a direct mechanical effect on delicately balanced surfaces. This explanation had to be abandoned when he found that, far from a white surface showing greater repulsion than a black surface because of greater reflection of light from the surface, it was the black surface that was more energetically repelled. This discovery forced him to modify his mechanical view of the effect of radiation.
