(4) The photo-electric dissipation showed by powdered fluorspar is dependent on the colour of the mineral, in such a way that the deepest blue, violet or green specimens are the most sensitive.
APPENDIX V.
PHOTO-ELECTRIC RESEARCHES OF
M. AUGUSTE RIGHI.[40]
M. Righi has observed the following facts: (1) That ultra-violet rays reduce to sensibly the same potential two metals placed near each other (plate and gauze parallel and close); (2) That several photo-electric couples of this kind can form a battery; (3) That a simple metallic plate charges itself positively under the influence of radiation; (4) That a voltaic arc formed with a zinc rod gives the strongest effect, while the sun gives none.
Besides these facts he finds:—
(a) That certain gases and vapours, such as coal-gas and CS₂, absorb the active rays strongly.
(b) That if the discharging body is easily movable it recedes like an electric windmill.
(c) A film of gypsum interposed between gauze and plate charges itself negatively on the side facing the negatively charged plate.
(d) Radiation produces its discharging effect even on non-conductors (ebonite and sulphur). With glass, resin and varnishes the action is feeble, or nearly nothing.
(e) If the experiment is made with a copper gauze and a zinc plate, the phenomenon nearly disappears on varnishing the gauze. His hypothesis is that radiation produces convection of negative electricity, the carriers being molecules of air.