Prof. Minchin's Experiments in Photoelectricity. 231 



cool, for which about ten minutes will suffice. When the 

 plate has cooled, it can, apparently, be kept in the dark, un- 

 immersed in any liquid, for any length of time before being 

 put into the acetone cell. A plate was thus kept for sixteen 

 days, and then, on being placed in the acetone cell, it was as 

 sensitive as if it had been immersed immediately after forma- 

 tion. It is a marked peculiarity of the seleno-aluminium cell 

 that, immediately after it has been set up, it is wonderfully 

 rapid in its response to light, and that on the withdrawal of 

 the light the E.M.F. at once disappears ; but after a few 

 days it is much slower in both respects — particularly the 

 latter — while its sensitiveness as regards the magnitude of the 

 E.M.F. developed is unimpaired. 



The dispersion of the residual effect is produced by the 

 means before described for the tin cells, viz., connexion with 

 a Daniell cell, the sensitive plate being now, of course, con 

 nected with the Zn pole of the Daniell. 



No sensitive and insensitive states due to vibrations, 

 mechanical or electromagnetic, have, so far, been observed in 

 the seleno-aluminium cells. 



Sign of the E.M.F. due to Light. — Unlike the tin-foil plates 

 described, the sensitive plate in a seleno-aluminium cell is 

 strongly negative towards the insensitive plate when the cell 

 is exposed to light. 



Effects of Different Colours. — The seleno-aluminium cells 

 differ from all other photoelectric cells that I have constructed 

 in their great sensitiveness to all parts of the spectrum, the 

 maximum effect being produced in the yellow near the 

 borders of the green. 



No very accurate experiment has yet been made on this 

 subject, because the Thomson quadrant-electrometer at my 

 disposal happens to be out of order ; but with Clifton's form 

 of the instrument, in which great sensitiveness has been 

 aimed at rather than accuracy or constancy, the following 

 numbers represent the relative electromotive forces produced 

 by the spectrum of an albocarbon light formed by a bisulphide- 

 of-carbon prism : — 



Red 109 



Border of red and yellow . . 117 



Yellow 130 



First edge of green . . . . 113 



Last edge of green . . . . 101 



Middle of blue . . . = 104 



End of blue 102 



The E.M.F. of a Daniell was represented by 408, and the 



S2 



