Prof. Mincum's Experiments in Photoelectricity. 225 



tioned, to show that in neither state of an impulsion-cell is 

 there a want of contact, is as follows : — An impulsion-cell 

 in which both plates hang from the top of the cell, and in 

 which the liquid did not rise nearly to the level of the junc- 

 tion of either platinum wire with the plate, was made part of 

 the circuit of a galvanometer and a Daniell cell by means of 

 a key ; its poles were connected with the electrometer, and 

 before pressing down the key which put the cell into the 

 galvanometer circuit, it was made insensitive by an impulse. 

 On pressing down the key, the needle of the galvanometer 

 was deflected, and this could not have happened if either 

 contact were broken. In fact, the indication of the gal- 

 vanometer was the same whether the cell was in the sensitive 

 or in the insensitive state. 



In two or three cells in which the alcohol had partially 

 evaporated, leaving a portion of the sensitive plate above the 

 liquid, it was found that, the whole plate having been 

 originally positive, the portion in the vapour gave a negative 

 E.M.F., while the lower portion continued positive. 



Among the numerous liquids tried in these cells was 

 butyric acid, which is efficacious ; but after a few days it 

 acts on the tin plate and tends to destroy the sensitive 

 surface. A plate which had been kept in a cell containing 

 butyric acid for some days was removed into one containing 

 propylic alcohol. The effect was that the upper portion 

 of the plate exhibited impulsion-effects — being alternately 

 positive and negative with impulses — while the lower portion 

 remained positive. 



In acetone, nitrate of ainyl, and glycerine these plates are 

 also sensitive — as indeed in water also ; but the E.M.F. in 

 these liquids is less than in methyl alcohol. 



It was found also that if the cell contained only the vapour 

 of alcohol, an E.M.F. was generated by light, but less than 

 that in the liquid. In aldehyde no E.M.F. due to light was 

 observed. 



Hydroxyl has the effect, after about twenty-four hours, of 

 giving to the plate a yellowish colour, and also of changing 

 the sign of the E.M.F. which exists in the dark. 



It has been found many times that one effect of removing a 

 sensitive plate from an alcohol cell to a cell filled with hydroxyl 

 or other liquids and then replacing it in the alcohol," was to 

 develop impulsion results ; and this fact shows that these 

 results cannot be due to any induced electrification on the 

 glass of the cell and an action of light on this electrification 

 — such an action as has been the subject of recent experiment 

 by a Russian physicist. 



