1893.] Waterhouse — Electrical action of Light upon Silver. 21 



sensitive after the first exposure, but after a few exposures lost all 

 sensitiveness. The current being so small, it was not thought necessary 

 to experiment with coloured glass or the spectrum. With the direct- 

 ing magnet placed as before described the deflection was increased to 

 about 6' 5 divisions. 



Tap Water. 



The tap water used was the filtered Hooghly water, supplied in the 

 town mains. It is fairly pure and free from lime salts, but chlorides 

 are present in moderately large proportion, the amount of chlorine 

 varying from '5 to 1'4 parts per 100,000, and at the time of the 

 experiments it would be about 1 to 1*2 parts per 100,000. The total 

 hardness varies from 3'15 to 11*5 parts and would be about 9 parts per 

 100,000 at the time of the experiments. In most of the cases observed 

 the exposed plate was distinctly positive to the unexposed, as with 

 distilled water ; but in some cases it was negative, and in one or two 

 instances the action was irregular. The plates were rather more sen- 

 sitive than they were in distilled water, the normal deflections without 

 the directing magnet varying from 1 to 7 divisions of the scale, but 

 usually they were between 2 and 4. 



In one case in which the plates had been in the cell for about 38 

 hours, and there was only a very slight cell-current, exposure to sun- 

 shine gave a deflection of + 4*5 divisions without the magnet, but with 

 it the deflection in bright sunshine rose to + 20 divisions, and even in 

 diffused light was -+- 5 divisions. Exposing under ruby glass gave a 

 deflection of + '5 ; yellow glass + "7 ; green glass + 1 ; blue glass + "5 in 

 diffused light, and -+■ 7'5 in sunshine. Trials were also made with the 

 grating spectroscope without the directing magnet, but the results were 

 not conclusive and the unsettled weather has, so far, prevented their 

 being repeated with the galvanometer in its most sensitive state. The 

 plates were found very sensitive to changes in the strength of the light, 

 but after repeated exposure to sunshine they seeemed to lose sensi- 

 tiveness. By the action of the water a greyish deposit of chloride 

 was formed and in some cases a darkened image of the exposed part 

 of the plate could be seen. It may be noted that my experience does 

 not agree generally with that of Hankel, who found that, of two silver 

 plates immersed in water the plate exposed to white clouds, or to the 

 setting sun, was negative. I find, however, that on one occasion when 

 fresh plates were exposed to daylight, the exposed plate was negative, 

 the deflection being about — 1*5 divisions of the scale. On again exposing 

 the same plates to sunshine the exposed plate was positive, and remaiued 

 so afterwards on further exposure. On two other occasions of expo- 



