1891.] loith thio^carh amides. 67 



Farther work with these curious salts and especially with a com- 

 pound salt of thio- carbamide and ammonium bromide, discovered by 

 Prof. J. E. Reynolds in 1868 and called by him tetrathiocarbamid- 

 ammonium bromide, which was found to be exceedingly active in pro- 

 ducing perfect reversals of the image with very short exposures, led 

 me to the belief that this complete change of deposit from the lights 

 to the shadows of the photographic image must be more or less due to 

 electro-chemical action. The subject of electro-chemistry is one of 

 which I have little knowledge, but I have been able to make some simple 

 experiments from which, though not conclusive, it seems probable that 

 my surmise is correct, and so far as they go, they seem to establish that 

 not only, as former observations by Lermontoff, Eder and Abney had 

 shewn to be probable, is the ordinary process of photographic development 

 of sensitive surfaces containing silver haloids, accompanied by electrical 

 action, but that the addition of these minute quantities of thio-car- 

 bamides to the developer, greatly increases the intensity of the electri- 

 cal action and produces a reversal of the current which should also 

 account for the reversal of deposit. 



With the aid of a very sensitive galvanometer, which has been 

 kindly lent me by the Rev. Er. Lafont, S. J., who also assisted me in 

 the experiment, it was found that when a pair of pure silver plates 

 coated with finely precipitated silver bromide, one of which had been 

 exposed to light and the other not, were connected to the galvanometer 

 so as to form a galvanic couple and immersed in the ordinary eikonogen 

 developer, the exposed plate formed the negative pole and the needle 

 was deflected to the left ; whilst in the developer containing a little 

 thio-sinamine, the exposed plate formed the positive pole and the needle 

 was deflected to the right. 



This experiment has been successfully repeated several times with 

 silver plates prepared in the same way and with other thio-carbamides, 

 also with silver plates bromised by dipping them in bromine water ; and 

 so far the occurrence of the reversal is well established. I propose to 

 repeat the experiment before you though I cannot be certain of success. 

 (The reversal was successfully shown with bromised plates, the image of the 

 needle and scale being projected on the wall.) 



I have also tried the same experiment with ordinary dry plates 

 rendered conductive in various ways, the best of which appears to be 

 gold leaf applied either on the face of the film or behind it. Gelatine 

 offers very great resistance to the current, and thoagh I have obtained 

 distinct evidence of currents in both directions, they are not always 

 observable, nor is it yet quite certain that they are caused by electrolytic 

 action within the gelatine film, and further investigation as to this 

 is necessar3\ 



