100 Dr. W. J. Russell. On the Action exerted by certain 



cracking which occurs can be very well shown on a photographic 

 plate. 



Pure water does not act on the plate, neither does pure alcohol or 

 pure ether, but the ordinary commercial specimens of the last two 

 bodies do, and often to a considerable extent. Alcohol which, pro- 

 duces a dark picture will, after digestion with, lime and careful dis- 

 tillation, be entirely inactive, and ether after careful purification also 

 becomes inactive. Moisture, if present, does not affect this result ; 

 thus the presence of certain impurities, and they appear to be some 

 of the most common ones, can be readily photographed, and approxi- 

 mately their amount determined by the darkness of the picture formed, 

 so that by this means can be determined whether, for instance, a 

 purification process is working well and whether it has completely 

 done its work. The pictures are easily obtained by placing some of 

 the liquid to be tested in a small glass dish, and a sensitive plate 

 above it. Obviously it is only certain impurities which will be indi- 

 cated in this way, but the reaction is otherwise of considerable 

 importance, for it gives a simple method of determining what bodies 

 soluble in these liquids, are capable of acting on a sensitive plate. 

 This matter will be treated of in a separate communication. 



That the vapour given off by these organic bodies is the imme- 

 diate cause of the action on the sensitive plate the above experiments 

 seem to show. At the same time it is remarkable that such a vapour 

 should readily pass through media such as gelatin, celluloid, &c., and 

 not by mere absorption, but in such a way as to produce a picture of 

 the surface from which it emanated. 



Passing on to the action which certain metals exert on a photo- 

 graphic plate, results have been obtained which are strikingly similar 

 to those just described. Substitute a piece of polished zinc for a piece 

 of Bristol board saturated with linseed oil, and similar effects are 

 produced on a photographic plate ; the time of exposure must, how- 

 ever, be longer. Although both magnesium and cadmium are 

 slightly more active than zinc, this last metal is the most convenient 

 one to experiment with, and has been used in most of the following 

 experiments. In addition to the above three metals, nickel, alumi- 

 nium, lead, and bismuth have the same property, but not so strongly 

 developed, of acting, both when in contact and when at a distance, on 

 a photographic plate. Cobalt, tin, and antimony can also act in the 

 same way, but their action is considerably feebler, and undoubtedly 

 other metals can act in the same way, but require much longer 

 exposure. Mercury, which in the former paper was stated to be the 

 most active of all the metals which had been tried, is now found to be 

 quite inactive; the metal used in the former experiments was impure. 

 This matter will be explained further on. 



As so strong a similarity exists between the effects produced by 



