Metals and other Bodies on a Photographic Plate. 107 



the above-mentioned organic bodies and the metals, the question 

 which naturally presents itself is, do they also give off a vapour 

 which directly or indirectly acts on the photographic plate ? The 

 following experiments show that such an action does probably occur. 

 Zinc which has been long exposed to the air is inactive. An expo- 

 sure out of doors for only three or four days diminishes very con- 

 siderably its activity, but covered up in doors after three weeks it 

 will still give a tolerably dark picture. If it has a bright but per- 

 fectly smooth surface it is active, but not strongly so ; rub the zinc 

 with coarse sand or emery paper, and it is then obtained in its state 

 of greatest activity ; the same applies to all the metals. If, when in 

 this condition, any of the active metals be placed in contact with a 

 photographic plate, a beautifully sharp picture of the scratched sur- 

 face is obtained. The great increase of the fresh metallic surface 

 produced by the rubbing may account for the increase of activity 

 which the scratching produces. If the zinc plate be raised only 

 slightly above the photographic plate, a sharp picture of the scratches 

 is still obtained, and of course as the distance is increased, so is the 

 indistinctness of the picture increased, and at last it fades into a 

 general cloudiness, and in this form the zinc plate can easily be made 

 to act through the distance of an inch or more. 



This action of the metals passes through the same media as do the 

 vapours from the organic bodies, and clear pictures can be obtained 

 through sheets of thin gelatin, &c. ; in fact what has already been 

 said with regard to the transmission of the activity of the organic 

 bodies applies to the metals ; gelatin, both thin and thick, allows the 

 action of the metal to pass through it ; celluloid and collodion do the 

 same, and so does gold-beaters skin and tracing paper. Reasoning, 

 then, from this strong analogy between the action of the organic 

 and the metallic bodies, it must be assumed that the above-mentioned 

 metals from a clean surface and at ordinary temperatures give off 

 vapour, and this vapour apparently acts when under the same circum- 

 stances in a like manner to the vapour given off by the drying oil. It 

 gives a clear picture of the metal surface from which it arose, and it 

 can permeate the same media as the organic vapours. The remark- 

 ably clear pictures of, for instance, a zinc surface, which can be 

 produced through a sheet, or even several sheets, of the thin gelatin 

 proves that the action is not one of mere absorption. 



To gain further knowledge on this point and test the porosity of 

 these different media, the power of hydrogen to diffuse through them 

 was tried by cementing specimens of the different substances on 

 glass tubes, which were filled with hydrogen and placed over water. 

 The action is somewhat remarkable, but requires further confirma- 

 tion. With the thin gelatin ordinary diffusion does not occur, 

 and a hardly, if any, perceptible rise of the water in the tube 



