Metals and other Substances on a Photog?xip7iic Plate. 425 



activity of the uranium compound by the density of the picture of 

 the pattern formed, but in place of obtaining in all instances a 

 negative of the perforated zinc, i.e., the action occurring where the 

 plate was exposed, and none where covered by the zinc, the reverse 

 took place, and the greatest amount of action occurred underneath 

 the zinc. This happened over and over again, and even when the 

 experiment was varied in different ways, so that the only explanation 

 of the action was that the zinc itself must be able to effect a change 

 of the same kind as the uranium, at all events, to act on a photo- 

 graphic plate, and further experiment with zinc alone proved this to 

 be the case : later on it became known to me that R. Colson had 

 already described this action of zinc in a paper in the ' Comptes 

 Rendus ' in January last, and had also found that similar results 

 could be obtained with cadmium and with magnesium. He 

 explains this remarkable action as due to vapour given off by these 

 metals. 



Both before and after seeing the account of Colson's work a laro-e 

 number of experiments have been made with zinc under different 

 conditions, and there is no doubt of the ease and certainty with 

 which the results can be obtained. The zinc, as Colson states, must 

 be bright ; if well rubbed with coarse sand-paper it is most active : 

 probabty this may, to some extent, arise from increase of surface ; if 

 cleaned with acid or with caustic alkali it is not so active, and zinc in 

 its ordinary condition after exposure to the air ceases to be active. 

 The salts also have no power of acting in this way. A polished 

 piece of zinc laid on a highly sensitive photographic plate will, under 

 certain conditions even in four or five hours, so act on it that on 

 development a complete picture of the zinc is produced, showing 

 the scratches or any ruled lines or faint pattern drawn on it, 

 or if flaws in the metal exist they are clearly seen. A slight 

 pattern produced on zinc by pressing on it a piece of white net and 

 then rubbing it down with fine emery or sand-paper will give a 

 picture in which the pattern is very evident. In fact, such a pattern 

 forms a satisfactory test of this action of the zinc. Very slight altera- 

 tions of the surface are shown in the picture. Absolute contact of 

 metal and plate is not necessary. If screens of different thicknesses 

 of any inactive substance be interposed between plate and metal, 

 thus preventing contact, the action still occurs ; if the screen be 

 very thin, a picture of the zinc surface is still obtained, but if 

 thicker, only a dark cloudy patch is formed. Still further, if a 

 thick piece of glass tubing an inch long be placed on a photographic 

 plate, and the upper end covered with a piece of polished zinc, in a 

 week to a fortnight distinct action will be found to have taken place 

 below the zinc. Since the action thon is not one of mere contact, 

 the next point was to ascertain whether it would be transmitted 



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