January 2, 1902] 



NA TURE 



201 



This and all the following pictures, when no statement to 

 the contrary is made, have been obtained by an action 

 of the paper on the photoj,'raphic plate for eighteen hours 

 at a temperature of 55 C. Fig. 2 shows that absolute 

 contact between the active paper and the photographic 

 plate is not necessary, for in this case a thick copper 



bright zinc plate, or a piece of Bristol board, or a glass 

 plate, that have been painted over with picture copal. 

 Fig. 4 is a picture of the paper of the Times produced 

 by placing it on a photographic plate and a charged slab 

 behind it for four minutes. Fig. 5 is the picture of a 

 writing paper 150 years old. The exposure to the hydro- 



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Fig. 3. 



screen with a hole in the centre was interposed between 

 the paper and the plate. This action of certain papers is 

 shown in an interesting way with many cardboards. 

 The following experiment. Fig. 3, was made with a 

 photographic mount. A small piece was cut off and on 

 it a long cut was made with a sharp knife passing through 

 the external paper and into the substance of the card- 

 board. The dark circular patch was where a cork borer 

 had been pressed about half way through the cardboard 

 and the upper part removed. The black ring is a cut 

 made by the same cork borer, but nothing removed. 

 The two dots are two pin holes. Thus it seems as if an 

 active vapour arising from the interior of the cardboard 

 collected in these cavities and acted on the plate. A 

 piece of cork acts in the same way ; all the holes give 



black pictures. In this figure the strong action of the 

 recently cut edge is very striking. 



Turning now to the case in which the action does not 

 arise from the paper itself, but from hydrogen peroxide 

 purposely placed behind the paper, we can obtain, again, 

 some interesting results. The best way of applying the 

 hydrogen peroxide is by means of a slab of plaster of 

 Paris. This should be cast on a plate of glass and be 

 about a quarter of an inch or slightly more in thickness. 

 After it has been allowed to dry it may be painted over 

 or dipped into a strong or weak solution of the peroxide 

 of hydrogen. The slab is again allowed to dry. It will 

 increase in activity for the first two days and after then 

 gradually decrease, and if a 15 per cent, solution has 

 been used it will be about ten days before it has lost its 

 power of acting on a photographic plate. Other sources 

 of hydrogen peroxide that are convenient to use are a 



NO. 1679, VOL. 65] 



gen peroxide in this case was eight minutes, as the piper 

 was thicker and less easily permeated by the peroxide. 

 In the same way it is easy to obtain a good picture of the 

 water-mark on a paper. 



This application of the hydrogen peroxide to the back 



of a paper shows an interesting change which paper 

 undergoes on being wetted. Take a paper which is- 

 easily permeated by hydrogen peroxide, wet it thoroughly 

 in water, then hang it up at the ordinary temperature untii 



tctc^anJIPurific 



Cleans 

 Swe«t 



perfectly dry, and it will be found to be quite opaque to 

 the hydrogen peroxide given off from a zinc plate or 

 from copal or turpentine. This opacity of the paper, 

 however, gradually passes off, and after two to three days 



