6o8 



NATURE 



[April 28, 1898 



ifor instance — were exceedingly active bodies ; and it is 

 interesting to note that with some of them an effect on 

 'the sensitive plate, analogous to what photographers 

 iterm reversal or solarisation, is readily produced. With 

 ■excessive action, a white in place of a black picture is 

 obtained, but with modified action an ordinary dark 

 picture is formed. All the fragrant essential oils are 

 active bodies, and all contain as an ingredient one or 

 more of the terpines. Now a characteristic property 

 •common to all the above-named bodies, and to others 

 which produce similar effects on a photographic plate, is 

 4;hat they are reducing or oxygen absorbing bodies ; con- 

 sequently it is probable that it is to this property that 

 they owe their power of acting on a photographic plate. 

 Bodies such as alcohol, ether, esters, benzene, petroleum 

 spirit, &c., exert no such action. Linseed oil is the most 

 active of the vegetable oils, and has the greatest oxygen- 

 absorbing power, olive oil the least, and it can produce 

 little or no action on a photographic plate. An interesting 

 test as to certain impurities in inactive bodies, for 

 instance in alcohol and in ether, can be founded on these 

 reactions. Ordinary commercial samples of these bodies, 

 when placed in a dish with a photographic plate above 

 them, yield a picture ; but on carefully applying the 

 •ordinary processes for purifying these bodies, the pictures 

 produced become fainter and fainter, and at last entirely 

 ■disappear, so that not only the process of purification can 

 be rendered visible step by step, but its completion be 

 proved. One remarkably interesting character of these 

 actions is, that they can take place through thin layers 

 •of certain solid substances ; for instance, through gelatin, 

 •celluloid, collodion, gutta-percha tissue, gold-beater's 

 skin, tracing-paper, &c., and naturally the action per- 

 «ieates paper and other strongly porous bodies. In 

 so doing it gives on the sensitive plate a picture of 

 the structure of the body. That the passing through 

 a medium such as gelatin is not one of mere absorp- 

 ition on one side and evaporation on the other, is 

 proved by the formation, after having passed through 

 ithe medium, of a clear picture of the surface, say hard- 

 ened copal varnish, from which the action arose ; and 

 ■even when more than a single layer of gelatin is inter- 

 posed between the active body and the plate, still a clear 

 picture is produced. On the other hand, bodies such as 

 .glass, mica, and selenite are perfectly opaque to the action, 

 and gum arable and paraffin in thin layers do not allow 

 the action to pass through. Experiments were described 

 in the lecture to show that it is a vapour given off by the 

 active bodies which is the immediate cause of the action 

 on the photographic plate, not a fluorescence emanating 

 from the active body. A card, for instance, painted with 

 •drying oil or copal varnish, smaller than the photographic 

 plate, and placed below a sensitive plate with the film 

 upwards, produces an action round the edge of the plate, 

 which creeps slowly and unevenly towards the centre. An 

 arrangement was also described with a series of mica 

 plates, overlapping one another so as to cut off all direct 

 ivievv of the active body from the sensitive plate, but so 

 arranged as to allow a space between each layer, so that 

 a vapour could work its way from the source of the action 

 to the sensitive film. Such an arrangement enclosed in 

 a box produced a definite picture. 



It was naturally to be expected that an inactive sub- 

 stance, such, for instance, as a piece of Bristol board, 

 could be made active by placing it in contact with 

 hardened drying oil or copal varnish, or simply by placing 

 '\\. over linseed oil or turpentine ; and the Bristol board, 

 although no change in the surface is visible, will now 

 produce a definite picture. High temperatures cannot, of 

 •course, be used with photographic plates. Many experi- 

 ments have, however, been made at 55*^ C, and the action 

 at this temperature as compared to that at ordinary tem- 

 peratures show that a very great increase of activity takes 

 ,place with the increase of temperature. 



NO. 1487, VOL. 57] 



Experiments similar to those which have been indi- 

 cated as having been made with organic bodies, have 

 also been made with the different active metals, and 

 similar results obtained. Zinc is, perhaps, the most con- 

 venient metal to experiment with. That it possesses this 

 property of acting on a photographic plate was first 

 published by M. Colson, but unknown to the lecturer 

 until after his first set of experiments were made. The 

 following is a list of the metals which have been found to 

 be most active, and they are arranged approximately in 

 the order of their activity. Magnesium, cadmium, zinc, 

 nickel, aluminium, lead, bismuth ; then follows cobalt, 

 tin, antimony, which are decidedly less active than the 

 foregoing ones ; and there are others which with 

 very long exposure show some amount of activity. 

 Again certain alloys, such as pewter and fusible metal, 

 are active bodies, whereas ordinary brass and german 

 silver are not so. Increase the amount of zinc in brass, 

 and it becomes an active alloy. The conditions under 

 which this change occurs are being investigated. From 

 the first experiments which were made it was concluded 

 that mercury was an exceedingly active metal, but it has 

 since been proved that it is entirely inactive, and that the 

 action previously obtained was due to the metal being 

 alloyed with a trace of zinc. That an exceedingly small 

 amount of zinc is capable of effecting this change in 

 mercury, is an interesting and important fact. This 

 action can also be utilised as a test for the absence both 

 of zinc and lead (for this metal acts in the same way as 

 zinc) in a specimen of mercury, and pictures were shown 

 which exhibited the effect of several of the ordinary 

 purifying processes on impure mercury. 



In order to show that the action exerted by the metals 

 is due to a vapour, as before stated, experiments similar 

 to those made with the organic bodies, and other ones 

 described in the lecture, have been carried out. For a 

 metal surface to be active it must be bright ; a piece of 

 dull zinc, for instance, exerts no action on the photographic 

 plate, but rub it with coarse sand or emery paper, and you 

 get a surface which will give an exact picture of every 

 line that is visible on the plate, and even when a sheet of 

 gelatin or celluloid is interposed, still a picture of the 

 metal surface is formed. As a further confirmation of 

 the view that a vapour is given off by the metal, it was 

 shown that a slow current of air passed over bright zinc 

 and allowed to impinge on a photographic plate, acted on 

 it and produced a picture. Interesting pictures of opaque 

 bodies are readily produced by placing a plate of polished 

 zinc behind them, and in this way, for instance, the 

 structure of different papers and the water-marks they 

 bear, the form of skeleton-leaves, &c., are obtained as 

 pictures. The opacity which certain solutions give to 

 paper, and the transparency which others communicate 

 to it, is of much interest, and further experiments are 

 being made on this branch of the subject. 



Another curious action was mentioned, which is that 

 zinc and other metals have the power of making certain 

 inactive liquids active. If, for instance, alcohol or ether 

 or acetic ether be digested for four days with bright zinc, 

 it will become active and capable of producing a picture ; 

 filtration and even distillation does not restore the 

 inactivity of the solution. 



It appears then that many substances, both organic and 

 metallic, are able to act on a sensitive photographic plate, 

 and that exceedingly small quantities of these active 

 bodies are sufficient to produce the effect. A piece of 

 board laid on a sensitive plate will give a good picture of 

 its structure, and even thin dry board 30 to 100 years old 

 can be made to give its likeness. Dry cinnamon and 

 many other bodies act in the same way. Other experi- 

 ments were described showing how an accumulation of 

 the active vapour from zinc could be demonstrated, and, 

 further, how the vapour was reflected from the sides of a 

 glass or paraffin tube, but absorbed by a paper one. The 



