432 Action of certain Metals, Sfc, on a Photographic Plate. 



and no doubt many other bodies have the property. Many of the 

 boxes in which photographic plates are packed are made of straw- 

 board, but as the action does not pass through glass, the plates are 

 but little or not at all acted on ; but if a plate be laid face upwards 

 in one of these boxes and left there for a week it will be very appre- 

 ciably affected. If a small piece of glass be laid on the plate, it 

 protects the film beneath, and shows clearly the amount of action 

 which has occurred. If a box of this kind be painted inside 

 with melted paraffin, this action does not take place. It happened 

 that a few months before making the above experiments others 

 were in progress in which black net was placed on a photographic 

 plate simply to show clearly whether the plate had been acted on, 

 and continually a reversed picture was obtained; this at the time 

 could not be accounted for, but now the experiment was made of 

 simply placing the black net on the photographic plate and leaving 

 it there for some days ; then on development a clear picture of the 

 net was produced. The action is due to some material in the black 

 dye, for white net does not act in the same way. 



The action of the vapour from a few liquids on a sensitive plate has 

 been tried. The plate was placed abdut half an inch above the liquid, 

 and a screen, with holes cut in it, was fastened against the plate. 

 Methylated spirit acted slightly on the plate ; pure alcohol and ether 

 had no action; benzene, coal-tar, crude wood spirit, linseed oil also, 

 had no action, but turpentine and oil of cloves produced a slight 

 amount of action. 



Such, in outline, is an account of the experiments which have 

 already been made on this subject. One point has led on to 

 another, and some of the results were so unexpected that the experi- 

 ments had to be repeated many times before full credence could be 

 given to them. On the present occasion it is desired to do little 

 more than record facts ; further experiments, it is hoped, may lead to 

 explanations not now evident. The supposition that all these active 

 substances, the metals as well as organic bodies, give off a vapour 

 capable of acting on a photographic plate, naturally suggests itself, 

 and that copal does give off a vapour which directly or indirectly is 

 active there can be no doubt. At the same time, it is at least 

 difficult to suppose that the activity of such a body as strawboard 

 should, after the treatment it has undergone, give off at ordinary 

 temperatures sufficient vapour to produce the effects described, and 

 the same applies to old dry wood, &c. Still more interest attaches 

 to the action of the metals ; do they emit a vapour so delicate in con- 

 stitution and in such a quantity that it can readily permeate celluloid 

 gelatine, &c, and produce a picture of the surface from whence it 

 came, or is it a form of energy (possibly what has been called dark 

 light) that these bod : es emit ? Zinc kept and polished in the dark 



