104 Dr. W. J. Russell. On the A ction exerted by cwtain 



glass painted with it or with picture copal, is placed on the bottom 

 of an ordinary plate box and a photographic plate larger than the 

 active body is suspended above it with the film upwards ; light is 

 excluded from the box, and the arrangement is left for, say, a fort- 

 night, then the plate when developed will be found to have been 

 acted on irregularly round its edge, at some parts considerably more 

 than' at others, but everywhere shading off and evidently in the way 

 which would occur if a vapour had rolled round the edge. Another 

 experiment which showed this kind of action very satisfactorily was 

 carried out as follows: A circular piece of the Bristol board was 

 saturated with drying oil, and at a little distance above it a smaller 

 circle of mica, which is perfectly opaque to the action, was placed, 

 and again above this was another piece of mica with a circular hole 

 smaller than the circular mica plate, and then the photographic 

 -plate was placed above. By this arrangement no direct action could 

 take place between the drying oil and the sensitive plate, but a 

 vapour could work its way between the mica plates and thus reach 

 the photographic plate; and this it did, for after an exposure of 

 three days, on developing the plate there was a dark ring formed 

 shading off towards the centre. Another and very simple experi- 

 ment illustrating this same point is to place a small circular glass 

 dish, with some drying oil in it, in the middle of a photographic plate, 

 and leave it there for a week. On developing the plate it will be 

 found that no action has taken place where the dish stood, but that 

 immediately beyond the outside of the dish much action has occurred, 

 and that the darkening gradually fades away. There is still another 

 way in which the action of these organic bodies has been tested, and 

 that is by transferring the active power of these bodies to a neutral 

 substance. If vapour be the immediate cause of the darkening of 

 the photographic plate, then it would be possible, if a piece of Bristol 

 board were suspended above drying oil, for instance, for the inactive 

 board to take up those vapours a,nd become photographically active. 

 This was found to take place. Bristol board of good quality is a 

 very useful substance in all these experiments, both as a screen and 

 as an absorbent. It is in itself an inactive body, and may be heated 

 in a water bath before using, to prevent the accidental presence of 

 any substance which might act on the plate. If a piece of the 

 Bristol board be suspended above drying oil, in the liquid or solid 

 state, or turpentine or picture copal for two or three days, or even 

 less, it becomes strongly active, and when placed in contact with a 

 photographic plate quickly darkens it. This action of the Bristol 

 board is well shown if a pattern be stamped upon it, which is easily 

 done by pressing against it a piece of white net (black net must not 

 be used as it is slightly active), then the charged Bristol board will 

 give an unmistakable picture. If turpentine be the active sub- 



