Jan., 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



II 



Another method, and the most usual, is to prepare a 

 plaster cast and render this impervious to water by 

 impregnating it with melted paraffin, a£ter which the 

 surface is coated with graphite to make it able to con- 

 duct the electric current. Sometimes instead of 



graphitising, it is coated with a thin film of silver by 

 chemical means. After having been rendered conduct- 

 ing, the cast is made the cathode in a plating bath and 

 metal deposited as already described. When a suffi- 



ciently thick deposit has been produced the cast is taken 

 out of the bath, the deposited metal removed and 

 backed up as already described. The deposited metal 

 gives a faithful reproduction of the original medal. 

 Fig. 4 was reproduced from a plaster cast. 



Instead of using plaster to make the cast, guii.i- 

 percha or mixtures of gutta-percha and other substances 

 are often employed. Fig. 5 is rather interesting. It was 

 made from a gutta impression. Happening to be 

 short of gutta, my assistant, Mr. \V. C. Prebble, to 

 whom my thanks are due for preparing the medallions 

 illustrated in this article, produced two golf balls, and 

 the gutta from the interior of these was employed for 

 making the matrix. The gutta was first kneaded in 

 hot water to render it plastic, and then carefully worked 

 on to the medal, after which it was pressed in a letter- 

 press; hydraulic presses are often used on a commercial 

 scale. The matrix so obtained was made con- 

 ducting with finely-powdered graphite and was then 

 placed in the depositing bath. This medal contained a 

 great amount of detail, and I think it shows how ex- 

 tremely useful a golf ball may be on occasion. On a 

 future occasion further illustrations of the uses and 

 applications of the electric current in reproduction 

 work mav be tji^'cn. 



-^^.^^^^ 



PhotogrsLphy. 



Pure and Applied. 



By Ch.ap.max Jones, F.I.C, F.CS., &c. 



Dr. RusseWs Experimenls. — The production of the 

 developable condition in silver bromide, when it is ex- 

 posed to the action of certain clean metals, notably 

 zinc, and other substances such as turpentine, boiled 

 oil, printers' ink, and sections of wood, is still obscure. 

 Dr. Russell has found that a very minute porportion 

 of the vapour of hydrogen peroxide is able to produce 

 a similar effect, and that hydrogen peroxide is produced 

 when many, if not all, of the substances found to be 

 active are exposed to the air, as they are in the experi- 

 ments. Again, Dr. Russell has shown that whatever 

 it is that affects the plate, it behaves in some ways like a 

 vapour or gas. It appears to be carried along a tube by a 

 current of gas, it creeps over the edges of plates when 

 they are placed with their glass sides towards the active 

 substance, and so on. If this was all that there is to 

 be said, we should probably rest satisfied with the idea 

 that hydrogen peroxide itself is the active agent. But 

 some of the experiments render it difficult to believe 

 that it is only the production of a vapour at the sur- 

 face of the active substances, which diffuses, as a 

 vapour would diffuse, towards the photographic plate. 

 Dr. Russell himself has several times drawn attention 

 to this difficulty, though latterly he has apparently 

 passed it over, considering that his experiments prove 

 that hydrogen peroxide is the active agent in spite of it. 



Dr. Russell has shown that gelatine is not porous; 

 tlierefore it may be assumed that if a vapour passes 

 through it, it must be absorbed on one side of the gela- 

 tine sheet, work through it, and be given off at the 

 other side. He has shown that it does take time to 

 pass through, but the difficulty is that a practically 

 sharp reproduction of the active surface is obtained 

 instead of a considerably blurred image, such as one 

 would expect from an active vapour passing through 

 such an obstruction. Dr. Russell has said "a good 



