NATURAL PHILOSOPHY. 231 



stead of glass or paper. This artificial material, which possesses all the 

 properties and beautiful finish of ivory, and allows of any subsequent tinting 

 of the image, and the obtainment of superior softness in the semitints, is 

 what is known in Trance as Pinson's artificial ivory, consisting of a com- 

 pound of gelatine and alumina. This material is prepared in the form of 

 slabs, for the photographer's use, in this way : The tablets or slabs are com- 

 posed cf gelatine or glue in its natural state, and are immersed in a bath of 

 alumina, which is held in solution by sulphuric or acetic acid ; by this means 

 a complete combination takes place between the alumina and the gelatine or 

 glue. The tablets or slabs should remain in the bath a sufficient time to 

 become thick enough for the purpose for which they are required, and to 

 allow the alumina to entirely penetrate them and incorporate itself there- 

 with ; they are then removed and allowed to dry or harden, when they may 

 be dressed and polished by any of the ordinary and well-known processes 

 for polishing ivory. 



Artificial ivory tablets, capable of bearing a fine polish, may also be made 

 by mixing alumina directly with gelatine or glue ; but this process is not so 

 satisfactory as the process hereinbefore described, since the thickening pro- 

 duced by the admixture of the alumina with the gelatine, renders the manu- 

 facture of the sheets both difficult and expensive. 



Another composition of artificial ivory which is employed, consists of 

 equal portions of bone or ivory dust, used either separately or combined, and 

 albumen or gelatine, the whole being worked into a paste, and afterwards 

 rolled out into sheets by suitable rolling or flattening mechanism. The 

 sheets are then allowed to harden by exposure to the atmosphere, and are 

 cut into slabs or tablets of the required size. But it is preferred to use two 

 parts of fine powdered baryta, and one part of albumen, well worked 

 together, and rolled out into slabs. The best plan hitherto discovered for 

 working the materials together, is that commonly used in the manufacture 

 of Parian marble ; this composition may also be used spread upon paper, if 

 desired. These slabs or tablets are then carefully scraped, to give them a 

 perfectly even surface. They are then washed with alcohol, to remove any 

 impurity therefrom, and arc prepared in the ordinary manner to receive posi- 

 tive pictures. The pictures having been printed, the entire slab or tablet 

 may be immersed for a few minutes in a weak solution of nitre-sulphuric 

 acid or nitre-hydrochloric acid, for the purpose of rendering the picture more 

 clear and brilliant. It is then fixed in the usual manner with hypo-sulphite 

 of soda, and is washed, and then dried on a marble or other slab, or under 

 pressure, to prevent it from warping. 



MISCELLANEOUS niPROYEMEXTS IN PHOTOGKAPHY. 



Ilallotypes. The following is a description of a patent recently granted 

 to J. Bishop Hall, of New York City, for a peculiar method of treating pic- 

 tures to produce a high degree of artistic and stereoscopic effect of objects, 

 applicable to photographs, engravings, lithographs, and- similar pictures. 

 The principle of the invention consists in combining two or more photo- 



