368 SCIENTIFIC AMUSEMENTS. 



already present on the developed plate exert a kind of special 

 attractive action, in virtue of which the additional silver particles 

 deposited from the intensifying solution are attracted towards 

 those already there, and are not deposited indiscriminately all over 

 the plate ; the result of which is that the extra silver thus 

 deposited does not form a uniform layer, but thickens up most 

 those parts of the picture where most silver is already present, and 

 so on in proportion, so that the gradations of light and shade 

 are thus regularly intensified. This action is closely akin to that 

 occurring during crystallisation of dissolved substances, especially 

 from a slightly supersaturated solution (Expt. 63) ; if suitable 

 nuclei, more particularly bits of solid crystals of the dissolved body, 

 be introduced, rapid separation by crystallisation of the dissolved 

 matter ensues, the substance thrown out of solution being deposited 

 on the surface of the introduced solid nuclei, causing them to 

 enlarge and grow bigger ; whereas little or no such separation takes 

 place in the absence of suitable nuclei ; e.g. (Expt. 58), a strong 

 aqueous solution of cane sugar readily deposits crystals of sugar- 

 candy on strings or sticks immersed therein ; but if kept in a 

 clean smooth glass vessel will often form a supersaturated solution 

 that will be long before it deposits any crystals at all. In some- 

 what similar fashion certain melted solids can be cooled down 

 considerably below their normal solidifying points without setting 

 solid, thus showing the phenomenon of supervision (Expt. 30) ; 

 but if a fragment of the substance in the solid state be brought in 

 contact with the superfused mass, the whole almost instantly 

 solidifies. In each case a sort of attractive action is exerted 

 whereby the presence of the solid particle acting as nucleus 

 facilitates, in one case the passage of the same kind of matter from 

 the liquid to the solid state (superfusion) ; in another case the 

 separation of the same kind of matter in the solid condition from 

 a solution thereof in a solvent (supersaturation) ; and in the third 

 case the separation of the same kind of matter from the condition 

 of chemical combination with something else (intensification of 

 photographic picture) ; the matter thus rendered solid being 

 deposited upon and attracted to the nucleus in preference to other 

 surrounding matters. 



This method of intensifying images is not conveniently ap- 

 plicable to gelatin bromide plates, because staining of the gelatin 

 is apt to occur ; in this case other methods are generally employed, 

 whereby the deposited silver is converted by simple chemical 

 action into more opaque substances of different nature. Thus, by 

 treating the plate with a solution of mercuric chloride the deposited 

 silver is partially converted into silver chloride, whilst mercurous 



