376 SCIENTIFIC AMUSEMENTS. 



about by exposure to light, either perceptible at once or capable of 

 being rendered visible on " development" with appropriate reagents 

 possessing the power of acting most rapidly on those parts where 

 the action of the light has been greatest. In similar fashion many 

 nonmetallic substances are known which are affected by light in 

 various ways. In one of the earliest invented photographic pro- 

 cesses the sensitive material was " bitumen of Judsea," which is 

 readily soluble in naphtha before being exposed to light, but much 

 less so afterwards, a change being produced somewhat analogous 

 to that occurring in the white of an egg when boiled ; previously 

 to heating the white will readily mix with water, but after heating 

 it becomes " coagulated," and insoluble in water. Gelatin, especi- 

 ally when mixed with certain metallic compounds (of which the 

 cliromates are some of the most convenient), is affected in a similar 

 fashion, so that a picture can be readily obtained by exposing to 

 .light paper or glass, &c., coated with a thin film of gelatin dissolved 

 in hot water to which a little potassium dichromate has been added, 

 and a little lampblack or other insoluble pigment very finely 

 divided ; wherever the light acts the gelatin is rendered insoluble, 

 but in the shadows it is hardly affected ; so that on treating the 

 exposed plate with warm water the gelatin is washed away in 

 these parts, but not in those where the light has acted. Accord- 

 ingly the picture appears as a negative, the residual film of 

 insoluble gelatin containing lampblack being thickest where the 

 light has acted most, when the action of the water is complete. 

 As chromatised gelatin is far less sensitive to light than ordinary 

 iodised collodion films, or gelatin bromide films, pictures are rarely 

 taken directly in the camera by this process ; negatives taken by 

 silver processes of one kind or another are generally first prepared, 

 and then prints (positives in reference to the original object) taken 

 therefrom by the gelatin process. Chromatised gelatin films are 

 apt to become spontaneously changed on keeping ; so that most 

 operators prefer to coat the paper employed with plain gelatin 

 containing no chromate, but only pigment ; and when required for 

 photographic purposes, to render the films sensitive to light by 

 dipping into a solution containing about 3 per cent, of potassium 

 dichromate and a little ammonia, and drying in the dark. Paper 

 thus coated with gelatin and carbon is prepared for sale by dealers 

 in photographic requisites, and only requires sensitising with 

 chromate solution to be ready for use. 



It is obvious that where the light acts the gelatin is rendered 

 insoluble, first on the outermost layers, and only later on in the 

 underlying ones, because the former tend to absorb largely the 

 active radiant energy, leaving but little to pass inwards ; conse- 



