Gold-coloured Allotropic Silver. 247 



exposed to the air ; the higher temperature aids the effect of 

 the light *. 



It appears, therefore, that the agency of light is somewhat 

 similar to that of the other forms of energy, bnt very much 

 slower. Experiments made for the purpose demonstrated 

 that it is the more refrangible rays that effect the change. 



With light, the production of the intermediate form is 

 never very satisfactory. Long exposures are needed, and 

 before the change to the intermediate form is complete, the 

 further alteration to white seems to commence. With heat 

 the changes are much better marked. 



When a film on pure paper has received an exposure of one 

 or two days of summer sunshine under an opaque design, the 

 exposed portions are sufficiently changed to have lost much of 

 their chemical activity, so that when the film is plunged into 

 a bath of potassium ferricyanicle, the effect given in the lower 

 figure of Plate III. is obtained ; the colour represented is 

 one of an immense variety of tints produced by this reagent 

 on the unaltered or active form. The upper figure of the same 

 Plate gives the effect of a very protracted exposure (as above 

 described] on pure paper (or glass) under glass. With 

 some kinds of sized paper, this effect is produced by a much 

 shorter exposure, apparently owing to the presence of traces 

 of a hyposulphite f, which appears to aid the action of light. 



These seem to be not merely new facts, but to belong to a 

 ne w class of facts. No instance has been hitherto known of an 

 element existing in so great a variety of forms, and passing 

 so readily under the influence of any form of energy from 

 one to another of them. 



* Since this was written I find that both Herschel and Hunt noticed 

 an analogous fact in the case of silver chloride, viz. that paper prepared 

 with it darkens more rapidly under glass than when freely exposed ; 

 without, however, suggesting the cause, which is the same in both cases. 

 I have observed that silver chloride darkens more rapidly when exposed 

 under warm water than under cold to the same light in vessels side by 

 side. 



t The behaviour of these varieties of paper led me to make inquiries 

 of an intelligent paper-manufacturer, from whom I learned that every 

 sort of paper pulp is now treated with chlorine. As any portion of the 

 bleaching material left in the paper would eventually destroy its strength, 

 it becomes necessaiw to add hyposulphite in excess to remove it. Accord- 

 ingly every specimen of sized paper that I have examined contained 

 hyposulphite, even the purest photographic papers were not free from it 

 though containing greatly less than most others. Apparently, the only 

 difference is that, with photographic paper more care is taken to avoid 

 any considerable excess of hyposulphite. 



