74 GUTHiLIE IIV.-On the Action of Light upon Chloride of Silver.. BY FREDERICK GUTHRIE,B.A. Ph.D. ASSISTANT IN THE LABORATORY OS' OWEN'S COLLEGE HANCHEBTER. THEblackening produced by light iipon chloride of silver in the moist state was ascribed by Sclieele to the liberation of chlorine and the deposition of metallic silver. He proved that the black- ened mass was only partially soluble in ammonia and that the portion which remained undissolved by this re-agent was soluble in nitric acid. Dan i ell and others considered the blackening to be due to the formation of oxide of silver imagining the clecom- position of the mater present by the chlorine to be accompanied by a corresponding oxidation of the reduced silver. 0them again have supposed the formation of' a subchlorids.ON THE ACTION OF LIGEfT UPON CHLORIDE OF SILVER. 75 The few experiments which I subjoin tend unmistakeably to support the view originally advanced by Scheele. (1.) Two or three grammes of dry chloride of silver were sealed in a glass tube and exposed to direct and diffused sunlight. There was increased tension in the tube. Chlorine was shown by the iodine test to be present in the free state. (2.)A portion of chloride of silver dried at loo' was intro- duced into a perfectly dry tube. The tube being then half-filled with pure and dry benzole and heated until the boiling of the benzole had expelled all the air was hermetically sealed and exposed with agitation to the light. The rapid blackening which the chloride here underwent proved the presence of oxygen to be unnecessary.(3.) Four or five grammes of moist chloride of silver were sealed in a tube. The tube was half filled with water and hermeti- cally sealed. After exposure to the light for ten or twelve days with frequent agitation it was opened the contents thrown upon a filter and mashed with cold water. On adding nitrate of silver to the filtrate a precipitate of chloride of silver was formed. The grey mass on the filter was treated with strong ammonia until the latter ceased to dissolve any more of the unaltered chloride. There remained on the filter a slaty grey body which in the dry state took the metallic lustre under the pestle. This body was solublc in nitric acid not reprecipitable by ammonia but precipitated by dilute hydrochloric acid.The non-precipitatiou of the nitric acid solution by the most gradual addition of ammonia seemed already to point to the absence of chlorine. (4.) About ten grammes of the moist chloride were introduced into a tube of 1+ft in length and 9 in. internal diameter. After adding water and sealing the tube was exposed as before. The supernatant liquid was poured off and the mass washed by decan- tation. The hydrochloric acid was thrown down by nitrate of silver. The chloride of silver was estimated on a weighed filter (dried at 100°C.) Ag C1 = 0.2125 grm. The washed material from the tube was digested with strong ammonia. The slaty-grey residue which subsided leaving the liquid above quite clear was collected upon a weighed filter and washed first with ammonia then with water (dried at 100°C.) Grey substance = 0.1756 grm.76 GUTHR1E This substance assumed the mctallic lustre under the burnislier. It dissolved in warm nitric acid dccomposing the latter. The substance together with the filter was thrown into strong nitric acid after digestion and due dilution it was filtered. The silver was thrown down by dilute hydrochloric acid collected on a weighed filtcr and estimated. (dried at 100OC.) Ag C1 = 0-2254grm. Supposing now the grey substance obtained to have bcen metallic silver it should have given 0.2333 grm. of chloride. The amount actually obtained though too small is yet sufficiently near to the calculated quantity to show that the original chloride of silver subjected to the light had really undergoue decom- position into chlorine and metallic silver.That the chloride of silver obtained from the hydrochloric acid found in the tube on breaking it open was somewhat smaller than that from the silver was probably due to an escape of a portion of the free acid by evaporation during manipulation. (5.) About twelve grammes of the chloride of silver mere intro- duced into a tube; the tube was then half filled with fuming nitric acid sealed and exposed as before. On opening the tube it was found to contain hydrochloric acid. The chloride was found to have undergone a blackening quite as deep as that which had taken place in chloride of silver surrounded by water which was exposed to the same light for the same time.On treating the contents as in experiment (at) there was found Grey substance . . = 0.1643 which gave . Ag C1 = 0.2040 The quantity supposing the substance to have been silver should have been Ag C1 = 0.2183 This is sufficiently near to show that the substance in question was nothing else than metallic silver. (6,) Confirmatory of experiment 5. (7.) The circumstances were as in experiment 5 excepting that the nitric acid employed was more dilute. On treating the contents as in experiment 4 I found Grey substance = 0.2207grm. and this gave Ag C1 = 0.2870 ON THE ACTTON OF LIGHT &C. instead of the calculated quantity Ag C1 = 0.2932 In experiments 4 5 6 the grey substance when dry assumed the metallic lustre under the burnisher.The fact that the chloride of silver was reduced to the metallic state even in the presence of nitric acid was quite unexpected. I found that neither by removing the affected mass from the light and agitating it nor even by warming it was the original white- ness restored. Indeed the silver was only very gradually attacked by boiling nitric acid unless the undccomposed chloride had becn previously removed by the action of ammonia. It seems as if the light in reducing the silver in spite of the nitric acid had thercby thrown it into a more passive state and that only after contact with the alkaline ammonia mas its original basic condition restored. The chloride of silver used in these experiments mas in every instance washed by decantation in order to avoid the presence of organic matter.