230 Progress of Invention. 



slow ; but it is something that pictures in their natural colours can 

 now be produced which undergo but little change in a year. Such 

 a result has been obtained by the method of M. Alp. Poitevin, and 

 on paper. He has now made public the methods he uses for the prepa- 

 ration of the paper. A film of ordinary chloride of silver is formed on 

 the surface of photographic paper which has not been albuminized, 

 by bringing one side of each sheet in contact with a solution o£ 

 chloride of sodium containing ten per cent, of the chloride, and 

 after drying, with a solution of nitrate of silver containing eight 

 per cent, nitrate ; or by brushing over one side of the paper uni- 

 formly with a mixture consisting of equal- volumes of a saturated 

 solution of bichromate of potash, and of a solution of sulphate of 

 copper containing ten per cent, of the sulphate, drying the paper in 

 the dark, applying the prepared surface to the solution of nitrate of 

 silver, removing the excess of nitrate by abundant washing with 

 water, and at the last washing adding, drop by drop, hydrochloric 

 acid until the red chromate is changed into white chloride of silver. 

 To obtain the violet subchloride, a small quantity of a solution of 

 chloride of tin, containing ten per cent, of the chloride, is poured 

 into a dish in which the prepared paper is immersed in water, and 

 without withdrawing the paper from the dish it is then exposed to 

 light in the shade. After about six minutes the required deep 

 violet tint is attained, and the exposure to light must be stopped. 

 The paper is then well washed, and dried in the dark. Its sensi- 

 bility in this state is very slight, and it may be kept for a consider- 

 able time in the dark. The paper thus prepared is best sensitized 

 by means of a mixture of bichromate of potash and sulphate of 

 copper. The fixing agent is water acidulated with sulphuric acid, 

 or, which is preferable, a very dilute solution of bichloride of 

 mercury, similarly acidulated with sulphuric acid. The acidulated 

 water dissolves certain compounds of silver which are formed in 

 spots ; and, after the picture has been dried in the dark, it is so 

 little sensitive to light that it will keep very well in an album, and 

 may without injury be looked at with artificial or even diffused 

 light. 



Production of Iron from Copper Ore. — Certain ores of copper 

 leave, as residue, after the copper has been extracted, a nearly pure 

 oxide of iron, which would be very valuable, as a source of iron, 

 were it not that its state of minute division renders its reduction 

 by the ordinary methods impossible. It has, however, been found 

 that the difficulty may easily be removed. For this purpose, it is 

 only necessary to agglomerate the oxide into masses of convenient 

 size ; which is effected, by drying it, until it contains only about 

 fifteen per cent, water, mixing it with five per cent, hydraulic 

 cement in powder, and sifting the mixture; then moulding the 

 latter by means of any of the machines ordinarily employed for making 

 bricks, considerable pressure being applied, and exposing for some 

 days to the air. The masses thus obtained are sufficiently cohesive 

 for reduction in any of the furnaces us,ed in the iron manufacture. 

 r i he addition of the hydraulic cement is an advantage, rather than 

 the contrary, since its materials serve as a flux. 



