NATURAL PHILOSOPHY. 181 



of copper without washing, it will assume different shades, according 

 to the degree of heat employed. If the reagent is a prussiate of 

 iron, the color of the picture will be a beautiful blue. 



Military Photography. The French Minister of War has recently 

 decided that in each corps of the army there shall be an officer skilled 

 in photography. In every campaign he is to follow the expeditionary 

 corps. To this officer is assigned two subordinates, in the capacity of 

 photographic aids, and six soldiers are detailed to serve as assistants. 

 The apparatus employed is necessarily limited, consisting of objec- 

 tives adapted to long distances, and which can be easily packed in a 

 single wagon. 



Restoration of Faded Photographs. The greatest defect of pho- 

 tography as an art is, that its pictures are more perishable than the 

 material which bears them. Many of them, indeed, have disappeared, 

 and left the paper on which they were drawn in all its original white- 

 ness. This fading of photographs has been ascribed, we believe 

 justly, to the imperfect removal by hot or cold water of the hypo- 

 sulphite of soda used in fixing them ; and for a long time photogra- 

 phers have endeavored to get rid of this injurious salt. It is fortunate, 

 however, for the credit of the art, that a method of reviving faded 

 photographs has been discovered, and the following process has been 

 published by MM. Davanne and Girard : " Place the print in a 

 solution of chloride of gold, and leave it in this bath for three or four 

 hours, if shielded from the light, or for a few minutes, if under the 

 influence of the solar rays. In other respects, follow the ordinary 

 course, pass through hyposulphite of soda, and the print, however 

 faded, will be revived." 



New Mode of Copying Engravings. M. Brettiger suggests a very 

 simple method of reproducing, by chemical means, an engraving from 

 a steel or copper plate. Dissolve in 1,500 parts of pure water 16 

 parts of pure, concentrated sulphuric acid, and to 200 parts of the 

 mixture add one-half part of iodide of cadmium. This last mixture is 

 poured into a dish, and the engraving is immersed in it, and left till 

 it has become thoroughly impregnated with the liquid ; it is then 

 placed upon folded sheets of white blotting-paper, on a plate of glass, 

 and the excess of moisture removed from the engraving. It is then 



o O 



placed printed side downward upon a sheet of writing or of positive 

 paper, and placed in a press. An impression is obtained as delicate 

 as that furnished by photographic processes. The iodide of cadmium 

 may be replaced by iodide of potassium. The reproduction is due to 

 the reduction of the iodine by the black in the ink of the engraving, 

 and the liberated iodine acts upon the starch with which the paper is 

 sized. The engraving will ive a second impression without beino - re- 



~ m O O 1 O 



turned to the solution. When the engraving has been used several 

 times, it is only necessary to wash it in water to remove the spots that 

 may have formed. Lithographs and ordinary printed matter cannot 

 be reproduced by this process, on account of the nature of the 

 printing ink, but writing ink succeeds very well. Unfortunately, 

 these beautiful impressions become blue all over, and are gradually 

 effaced, even if covered with a coat of varnish. 



A utographs of the Sun. At the meeting of the British Asso- 

 ciation, 1862, Prof. Selwyn showed several " autographs of the sun," 



16 



