NATURAL PHILOSOPHY. 131 



as well as silver. 4. I have endeavored to show that a purely 

 physical cause, to wit, mechanical pressure, is capable of produc- 

 ing a developable impression, thereby answering the objection of 

 the inadequacy of a physical influence to create a basis of devel- 

 opment. And, finall}', I may remark that although the chemical 

 theory is suj^ported by some distinguished chemists of the present 

 day, I am not aware that there is a single well verified experi- 

 ment which can be brought forward in support of that view. In 

 the absence of such, I have been necessarily obliged to confine 

 myself to the affirmative side of the question, in sujiport of the 

 existence of a physical image, distinct from chemical reduction, 

 and though often accompanied by it, yet never necessarily," 



PHOTOGRAPHY IN NATURAL COLORS. 



M. Poitevin has lately succeeded in producing photographs on 

 paper in their natural colors. He prepares his sensitive paper in 

 the following way : Having obtained a layer of violet subchloride of 

 silver on the j)aper, by the action of light on the white chloride in 

 the presence of a reducing agent, he applies to the surface of the 

 paper a liquid composed of one volume of a saturated solution of 

 bichromate of potash, one volume of a saturated solution of sul- 

 phate of copper, and one volume of a solution containing five per 

 cent, of chloride of potassium. This paper is dried and kept in 

 the dark : it will keep good for several days. In this mixture, the 

 bichromate of potash is the principal agent ; the sulphate of cop- 

 per facilitates the action, and the chloride of potassium preserves 

 the whites which are formed. In copying paintings on glass, the 

 exposure to direct light need only last five or six minutes ; but the 

 time must, to some extent, depend on the transparency of the pic- 

 ture to be copied, and it is easy to watch the development of the 

 image on the paper. The paper is not sufficiently sensitive for 

 use in the camera. To preserve the pictures, it is only necessary, 

 first, to wash them Avith water acidulated with chromic acid, then 

 to treat them with water containing bichloride of mercury, after- 

 wards with a solution of nitrate of lead, and, lastly, well wash 

 them with water. After that they will not change in ordinary 

 light, but will, hoAvever, turn brown in direct sunlight. — Quart. 

 Journ. of Science, April, 1866. 



PRINTING PHOTOGRAPHS IN COLORS. 



Mr. J. A. Gatty read, on the 4th of October, at a meeting of the 

 Manchester Literary and Philosophical Society, the subjoined 

 paper, describing a process for obtaining colored photogra^ihs : — 



" My process is based upon the property possessed by ferrocy- 

 anide of potassium, of forming clear solutions with certain me- 

 tallic salts, producing insoluble compounds when the mixture is 

 brought into contact with a deoxidizing agent: the ravs of the 

 sun acting as such, a perfect precipitation takes place upon pajjer 

 or other material ijrepared with the above-named solution. In 

 producing the specimens sent herewith, I applied to the paper a 



