NATURAL PHILOSOPHY. 1G1 



The collodion was, perhaps, two months old, nearly colorless, and gave a 

 thin coat. It was prepared by Mr. Robinson, chemist, who assured me that 

 lie had iodized it with iodide of potassium with a little bromine. 



The bath was a neutral solution of crystallized nitrate of silver. I had 

 developed with a solution formed of two grains of pyro-gallic acid, twenty 

 drops of acetic acid dissolved in one ounce of water, and fixed by a concen- 

 trated solution of cyanide of potassium. 



M. Raymond remarks that if, as soon as a collodion picture begins to 

 develop clearly under the combined action of pyre-gallic and acetic arid, it 

 be exposed to the light without previous drying, it is rapidly changed into a 

 positive picture, and takes more or less perfectly the colors of the model. 

 The stronger the light is, and the less the development of the picture, the 

 more rapid, but at the same time the less perfect, is the transformation. A 

 picture accidentally made in this way was completed in a quarter of an hour, 

 and lasted some months with scarcely any loss of brilliancy; and even now, 

 after more than two years time, is not completely effaced. 



In reference to this communication of M. Muguet to the Academy (Paris), 

 M. Bertsch reminded the hearers that every photographer had frequently 

 observed, that when the development of a positive was arrested at a certain 

 point, and the picture placed upon a black ground, effects were obtained, 

 which imitated the natural color very well. The whole picture takes on a 

 rose-color, which imitates tolerably well the tones of the face; and as the 

 hair and dress, which are darker than the face, are but slightly brought out, 

 they allow the black color of the face to appear through them, and thus pro- 

 duces the appearance of a coloring which does not in reality exist. Cosmos. 







PHOTOGRAPHIC INFLUENCE OF RADIANT HEAT. 



Mr. Crookes, editor of the London Photographic News, acting upon a hint 

 conveyed by M. Niepce de St. Victor's recent experiments on photographic 

 printing, has succeeded in reproducing, by means of radiant heat, and with- 

 out the previous exposure of any of the materials to sun-light, the experi- 

 ment of photographing in the dark, which M. Niepce supposed to be accom- 

 plished by the action of light stored up in hermetically sealed tubes. Having 

 lined a tin tube with paper soaked in tartaric acid, Mr. Crookes proceeded as 

 follows : 



A little water was introduced inside the tube, so as to well moisten the 

 paper, and the excess poured out. The tube was again closed, and heated 

 to a temperature too high to be borne by the naked hand. It was then 

 opened directly, and applied face downwards upon a sheet of ordinary sen- 

 sitive chloride of silver paper, a piece of a handbill having previously been 

 laid on to serve as a negative. It was suffered to remain in that position 

 about ten minutes. The result was precisely similar to that accomplished by 

 M. Niepce. The circle of the sensitive paper which was covered by the 

 mouth of the tube became visibly blackened in those parts which were un- 

 protected by the piece of handbill, the letters on which were impressed, white 

 on a black ground, and distinctly legible. This, therefore, proves conclu- 

 sively that light has nothing whatever to do with the operation, inasmuch as 

 the icliole of the manipulations we have described were performed at night by 

 the light of a small lamp. The whole of the materials employed had also 

 been kept in darkness for some time previously. 



Mr. Crookes thinks that "the heat, combined it may be with a chemical 



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