142 ANNUAL OF SCIENTIFIC DISCOVERY. 



notices particularly the action of light upon the oxalate of peroxide of iron. 

 The golden yellow solution of this salt undergoes no change when kept in 

 total darkness, but on exposure to a lamp or to daylight is decomposed with 

 evolution of carbonic acid and pecipitation of oxalate of the protoxide as a 

 lemon-yellow powder. In sunlight it effervesces violently. The indigo ray 

 is especially active in producing this effect, and undergoes absorption in 

 doing so, since a sunbeam which has passed through one layer of solution is 

 incapable of affecting another. The author points out several methods of 

 employing this salt in photometry, the most advantageous of which is to col- 

 lect and measure the quantity of carbonic acid absorbed in a given time. 

 The solution is sufficiently sensitive for all ordinary purposes. When great 

 sensitiveness is required the author recommends the use of the tithonometer 

 invented by him in 1843, and since employed, in a modified form, by Bunsen 

 and Roscoe. 



IMPROVEMENTS IN PHOTOGRAPHY. 



Sensitiveness of Photographic Reagents. The sensitiveness of the reagents 

 used in photography is well known. But there are others more sensitive still, 

 which often interfere with photographic results. Photographers are aware 

 that they do not succeed well in the vicinity of a perfumery, and that even 

 the effluvia from the hand that has been wet with an essence, will prevent 

 success when it was thought to be sure. It has even been supposed that in 

 clear weather, when the atmosphere was full of vegetable emanations and 

 vapors drawn from the soil by heat, photographs do not succeed as well, 

 and that these influences must be avoided for the finest results. Evidently 

 the emanations from the soil and plants arc themselves sensitive to the 

 chemical rays, and more so than the photographic reagents, especially chlo- 

 ride of silver; and hence, as far as they are not destroyed, may monopolize 

 the chemical action. 



These thoughts are suggested by a fact recently made known by Mr. 

 Ford. He had always had perfect success ; but afterwards, in spite of all 

 precaution, failed of good pictures. After long seeking the cause, he finally 

 found that it was owing to his room being near a storehouse of " noir ani- 

 mal," for economical uses, the effluvia was injurious to the photoraphic 

 liquids, the principal of which were nitrate of silver, pyrogallic acid, and 

 hyposulphite of soda. He moved to another place, and had no more diffi- 

 culty. 



Novel application of Photography. M. Persoz, Professor of Chemistry at the 

 Conservatoire des Arts et Metiers of Paris, has just published a most inter- 

 esting discovery of his, by which photography may be applied to the orna- 

 menting of silk stuffs. The bichromate of potash is a substance commonly 

 used in photography, being extremely sensitive to light. If a piece of silk 

 stuff, impregnated with this salt, be exposed to the rays of light penetrating 

 through the fissures of the window-blinds in a closed room, the points where 

 the stuff has received these rays of light will assume a peculiar reddish tint. 

 Now, suppose a piece of metal or of strong paper to be cut out after a given 

 pattern, and to be laid upon a piece of silk prepared as before ; if exposed to 

 the sun, or, better still, to simple daylight, the pattern will be reproduced in 

 a few instants. The pale red, which the parts acted upon by the light assume, 

 is so permanent that nothing can destroy it; nay, it will fix other colors, 

 such as madder, campeachy, etc., just like a mordant, and in that case it 



