574 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



similarly steeped in sulphate of iron, will serve for the detec- 

 tion of the second class of impurities, every particle of which 

 will be indicated by a fine black spot. An adulteration 

 amounting only to one part in a thousand parts of madder 

 may thus be detected. 2 A^ February 1, 1873, 83. 



REDUCING THE INTENSITY OF A NEGATIVE. 



According to Letalle, a negative can be diminished in in- 

 tensity by first washing it and then covering it with a solu- 

 tion containing fifteen grains of chloride of gold and 0.53 of 

 a quart of water. The operation is to be repeated until the 

 plate has obtained a proper tone. A quantity of nitric acid 

 is then to be poured on one corner, enough to cover the 

 whole plate, when the silver is immediately dissolved and 

 almost disappears. It is then to be re-enforced with sulphate 

 of iron, and then re-appears with a great degree of transpar- 

 ency. Pyrogallic acid may be made use of to bring it up 

 both to the desired degree of intensity and to that of trans- 

 parency. The image which remains after the application of 

 the nitric acid, on account of its great transparency, is well 

 adapted for taking large pictures. 3 B, April 24, 1873, 720. 



GUMMATE OF IRON PFIOTOGRAPHIC PAPER. 



The British Journal of Photography announces a remarka- 

 ble compound of gummic acid and sesquioxide of iron which 

 renders paper coated with it sensitive to light. To prepare 

 it we are directed to add ammonia carbonate, carefully and 

 with continual stirring, to a solution of sesquichloride of iron, 

 until the mixture effervesces violently. Filter the liquid, 

 saturate the paper with it, dry in the dark, and then cover 

 it with a tolerably thick film of gum-arabic, and a layer of 

 guramate of iron will immediately form npon it. Paper thus 

 prepared does not become deep yellow for some time, and, if 

 perfectly dry, remains very flexible for a long period, and re- 

 tains a fine gloss. 8 (7, May 1, 1873, 150. 



PREPARATION OF COTTON FOR COLLODION. 



M. Adolph Martin, of Paris, has lately suggested a new mode 

 of preparing cotton for collodion, which is extremely soluble 

 and well adapted for many purposes of photography. For 

 this purpose he takes two parts of fiulphuric acid of 66 Fahr., 



