M. TECHNOLOGY. 575 



to which he adds one part of dry nitrate of potash. When the 

 mixture reaches the temperature of 130 Fahr., the cotton is 

 introduced in small bunches, taking care to allow it to be- 

 come moistened as rapidly as possible. After seven or eight 

 minutes, the whole is turned into a large quantity of water 

 and repeatedly washed. These washings are continued until 

 the cotton becomes completely neutral, and it is then carded 

 with copper cards to remove all its pulverulent matter. The 

 proper proportions are eight grammes of cotton to 300 

 grammes of the mixture above mentioned. 3 -S, Ajwil 24, 

 1873,718. _____ . 



PEESISTENT ACTINISM OF A BICHROMATE IMAGE. 



According to Marion, of Paris, as announced in JVature, if 

 a bichromate photographic image, printed in the sun, be 

 brought into contact with another bichromate surface in the 

 dark, a similar impression will be made upon the latter. In 

 fact, a carbon picture fresh from the frame can be employed 

 as a printing-block, from which any number of impressions 

 can be obtained, as if a sufficient quantity of sunlight had 

 been stored up in the original impression to produce an act- 

 ive effect by merely bringing the surface in contact. 



This discovery, if verified, is considered of very great im- 

 portance, since it is alleged that if a single photograph be 

 printed in the sun, we can from this procure a large number 

 of copies, all of which will be as delicate and vigorous as the 

 original. For this purpose a sheet of gelatine, sensitized with 

 bichromate of potash, is put under a negative and printed ; 

 it is withdrawn from the printing-frame, and immersed in a 

 weak solution of bichromate of potash, which swells up those 

 portions of the surface that have not been attacked by light, 

 and thus produces a picture in relief. The sheet of gelatine 

 is then put into a press, and impressions from it taken on 

 sensitive carbon tissue, the block being moistened, from time 

 to time, with bichromate solution. The copies thus produced 

 upon the tissue are not fully printed, and can not be devel- 

 oped at once ; they are simply incipient, or nascent pictures, 

 and require preservation in the dark for some hours, to allow 

 the action of -the light to continue, exactly in the same way 

 as if the carbon tissue had been exposed to sunlight for a few 

 minutes. When the prints have been kept sufficiently long, 



