NATURAL PHILOSOPHY. 197 



pered again, after another printing, the same operation is repeated, 

 so that a considerable number of copies may be printed without much 

 injury to the engraving. Humphrey's Journal. 



REPRODUCTION OF ENGRAVINGS. 



In 1847, M. Niepce published a memoir on the action of different 

 vapors, and amongst others of that of iodine. He stated that the 

 vapor of iodine attached itself to the black portions of an engraving 

 to the exclusion of the whites, so that the picture could be reproduced 

 on paper sized with starch, or on glass coated with this substance. 



He now proposes to render them unalterable by the following pro- 

 cess : If a design obtained on starched paper or glass, in the manner 

 described by him in 1847, be plunged into a solution of nitrate of 

 silver, it will disappear ; if the paper or glass be now exposed to the 

 light for a few seconds, the iodide of silver, into which the iodide of 

 amidone which formed the primitive design has been converted, being 

 much more sensitive than the nitrate of silver with which the rest of 

 the surface is imbued, is acted upon much more rapidly ; if the paper 

 or glass be then dipped into a solution of gallic acid, the design is 

 immediately reproduced, and it is then treated with hypo-sulphite 

 exactly as is done with photographic pictures. By this process the 

 design becomes as permanent as these latter, and it will probably be 

 adopted in many cases. 



M. Bayard has applied this process to the reproduction of old 

 engravings, by first forming a negative upon albuminised glass, by 

 means of the previously iodized engravings placed upon it. Comptes 

 Rendus, March, 1853. 



COLORED DAGUERREOTYPES. 



A late number of Humphrey's Journal contains an article by James 

 Campbell, of Dayton, Ohio, giving the results of some experiments 

 made to produce colored daguerreotypes, from which we make the fol- 

 lowing very interesting extracts : 



M. Becquerel and Kiepce de St. Victor have proved that if chloride of 

 silver containing a slight trace of copper, be exposed to the prismatic 

 spectrum, or to the rays of different colors, while undergoing this re- 

 duction, it is susceptible of coloration after a protracted exposure. 

 From this it would seem that this process might be much accelerated, 

 if we were careful to aid nature in her operations, instead of trying 

 mere haphazard experiments, not based on rational theory. I will 

 show by a few experiments that this may be done, and to avoid being 

 too prolix, will, at present, speak of the chloridated silver plate, unac- 

 celerated by iodine, bromine, fluorine, chrome, or their compounds. 



If the plate, covered with the enameled chloride of silver prepared 

 by Niepce's process, be exposed to a current of hydrogen while re- 

 ceiving the image, the process will be much accelerated, and the 

 image will be impressed in from half an hour to an hour ; according 



