NATURAL PHILOSOPHY. 141 



black of natural objects, represented in a colored picture, reflects 

 its own tint, or, if we may say so, its own rays, endowed, it 

 would appear, like all the others, with chemical action, for the 

 apparent reason that the hole could not reflect any rays, and its 

 blackness is the result only of the absence of all rays. The same 

 thing may be said of the white, but less extraordinarily ; for the 

 white being the result of all the rays of light united, it may be 

 more easily understood that the chemical action of the white 

 would be the compound result of the various rays of which it is 

 composed, and that result is the same as that which gives us the 

 sensation of white. Certainly the reproduction of -black and 

 white by M. Niepce de St. Victor is a most extraordinary fact un- 

 folded by his beautiful discovery, and perhaps more surprising 

 than the reproduction of all the colors themselves. British Jour- 

 nal of Photography. 



CHEMICAL FORCE OF THE SOLAR LIGHT. 



This, or actinism, upon which organic nature depends so largely 

 for life and health, varies very much under different conditions ; 

 so much so that within the tropics it is said to be very difficult 

 to obtain good photogranis. The apparatus for determining 

 it relatively has lately been improved by Bunsen and Roscoe, 

 in a manner analogous to that for measuring ozone. After 

 many experiments they have succeeded in preparing paper of a 

 standard sensitiveness, in which the tint produced in a given 

 number of seconds varies in exact proportion to the intensity of 

 chemical force in the light employed. It was found in France 

 that the actinic intensity varied from 1 to 20 between December 

 and June. 



VITRIFIED PHOTOGRAPHS. 



De Mothay and Marechal have produced a new method for fix- 

 ing vitrified photographic images in porcelain, enamel, glass, etc. 

 The article is first varnished with a solution of 4 parts of caout- 

 chouc in 100 of benzole, with the addition of one part normal collo- 

 dion. After diying, a second coating of iodized collodion is 

 poured over the first, and unites intimately with it. It is then 

 immersed in a bath of nitrate of silver, and the image is produced 

 either by camera or superposition, developed by any of the usual 

 agents, and fixed by two successive baths, one containing a solu- 

 tion of an iodocyanide, and the other an alkaline cyanide. It is 

 next steeped for some instants in a solution of protoxide of iron, 

 pyrogallic acid, or any other substance that will reduce the salts 

 of silver. The image is intensified by the action of pyrogallic, 

 gallic, or formic acid, or sulphate of protoxide of iron mixed with 

 an acid solution of nitrate of silver ; requiring 4 to 6 applications 

 for images to be seen by reflection, and 12 to 15 for44iose to be 

 seen by transparency. During this operation the image is washed 

 three or four times in alternate baths containing iodocyanides and 

 alkaline cyanides, and then, immediately afterward, in sulphate 

 of protoxide of iron, pyrogallic acid, or other reducer of salts of 



