remarkable Properties of Iodine^ <$'c- 213 



subsequently laying it upon blue litmus paper, the white parts 

 of the engraving are copied in white; whilst if the engraving 

 is placed in contact with solution of chlorine or the vapour 

 which it exhales, the black parts are copied in red. But to 

 obtain these results, especially with chloride of lime, the tem- 

 perature must be raised to about 104<° F. The same effects 

 are produced upon silver and copper. 



On Photography upon Glass. 



Although this essay is merely preliminary, I publish it in 

 its present state, not doubting that it will make rapid progress 

 in more practised hands than mine, and by those who will be 

 enabled to experiment under better circumstances than I have 

 been enabled to do. 



I shall point out the means which I have employed, and 

 which have yielded satisfactory although not perfect results : 

 as everything depends upon the preparation of the plate, I 

 shall describe the best method of preparing the starch. 



I take five grammes of starch, which are mixed first with 

 five grammes of water and afterwards diluted with ninety-five 

 grammes, and lastly thirty-five centigrammes of iodide of 

 potassium dissolved in five grammes of water. The whole is 

 placed upon the fire : when the starch is dissolved, it is allowed 

 to cool, next strained through linen, and then allowed to flow 

 upon the plates of glass, taking care to cover the entire surface 

 as equally as possible. After wiping them beneath, I place them 

 upon a jierfectly horizontal plane, so as to dry them as quickly 

 as possible in the sun or by means of a stove, in order to obtain 

 a layer which is not fissured; that is to say, to prevent the glass 

 from being covered with circles in whidi the coating is thinner 

 than elsewhere (which are produced, in my opinion, by the 

 iodide of potassium). The starch should always be prepared 

 in a porcelain vessel, and the quantity, five grammes, which 

 has been mentioned, is sufficient to coat ten plates, of the size 

 known in commerce by the name fVun quart*. It is thus seen 

 that a large number of plates are easily prepared at once. 

 Moreover, bubbles of air must be excluded, as these would 

 make so many smhil holes in the impressions. The plate being 

 thus prepared, when we wish to proceed, it is merely requisite 

 to apply the aceto-nitrate to it, by means of a piece of paper 

 plunged several times into this compound; a second piece of 

 paper moistened with distilled water is subsequently laid upon 

 the plate. A second method consists in previously impreg- 

 nating the layer of starch with distilled water, before laying 

 on the accto-nilrate ; in the latter case the image is much 

 * Of iilioiit A\ inches l)y 3.>. 



