214 On some remarkable Properties of Iodine, ^c. 



blacker, but the exposure to the light should be continued a 

 little longer than in the method first indicated. The plate is 

 afterwards placed in the camera obscura, and retained there 

 perhaps a little longer than when paper prepared by Blan- 

 quart's process is used*. However, I have obtained very black 

 impressions in twenty or twenty-five seconds in the sun, and in 

 a minute in the shade t- 



The operation is then conducted as for the process on 

 paper, i. e. gallic acid is used to bring out the impression and 

 bromide of potassium to fix it. 



This is the first process which I used ; but having tried 

 albumen (white of egg), I obtained a remarkable superiority 

 in every respect, and I believe the preference must be given to 

 the latter substance. The plates were prepared as follows : — 

 I took the most transparent part of the white of egg J (the 

 liquid portion), in which I placed some iodide of potassium; 

 then, having allowed it to flow upon the plates, I left it to dry 

 at the ordinary temperature (if this was too high, the layer of 

 albumen cracked). When about to use it, the aceto-nitrateis 

 applied by pouring it upon the plate, so as to cover the entire 

 surface at once ; but it is better to plunge it into this composi- 

 tion so as to obtain a perfectly uniform layer. The aceto- 

 nitrate renders the albumen insoluble in water, and makes 

 it strongly adherent to the glass. With albumen, rather 

 longer exposure to the light is required than when starch is 

 used. The action of the gallic acid is also slower; but by 

 way of compensation, we obtain remarkable distinctness and 

 delicacy of the lines, and which, in my opinion, will someday 

 be brought to the perfection of an image upon a plate of 

 silver. 



I tried gelatinous matters: they also yield very distinct im- 

 pressions (especially if filtered, which is essentially requisite 

 i'or all substances), but they are too readily dissolved by water. 

 If starch is employed, the finest should be chosen ; my expe- 

 riments were made with that manufactured by Groult. 



By employing the means which have just been pointed out, 

 negative impressions are obtained. I have not made any po- 

 sitive impressions ; but I presume that they can be obtained 

 as on paper, or by placing the substances in starch, and not in 

 albumen; which it is not even necessary to immerse in the so- 

 lution of common salt. With the latter substance, the plate 

 must be plunged into the silver-bath. 



* Mr. Blanquart's process is no other than the Calotvpe invented by 

 Mr. H. F. Talbot— Edit. 



f By slightly heating the plate, less time is required. 

 I The fresher the white is, the more viscid it is. 



