Mr. G. Shaw on some Photographic Phccnomena. 449 



been properly prepared, and which had been exposed to light. 

 From this it will be evident, that a plate properly prepared 

 in the first instance, and then exposed to light, may, by sub- 

 sequent exposure to the vapour of bromine, have the impres- 

 sion produced by light isoholly destroyed] and yet, by the ac- 

 cumulation of bromine may exhibit, on exposure to mercury, 

 an appearance similar to that due to light. In other words, it 

 is impossible (in the case supposed) to distinguish between an 

 effect produced by light and an effect due to excess of bromine. 

 By using iodine in place of bromine, there is no risk of pro- 

 ducing the appearance which accompanies excess of bromine; 

 but on the other hand, by augmenting the quantity of iodine, 

 the sensitiveness of the plate is diminished. These difficulties 

 were overcome by using a solution containing both iodine and 

 bromine, in such proportions that theevaporationofeach should 

 take place in the proportion in which they produce on silver 

 the most sensitive surface. The solution employed was made 

 by adding alcoholic solution of iodine to a solution of chlorate 

 of potash until the latter would take up no more of the for- 

 mer, and to each ounce, by measure, of this solution ten 

 drops of a saturated solution of bromine in water were added. 

 The solution of chlorate of potash was made by diluting one 

 part of a saturated solution of the salt with ten parts of water. 

 The use of the chlorate is simply as a solvent of iodine. In 

 the subsequent experiments the plate was exposed to the 

 vapour of this mixture of iodine and bromine with precisely 

 the same effect as when either was used separately, and with- 

 out the inconvenience or uncertainty which attended their 

 use. 



A number of preliminary experiments, the detail of which 

 would be uninteresting, appeared to indicate, that not only 

 is the effect of light on a Daguerreotype plate destroyed by 

 iodine or bromine, but that the plate is restored to its original 

 condition; in other words, that its sensitiveness to light is 

 restored. In order to determine this point, the following ex- 

 periments were made. 



A prepared plate was exposed to light, and afterwards to 

 the mixed vapour*; mercurial vapour produced no effect 

 upon it after a long exposure ; the plate on removal from the 

 mercury box was a second time exposed to light, and again 

 introduced into mercurial vapour. The appearance of the 

 plate was very little changed, and it was concluded that no 

 effect, or, if any, very little, was produced by the second ex- 



* I shall hereafter call the mixed vapours of iodine and bromine pro- 

 duced in the way described in the last paragraph but one, mixed vapour, 

 in order to avoid circumlocution. — G. S. 



Phil. Mas. S. 3. Vol. 25. No. 168. Dec, 1844. 2 G 



