TO BOTANICAL TURPOSES. 189 



silver, has attracted so much notice, and produced so much 

 popular excitement, that a few observations on this interest- 

 ing process will not perhaps be considered out of place in 

 your Magazine. I venture to occupy your pages with the 

 less reluctance, because I feel that the application of this he- 

 liographic or photogenic art will be of immense service to the 

 botanist, by enabling him to procure beautiful outline draw- 

 ings of many plants, with a degree of accuracy which, other- 

 wise, he could not hope to obtain. 



That light will act on chloride of silver is by no means 

 a novel discovery, and paper prepared with it was long ago 

 used by Ritter and Wollaston, in testing the chemical action 

 of the rays of the solar spectrum ; still, in this country it was 

 not, I believe, applied to any purpose likely to be of use to 

 the naturalist and traveller, until brought into notice by the 

 researches of Mr. Talbot. It is not a little amusing to ob- 

 serve how many pretenders to the discovery have started up. 

 since the announcement of Mr. Talbot's discovery, and that 

 of M. Daguerre in France. The latter gentleman has, through 

 M. Arago, at a late meeting of the French Institute, announ- 

 ced his mode of preparing a sensitive paper, far exceeding 

 that of Mr. Talbot in delicacy, but otherwise possessing the 

 same property of indicating intensity of light by depth of 

 colour, and consequently differing from that marvellous pre- 

 paration which he is said to possess, and which represents 

 shadows by depth of colour, precisely as in nature. 



M. Daguerre prepares his heliographic paper by immersing 

 a sheet of thin paper in hydrochloric ether, which has been 

 kept sufficiently long to be acid ; the paper is then carefully 

 and completely dried, as this is stated to be essential to its 

 proper preparation. The paper is next dipped into a solu- 

 tion of nitrate of silver, (the degree of concentration of which 

 is not mentioned), and dried without artificial heat in a room 

 from which every ray of light is carefully excluded. By this 

 process it acquires a very remarkable facility in being black- 

 ened on a very slight exposure to light, even when the latter 

 is by no means intense, indeed by the diffused daylight of 

 early evening in the month of February. This prepared pa- 

 per rapidly loses its extreme sensitiveness to light, and finally 

 becomes not more readily acted upon by the solar beams than 

 paper dipped in nitrate of silver only. M. Daguerre renders 

 his drawings permanent by dipping them in water, so as to 

 dissolve all the undecomposed salt of silver. 



This process is very inconvenient, for many reasons, among 

 which are the difficulty of procuring, as well as the expense 

 of, hydrochloric ether : on this account I prefer Mr. Talbot's 



Vol. III.— No. 28. n. s. u 



