8 LECTURES TO SCIENCE TEACHERS. 



electric light. To render it sensitive before exposure in the 

 camera Talbot brushed over it a solution of silver nitrate and 

 gallic acid, which I will get my assistant to do with our 

 prepared paper. The gallic acid plays a most important part 

 in the process, so much so indeed that I ought to mention 

 that its utility was discovered previously to the Talbot-type 

 process, by the Eev. J. B. Eeade, a gentleman who but a few 

 years since has passed away from amongst us. The method of 

 his discovery was systematic. When securing images in the 

 solar microscope he remembered that Wedgwood had produced 

 images on white leather and paper on which had been brushed 

 silver nitrate, and had found that his leather was more sensi- 

 tive than paper. It occurred to Mr. Eeade that the dressing 

 used for the leather might have some important property, 

 so he applied a solution of nutgalls to his paper, and found 

 the necessary exposure to light was greatly shortened. He 

 also discovered that this same nutgall solution had the power 

 of developing the image. This, however, was a chance dis- 

 covery. One day whilst engaged in producing photographically 

 an image of the Trientalis Europsea, he was compelled through 

 circumstances to put on one side the paper which had not 

 been sufficiently exposed to give a visible impression. Placed 

 in the dark, this paper was left till next day, when, on 

 glancing at it, he found a perfectly distinct image. The 

 gallic acid had played a part hitherto not dreamt of. 



Eeverting to the calotype process, we find that Fox Talbot 

 employed the gallic acid with silver nitrate to render the 

 paper sensitive (the former being what is termed an ac- 

 celerator) ; and the discovery that an invisible image could 

 be rendered visible by the same solutions was also utilised, 

 for after exposure to the light the image was brought out by 

 them. 



Our own piece of Talbot-type paper is ready, it having 

 been treated as indicated, and the excess of moisture blotted 

 off on blotting paper. After placing it behind the same 

 negative which we employed in illustrating the daguerreotype 

 process, we will expose it to the beam of the electric light. 

 A couple of seconds is a sufficient time to have produced an 

 invisible image, and we will at once proceed to render it 

 visible with a solution of gallic acid and nitrate of silver. 

 Tiiis we dab on with a tuft of white wool, and the picture 

 begins to appear. After a little patient manipulation the 



