26 LECTURES TO SCIENCE TEACHERS. 



with, which man was acquainted as photographic action was 

 that of the tanning due to the sun, and in very primitive 

 days no doubt it was more marked than it would be at 

 the present day. The next change most probably would be 

 noticed by the fair sex, who used coloured materials for their 

 dresses, and ladies soon found out that silks, calicoes, or 

 ribbons of certain colours materially changed under the action 

 of light. Here I have three pieces of different coloured 

 materials^ on which you are able to produce an image by 

 the fading of the dyes. These have been exposed under a 

 negative to the action of light, not heat. The usual explana- 

 tion about the fading of these colours is that there is something 

 given off like the scent from a rose ; but when you see a map 

 absolutely printed by light on them, you can have no doubt 

 as to the action which has produced it. If it were that a sort 

 of essence is given off, a negative placed over these 

 colours whilst in the light would produce no defined result 

 whatever, they would fade equally under it ; we therefore 

 cannot help concluding that some chemical change has taken 

 place. Again, you will find most unlikely substances, such 

 as glass, change under the influence of light. My friend Mr. 

 Dallmeyer has some beautiful specimens of glass, which 

 have been altered in this way ; flint glass being changed 

 to a yellow colour and crown glass to a purple tint. Again, 

 we know that there are elements which are affected by light, 

 and amongst them I may mention selenium, a body whose 

 resistance to the passage of a current of electricity it has been 

 proved is diminished by the impact of light. Thus in 

 darkness a piece of selenium 1 '5 in. X '5 in. x '05 in. offered 

 333000 units of resistance to the passage of the current. 

 Whilst in the diffused light it offered nearly 270000 units. 

 It was also found that the resistance was decreased most 

 in the least refrangible portion of the visible spectrum. In 

 the blue it was only 279000 units, in the yellow 277000 

 units, and in the red 255000 units of electrical resistance. 

 I have brought this forward to show to you that a simple 

 elementary body may be acted upon by light. 



We also find that the colouring matter of flowers and of 

 leaves is affected by light. Mrs. Sornerville and Sir John 

 Herschel made a long series of experiments with it. If you 

 take the leaves, say, of common cabbage, and place them in 

 alcohol, a certain coloured resin is extracted, which is known 



