30 LECTURES TO SCIENCE TEACHERS. 



The other most interesting compounds to which I would 

 call your attention are those of vanadium. Professor Eoscoe 

 found, during some recent researches, that certain vanadium 

 salts were sensitive to light. Here is the first vanadium 

 print ever produced. It was developed by silver nitrate 

 in a manner similar to that employed with the uranium print. 



I am obliged to pass over some other metallic compounds, 

 but 1 must mention the potassium dichromate, or rather 

 the chromium salts. These salts are the great handmaidens 

 of photographic printing processes at the present day. When 

 you brush a solution of potassium dichromate over paper and 

 expose it to the light, you will find the paper becomes 

 darkened where the light has acted, and an oxide of chromium 

 has been formed, the organic matter in the paper having 

 reduced the potassium dichromate to that state. I have here 

 such a piece of paper which has been exposed under a 

 negative, and washed afterwards ; and you see the green 

 coloration of the chromium oxide. 



Not only is the potassium dichromate reduced to the state 



FIG. 8. 



of oxide, but it also oxidises the organic matter with which 

 it is in contact. If you take gelatine or any similar colloid 

 body and add fco it a solution of potassium dichromate, dry 



