28 LECTURES TO SCIENCE TEACHERS. 



pounds, and to him are due a variety of most interesting 

 processes, examples of one or two of which. I shall endeavour 

 to show yon. 



Whilst mentioning the above great philosopher, I should 

 like to point out to you the instrument with which he 

 operated when experimenting on the effect that light pro- 

 duced on different organic and metallic compounds. 



FIG. 7. 



A is a glass prism which could be rotated in its frame 

 round an axis D, so that the sun's rays would be dispersed 

 in any given direction. B is a lens denning the spectrum 

 (which, it must be recollected, was a mixed one, and not 

 pure). C the screen on which the spectrum was raised, and 

 on which the compound to be tried was placed. At E was 

 marked a line on which one particular part of the spectrum 

 was invariably caused to fall. When used the frame-work 

 was covered with a black velvet cloth. The absolute results of 

 the experiments on the different compounds are shown in an 

 old book containing a list of Fellows of the Royal Society, 

 over whose distinguished names they have been pasted. 

 This book is perhaps one of the most interesting exhibits 

 in the Loan Collection. 



Reverting to the iron salts I may point out that those of 

 them which are in the ferric state are the most readily acted 

 upon by light ; the ferrous salts, as a rule, not being sensi- 

 tive. A variety of ferric salts may be formed, such as ferric 

 chloride, or ferric oxalate ; or you may have a compound of 

 ferric citrate with citrate of ammonia, and so on. Of all 

 the compounds of iron, Sir John Herschel found that this 

 latter ferric salt, when employed with the ammonium citrate, 



