of the more refrangible Solar Rays. 171 



The lens concentrated the violet rays, and produced a mag- 

 netic effect in a shorter time than the prism ; but the rapid 

 motion of the sun made it difficult to keep the needle in the 

 focus. The effect was produced with equal facility by throw- 

 ing the spectrum on the floor of the room ; but success could 

 not always be depended upon even when the weather seemed 

 most favourable. 



I now made the following experiments with blue glass : — 

 Three needles free from magnetism, having "one half covered 

 with paper, were laid horizontally on the 'stone outside of a 

 south window, under a dark blue glass coloured by cobalt, 

 in a very hot sun; after remaining in this position between 

 three and four hours, they were found to have become feebly 

 magnetic; the uncovered part being a north pole. On ex- 

 amining these needles the following day, they had lost their 

 magnetism, a circumstance which had not before occurred, 

 though it was observed sometimes to take place afterwards, as 

 the force of the sun diminished from the advance of the sea- 

 son. There was no iron near, and the magnetic needle when 

 placed on various parts of the stone, showed no magnetic in- 

 fluence in it. Next day the experiment was repeated with 

 this difference, that the needles were left exposed to the sun, 

 under the blue glass, six hours ; and then the needles had not 

 only acquired very sensible magnetism, but still retain it, at 

 the distance of nearly six months. Pieces of clock-spring, 

 which had been heated as formerly mentioned, also became 

 magnetic under the blue glass. 



I was desirous of ascertaining whether this kind of glass 

 suffered the chemical rays to pass, and thereby occasion these 

 changes in the steel, therefore I employed a liquid holding 

 muriate of silver in suspension, as a test, in the following 

 manner : A piece of writing paper dipped in the liquid was cut 

 into two equal parts, of which one was placed under the blue 

 glass, and the other under a white glass, as nearly at the same 

 time as possible; but the one did not become black sooner 

 than the other ; nor on comparing them could any difference 

 be perceived in intensity of colour, both having been equally 

 exposed to the chemical rays; The experiment was repeated 

 with the same result. 



On the 26th of August, the thermometer at noon being 66°, 

 two neutral pieces of clock-spring were exposed to the sun, 

 one under a thicker piece of the same blue glass, as in the 

 former experiment, and the other under green glass ; both ac- 

 quired polarity. 



31st of August, the thermometer at noon 68°. Having thus 



Y 2 succeeded 



