136 Mrs. M. Somerville on the magnetizing power 
than the prism ; but the rapid motion of the sun made it 
difficult to keep the needle in the focus. The effect was pro- 
duced with equal facility by throwing 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 exa- 
mining 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 
season. There was no iron near, and the magnetic needle 
when placed on various parts of the stone, showed no mag- 
netic influence 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 hold- 
ing muriate of silver in suspension, as a test, in the following 
manner : a piece of writing paper dipped in the liquid was 
