Magnetizing Power of Violet Rays.—The Mississippi. 155 
has suggested a more satisfactory theory, being less at variance 
with facts ; for the rainbow is often seen where no rain is falling. 
He, like Newton, makes the effect to depend upon refraction : 
but he aseribes-the refraction to the rays coming through the 
lower edge of a cloud posited between the beholder and the sun, 
which, acting like the prism, divides the rays of light, and exhi- 
bits on the dark opposite sky, as on a curtain, a portion of the 
solar spectrum. The cause and effect which he points out are 
so closely connected, that, especially after a little practice, the 
appearance of the bow may often, from the state of the clouds, 
be predicted with great certainty. 
MAGNETIZING POWER OF VIOLET RAYS. 4 
The discovery of M. Morichini respecting the power which 
was doubted by many, h&s been confirmed by Professor Playfair ; 
who gives the following account of an experiment which he wit- 
nessed :— 
** After having received into my chamber a solar ray, through 
a circular opening made in the shutter, the ray was made to fall 
upon a prism, such as those which are usually employed in expe- 
riments upon the primitive colours. The spectrum which resulted 
from the refraction was received upon askreen ; all the rays were 
intercepted except the violet, in which was placed a needle for 
the purpose of being magnetized. It was a plate of thin steel, se- 
lected from a number of others, and which, upon making the 
trial, was found to possess no polarity, and not to exhibit any at- 
traction for iron filings. It was fixed horizontally on the support 
by means of wax, and in such a direction as to cut the magnetic 
meridian nearly at right angles. By a lens of a sufficient size, 
the whole of the violet ray was collected into a focus, which was 
carried slowly along the needle, proceeding from the centre to- 
wards one of the extremities, and always the same extremity ; 
taking care, as is the case in the common operation of magne- 
tizing, never to go back in the opposite direction. After ope- 
rating fifty-five minutes, the needle was found to be strongly 
magnetic ; it acted powerfully on the compass, the end of the 
needle which had received the influence of the violet ray repelling 
the north pole, and the whole of it attracting, and keeping sus- 
pended, a fringe of iron filings.” 
THE MISSISSIPPI. 
A party of scientific men have built a steam-boat at Pittsburg, 
for the purpose of exploring the productions of the numerous ri- 
vers which empty their streams into the Mississippi. They pro- 
pose setting out next month (March), and expect to be absent 
for at least three years, 
AFRICA. 
