1 70 Mrs. Somerville on the magnetizing Power 



shorter time under these circumstances ; yet in fact, there 

 seemed to be no difference ; most of the needles became mag- 

 netic, some in longer, others in shorter periods, varying from 

 about half an hour to four hours, but depending on circum- 

 stances which I have not yet been able to detect, further than 

 that a number of results induced me to believe, that the ex- 

 periments were more successful from ten to twelve, or one 

 o'clock, than later in the day. The portion of the needle ex- 

 posed was almost always a north pole, whether it pointed up- 

 wards or downwards : in a few instances in which the contrary 

 occurred, it may possibly have arisen from some previous dis- 

 position in the needle to magnetism, too slight to be observed. 



The distance of the needle from the prism was frequently 

 varied by fixing the needle to the wooden pole of a fire-screen, 

 but without material variation in the effect. I found it unneces- 

 sary to darken the room ; it was sufficient to place the prism 

 so as to throw the spectrum on any place out of the sun's rays. 



My next object was to endeavour to ascertain whether any 

 other of the more refrangible rays had the same property as 

 the violet. A set of needles carefully examined as before, were 

 therefore subjected to the different rays of the solar spectrum : 

 the needles exposed to blue and green rays, sometimes ac-? 

 quired the magnetic property, though less frequently, and re- 

 quiring longer exposure than when the violet rays were used ; 

 but the magnetism seemed to be equally strong in these as in 

 the examples of the violet rays. The part exposed became a 

 north pole. The indigo rays succeeded almost as well as the 

 violet. 



Pieces of clock and watch-spring were next tried, under the 

 idea that they might, possibly from their blue colour, be more 

 susceptible of magnetic influence, — and it was the case; their 

 greater extent of surface, however, or their softness, may have 

 contributed to this susceptibility. The pieces of spring were 

 from two to three inches long, and from the eighth to the half 

 of an inch broad. It was difficult to procure watch and clock- 

 spring free from magnetism ; it even happened on one occa- 

 sion, that although the roll of spring was neutral, the pieces 

 into which it was cut became magnetic ; in one instance the 

 pieces were heated so as to deprive them of magnetism and 

 colour : they then required longer exposure to the rays, in or- 

 der to acquire polarity. Large bodkins were exposed to the 

 violet rays without effect, the mass perhaps being too great, 

 When needles already magnetic were tried, their magnetism 

 was increased. Dr. Wollaston was so kind as to lend me a 

 very large lens, having its centre covered with paper, which 

 he had used in his investigations respecting the chemical rays, 



The 



