6 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XIX.) 
posing within itself of their force. Thus, when the two contrary poles were on one 
side of the polarized ray (2150.), and the heavy glass in its best position between 
them and in the ray (2152.), the bringing of a large piece of iron near to the glass 
on the other side of the ray, caused the power of the diamagnetic to fall. This w r as 
because certain lines of magnetic force, which at first passed through the glass 
parallel to the ray, now crossed the glass and the ray ; the iron giving two contrary 
poles opposite the poles of the magnet, and thus determining a new course for a 
certain portion of the magnetic power, and that across the polarized ray. 
2169. Or, if the iron, instead of being applied on the opposite side of the glass, 
were applied on the same side with the magnet, either near it or in contact with it, 
then, again, the power of the diamagnetic fell, simply because the power of the magnet 
was diverted from it into a new direction. These effects depend much of course on 
the intensity and power of the magnet, and on the size and softness of the iron. 
2170. The electro-helices (2190.) without the iron cores were very feeble in power, 
and indeed hardly sensible in their effect. With the iron cores they were powerful, 
though no more electricity was then passing through the coils than before (1071 •)♦ 
This shows, in a very simple manner, that the phenomena exhibited by light under 
these circumstances, is directly connected with the magnetic form of force supplied 
by the arrangement. Another effect which occurred illustrated the same point. 
"When the contact at the voltaic battery is made, and the current sent round the 
electro-magnet, the image produced by the rotation of the polarized ray does not 
rise up to its full lustre immediately, but increases for a couple of seconds, gradually 
acquiring its greatest intensity ; on breaking the contact, it sinks instantly and dis- 
appears apparently at once. The gradual rise in brightness is due to the time which 
the iron core of the magnet requires to evolve all that magnetic power which the 
electric current can develope in it ; and as the magnetism rises in intensity, so does 
its effect on the light increase in power ; hence the progressive condition of the 
rotation. 
21/1. I cannot as yet find that the heavy glass (2151.), when in this state, i. e. with 
magnetic lines of force passing through it, exhibits any increased degree, or has any 
specific magneto-inductive action of the recognized kind. I have placed it in large 
quantities, and in different positions, between magnets and magnetic needles, having 
at the time very delicate means of appreciating any difference between it and air, 
but could find none. 
2172. Using water, alcohol, mercury, and other fluids contained in very large 
delicate thermometer-shaped vessels, I could not discover that any difference in 
volume occurred when the magnetic curves passed through them. 
2173. It is time that I should pass to a consideration of this power of magnetism 
over light as exercised, not only in the silicated borate of lead (2151.), but in many 
