10 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XXV.) 
2729. In order to compare the expected result with the real result due to change 
of volume, I took a bar of iron 7 inches long, and placed it so that the ray from the 
luminous object in passing to the eye should proceed by the side of the bar at not 
more than -^oth of an ineh from it, and then raised the temperature of the bar 
gradually, until by expanding the air in contact with it, the course of the ray of light 
was sensibly affected ; to do this it required to be exalted many degrees. When the 
air of the place was at 60° and the iron raised to 100° Fahr., the effect was not 
distinct. Hence it seemed, that observation of the expected change of volume of the 
air would be rendered far more sensible by some arrangement,' measuring that change 
directly, than by such means as those referred to above, dependent on refractive force ; 
for it is certain that the change of volume, in a very small quantity of air, raised from 
60° to 100°, would be very evident by the former method. On the other hand, it was 
just possible that if the air or gas was affected by the magnet, it might only be in 
that film immediately contiguous to the pole ; and also that great differences in the 
degree of change might exist along the edge of a solid angle, and along the sides of 
the planes forming that angle. Hence the assumed necessity for examining those 
parts by a ray of light ; and every precaution was taken, by inclining the course of the 
ray a little more or less to the sides or edges of the poles, and by making the sides or 
edges very slightly convex, to include every variation -of the experiment, that might 
help to make any magnetic or diamagnetic effect, whether special or local, or general, 
manifest ; but without effect. 
2730. I proceeded, as these attempts had failed, to endeavour to determine and 
compare the volume of air subjected to the magnetic force, before and after its sub- 
jection ; and there seemed to be the greater hope of obtaining some results in this way, 
provided any such change was a consequence of the action of magnetic power, be- 
cause air and gases, at a considerable distance from the surface of the magnet, are 
known to be strongly affected diamagnetically, and because Plucker had already 
said he had obtained such change of volume (2722.). 
2731. The first instrument construeted for this purpose was of the following kind. 
Two blocks of soft iron, each 1 inch thick and 3 inches square, having filed and flat- 
tened surfaces, were prepared ; and also a sheet of copper, of an inch in thick- 
ness and 3 inches square, having its middle part cut away to within 0'3 of an inch 
of the edge all round. This plate or frame was then placed between the iron blocks, 
and the whole held together very tightly by copper screws, so as to make an air- 
chamber tj^th of an inch wide and 2‘4 inches square, having the faees of the blocks, 
which were to become the magnetic poles, for its sides. Three apertures and 
corresponding passages gave access to the interior of this chamber ; small stop- 
cocks were attached to each. By two of these, any gas, after it had been properly 
dried, could be sent into the chamber, or swept out of it, by any other entering gas ; 
