250 FRAGMENTS OF SCIENCE. 
statue, on the contrary, has its feet a south pole, and its 
head a north pole, and there is no doubt that the same 
remark applies to its living archetype; each man walking 
over the earth's surface is a true diamagnet, with its poles 
the reverse of those of a mass of magnetic matter of the 
same shape and position. 
An experiment of practical value, as affording a ready 
estimate of the different conductive powers of two metals 
for electricity, was exhibited in the lecture, for the purpose 
of proving experimentally some of the statements made in 
reference to this subject. A cube of bismuth was suspended 
by a twisted string between the two poles of an electro- 
magnet. The cube was attached by a short copper wire to 
a little square pyramid, the base of which was horizontal, 
and its sides formed of four small triangular pieces of 
looking-glass. A beam of light was suffered to fall upon 
this reflector, and as the reflector followed the motion of 
the cube the images cast from its sides followed each 
other in succession, each describing a circle about thirty 
feet in diameter. As the velocity of rotation augmented, 
these images blended into a continuous ring of light. At 
a particular instant the electro-magnet was excited, cur- 
rents were evolved in the rotating cube, and the strength 
of these currents, which increases with the conductivity of 
the cube for electricity, was practically estimated by the 
time required to bring the cube and its associated mirrors 
to a state of rest. With bismuth this time amounted to a 
score of seconds or more: a cube of copper, on the contrary, 
was struck almost instantly motionless when the circuit 
was established. 
CHAPTER XIV. 
PHYSICAL BASIS OF SOLAR CHEMISTKY.* 
OMITTING all preface, attention was first drawn to an 
experimental arrangement intended to prove that gaseous 
bodies radiate heat in different degrees. Near a double 
screen of polished tin* was placed an ordinary ring gas- 
burner, and on this was placed a hot copper ball, from 
* From a discourse delivered at the Royal Institution of Great 
Britain, June 7, 1861. 
