. A marble 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, currents 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.

********************

XIV. PHYSICAL BASIS OF SOLAR CHEMISTRY.

[Footnote: From a discourse delivered at the Royal Institution of
Great Britain, June 7, 1861.]

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 which a column of heated air ascended. Behind the screen, but so
situated that no ray from the ball could reach the instrument, was an
excellent Thermo-electric pile, connected by wires with a very
delicate galvanometer. The pile was known to be an instrument whereby
heat is applied to the generation of electric currents; the strength
o