250 FRA GMENTS OF SCIENCE. 



statue, on the contrary, lias 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 CHEMISTRY.* 



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. 



