380 . FRAGMENTS OF SCIENCE. 



to be polar, the disposition of the force being the same as 

 that of bismuth and the reverse of that of iron. When 

 a bar of iron is set erect, its lower end is known to be 

 a north pole, and its upper end a south pole, in virtue 

 of the earth's induction. A marble statue, on the con- 

 trary, 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. 



