Dec. 1845.] Action of Magnets on Heavy Glass. 399 



pended substance could be varied ac pleasure, by bending any 

 part of the wire at the instant into the hook form. A glass 

 cylinder placed between the magnetic poles was quite sufficient 

 to keep the suspended substance free from any motion, due to 

 the agitation of the air. 



2250. It is necessary, before entering upon an experimental 

 investigation with such an apparatus, to be aware of the effect 

 of any magnetism which the bodies used may possess; the 

 power of the apparatus to make manifest such magnetism is 

 so great, that it is difficult on that account to find writing- 

 paper fit for the stirrup above-mentioned. Before therefore 

 any experiments are instituted, it must be ascertained that the 

 suspending apparatus employed does not point, i. e. does not 

 take up a position parallel to the line joining the magnetic 

 poles, by virtue of the magnetic force. When copper sus- 

 pensions are employed, a peculiar effect is produced (2309.), 

 but when understood, as it will be hereafter, it does not in- 

 terfere with the results of experiment. The wire should be 

 fine, not magnetic as iron, and the form of the suspending 

 cradle should not be elongated horizontally, but be round or 

 square as to its general dimensions, in that direction. 



2251. The substances to be experimented with should be 

 carefully examined, and rejected if not found free from mag- 

 netism. Their state is easily ascertained; for, if magnetic, 

 they will either be attracted to the one or the other pole of 

 the great magnet, or else point between them. No examina- 

 tion by smaller magnets, or by a magnetic needle, is sufficient 

 for this purpose. 



• 2252. I shall have such fre- 

 quent occasion to refer to two e 

 chief directions of position across 

 the magnetic field, that to avoid /////// /2 



periphrasis, I will here ask leave '////, 



to use a term or two conditionally. 



One of these directions is that »* 



from pole to pole, or along the 



line of magnetic force ; I will call it the axial direction : the 



other is the direction perpendicular to this, and across the 



line of magnetic force; and for the time, and as respects the 



space between the poles, I will call it the equatorial direction. 



Other terms that I may use, I hope will explain themselves. 



% ii. Action of magnets on heavy glass. 

 2253. The bar of silicated borate of lead, or heavy glass 

 already described as the substance in which magnetic forces 

 were first made effectually to bear on a ray of light (2152:), 



