CHAP. II.] ELECTRICITY AND MAGNETISM. 77 



Gilbert suggested that the force of a magnet might be 

 measured by making it attract a piece of iron hung to 

 one arm of a balance, weights being placed in the scale- 

 pan hanging to the other arm ; and he found, by hang- 

 ing the magnet to the balance and placing the iron 

 beneath it, that the effect produced was the same. The 

 action and reaction are then equal for magnetic forces. 



84. Attraction across bodies. If a sheet of 

 glass, or wood, or paper, be interposed between a magnet 

 and the piece of iron or steel it is attracting, it will still 

 attract it as if nothing were interposed. A magnet 

 sealed up in a glass tube still acts as a magnet. Lucre- 

 tius found a magnet put into a brass vase attracted iron 

 filings through the brass. Gilbert surrounded a magnet 

 by a ring of flames, and found it still to be subject to 

 magnetic attraction from without. Across water, vacuum, 

 and all known substances, the magnetic forces will act ; 

 with the single exception, however, that magnetic force 

 will not act across a screen of iron or other magnetic 

 material. If a small magnet is suspended inside a 

 hollow ball made of iron, no outside magnet will affect it. 

 A hollow shell of iron will therefore act as a magnetic 

 cage^ and screen the space inside it from magnetic 

 influences. 



85. Magnetic Substances. A distinction was 

 drawn by Gilbert between magnets and magnetic 

 substances. A magnet attracts only at its poles, and 

 they possess opposite properties. But a lump of iron 

 will attract either pole of the magnet, no matter what 

 part of the lump be presented to the magnet. It has no 

 distinguishable fixed "poles," and no magnetic "equator." 

 A true magnet has poles, one of which is repelled by the 

 pole of another magnet. 



86. Other Magnetic Metals. Later experimenters 

 have extended the list of substances which are attracted 

 by a magnet. In addition to iron (and steel) the follow- 

 ing metals are recognised as magnetic : 



