SPECIFIC GRAVITY 63 



rubes, with only the alternate solid angles similarly replaced 

 (figs. 40, 4i, page 37.) Each solid angle, on heating the 

 crystals, becomes an electric pole ; the angles diagonally 

 opposite, are differently modified and have opposite polarity 



Magnetism. — Lodestone includes certain specimens of an 

 ore of iron, called magnetic oxyd of iron, having the power 

 of attraction like a magnet; it is common in many ore beds 

 where this ore of iron occurs. When mounted like a horse- 

 shoe magnet, a good lodestone will lift a weight of many 

 pounds. This is the only mineral that has decided magnetic 

 attraction. But several ores containing iron are attracted by 

 the magnet, or, when brought near a magnetic needle, will 

 cause it to vibrate ; and moreover, the metals nickel, cobalt, 

 manganese, palladium, platinum and osmium, have been 

 found to be slightly magnetic. 



Many minerals become attractable by the magnet after 

 being heated, that are not so before heating. This arises 

 from a partial reduction, developing the protoxyd of iron. 



SPECIFIC GRAVITY. 



The specific gravity of a mineral is its weight compared 

 with that of some substance, taken as a standard. For solids 

 and liquids, distilled water at 60° F. is the standard ordinarily 

 used ; and if a mineral weighs twice as much as water, its 

 specific gravity is 2 ; if three times, it is 3. It is then 

 necessary to compare the weight of the mineral with the 

 weight of an equal bulk of water. The process is as follows : 



First weigh a fragment of the mineral in the ordinary way, 

 with a delicate pair of scales : next sus- 98 



pend the mineral by a hair or fiber of 

 silk to one of the scales, immerse it thus 

 suspended in a tumbler of water, (keep- 

 ing the scales clear of the water,) and 

 weigh it again : subtract the second weight 

 lrom the first, to ascertain the loss by im- 

 mersion, and divide the first by the dif- 

 ference obtained : the result is the spe- 

 cific gravity. The loss by immersion is 



What ore is at times possessed of magnetic attraction 1 What i* 

 said of other minerals as regards magnetism ? What is specific gravity t 

 Explain. Mention the mode of ascertaining specific gravity. 



I 



