USES OF MICA. 73 



glass and horn ; but as horn burns and glass cracks when exposed to 

 a flame, mica is still retained to some extent for lantern uses. 



As the transparency of mica is not affected by sudden exposure to 

 heat, or by alternate heating and cooling, and as it is not readily 

 attacked by vapours and gases, it is largely used in anthracite and gas 

 stoves, from which a cheerful glow can be obtained if necessary, with- 

 out exposure to direct heat. Its transparency for light, combined with 

 opacity for radiant heat, creates for it a special usefulness as fire- 

 screens in the peep-holes of furnaces, or as hand- screens in the labor- 

 atory and workshop for inspecting operations proceeding in highly- 

 heated furnaces whose heat would otherwise be intolerable. 



As lamp-chimneys, exposed to cold air draughts or rain-drop 

 splashes, such as those outside drapers' show-windows in dangerous 

 proximity to inflammable materials, or in the case of lamps giving out 

 great heat, as in the case of the incandescent burners, mica is a con- 

 venient substitute for glass. Numbers of other uses for mica, depend- 

 ing on its non-inflammability and flexibility, have been devised; thus 

 the electric cables for street installations are sometimes rolled around 

 with mica films kept in place by tarred twine, whilst the Indian Mica 

 Company have lately proposed to make envelopes of the mineral to 

 preserve valuable documents from fire and insects. 



By far the largest quantity of sheet mica is used for electrical 

 purposes, for which the principal consumption obtains in America. Its 

 highly insulating properties, combined with flexibility, indifference to 

 sudden exposure to high temperatures, and the ease with which sheets 

 can be cut to any shape, render it of great value in covering various 

 portions of dynamos and other electrical machines. For similar and 

 related uses, thin films, down to one-thousandth of an inch in thickness, 

 have lately been used for making the so-called micanite in which the 

 films are made to adhere to one another by a fusible and highly insula- 

 ting cement. The relamination of mica in this artificial preparation is 

 said to increase its insulating property. Plates of micanite when 

 heated can be bent to any form to make, for instance, cylinders for 

 armature shafts and cores, commutator shells and field-magnet cores. 



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