MICROSCOPIC OBJECTS; 93 



covered over it with some Canada balsam between the 

 two. 



Iron. — This metal as employed in the arts and manu- 

 factures varies very much in purity and quality. In 

 some cases where great strength is required with a mini- 

 mum of weight, it is important to select the hardest 

 kinds the market affords, as for plough- shares, which 

 when used on stiff soils are subject to great strain ; hence 

 for these implements the Derbyshire iron is preferred. 

 The hot blast iron, which has very inferior cohesion from 

 its containing particles of carbon, is unfit to be used for 

 cannon and similar purposes, but, being much cheaper 

 than the other sorts, is sometimes substituted for the 

 better kinds. To examine the quality of iron by the 

 microscope a piece should be broken off, and its fractured 

 sm'face, wdiile clean and free from oxidation, submitted 

 to that instrument as an opaque object. A strong light 

 should be throv^ni upon it, and all the specimens should 

 be examined under the same magnifying power. So im- 

 portant has a microscopic examination of this metal been 

 deemed by the Government, that Her Majesty's Honor- 

 able Board of Ordnance have directed Mr. Pritchard to 

 construct a microscope for the Arsenal at Woolwich. 



Iron, Elba. — This ore, from the richness of the 

 coloured oxides on its surface, and the grouping of its 

 crystals, afford a good opaque object for a low power, 

 say 30 diameters. 



Jasper. — Of the several kinds of this stone, the agate 

 jasper, when cut into thin plates about -^q^ of an inch 



