145 
they present under the microscope. Referring to the che- 
mical analysis of coal ash, we find that it sometimes contains 
silica, magnesia, alumina, sesqui- oxide of iron, lime, soda, 
potash, sulphate of calcium, anhydrous sulphuric acid, anhy- 
drous phosphoric acid, sulphur, and sometimes traces of 
copper and lead. The vegetable origin of coal is now 
generally admitted, and doubtless some of the substances I 
have just named have been taken up by the coal plants, 
whilst other portions may have collected in the locality where 
the coal was formed. As this is not immediately connected 
with our present inquiry, I proceed to speculate as to the 
constitution of these globular bodies. The transparent 
spheres I imagine to be silicates of soda or potash; the 
opaque white are most likely silicate of soda or potash 
combined with lime and alumina ; the yellow and brown 
are silicates coloured by iron in different proportions. The 
black globes are not all alike in composition ; some of these 
are silicates coloured by carbon, others are iron balls coated 
externally with a silicate. Many of these rusty cannon 
balls are probably ferrous oxide formed by the action of 
heat on the iron pyrites in the coal. There are also balls of 
black magnetic oxide: the perforated shells are probably 
ferrous sulphides, The globular form of these bodies 
suggests that they have been thrown off in scintillations, 
such as are seen during the combustion of iron in oxygen 
gas, and whilst in a fluid state they assume a spheroidal 
form. They are carried by the draught into the flue, and 
being of greater specific gravity than the carbonaceous 
matter forming the smoke, they fall before the current of 
air has reached the chimney. Some of the dust has been a 
considerable time in the flue, exposed to the intensely 
heated circulating flame ; the reducing action of this would 
probably convert some of the oxide into metallic iron. 
Many of these balls have the appearance of reduced oxides. 
The flue dust contains a larger amount of ferruginous 
