OTHER PROPERTIES OF CAST IRON. 173 
ter preparations for light machinery. Cylinders, 
air pumps, and pistons of steam engines have also 
(in practice) their peculiar compounds ; and it 
is important in all these operations to have con- 
firmed data (the results of actual experiment) 
for directing the labours of the architect, engi- 
neer, and mechanic. 
Tredgold in his essay on the strength of cast 
iron seems to have been aware of the deficiencies 
under which the labours of the iron founder 
have been conducted ; he describes the proper- 
ties of the iron* but gives no proportions for the 
mixtures ; nor have we at the present time any 
guide beyond what is indicated by the appear- 
ance of the fracture. The amalgamation of the 
different metals, however important in practice, 
is generally left to chance; or at best to the 
* Soft iron yields easily to the file, when the external crust 
is removed, and is slightly malleable in a cold state. 
White cast iron is less subject to be destroyed by rusting 
than the grey kind, and it is less soluble in acids ; therefore it 
may be usefully employed when hardness is necessary, and when 
its brittleness is not a detect ; but it should not be chosen for pur- 
poses where strength is necessary. 
White cast iron, in a recent fracture, has a white and ra- 
diated appearance, indicating a crystalline structure ; it is very 
brittle and hard. Gray cast iron has a granulated fracture of a 
gray colour with some metallic lustre; it is much softer and 
tougher than the white cast iron.—Tredgold’s Essay, p 7. 
