36 Dr. Playfair’s Lecture on the Great Erhibition of 1851. 
Prussian blues The naphthaline, an enemy to the gas-manufac- 
turer by choking the pipes, may be made into a beautiful red 
coloring matter, closely resembling that from madder. This, by 
its transformation, promises an important, though hitherto not 
yet realized useful product.. Coal, when distilled at a lower 
temperature than that requiredto form gas, produces an oil con- 
taining paraffin, largely used asan antifrictional oil for light 
machinery. 
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