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EXPERIMENTAL PETROLOGY. 
BY COSMO JOHNS, M.I.MECH.E.. F.G.S. 
{Pamper read at the Annual Meeting of the Yorkshire Geological 
Society, Wakefield, November 2nd, 1905, and since revised.) 
No one who has contrasted the wonderful progress made 
in the investigation of slags and alloc s during the last ten years 
with the present state of petrological research after nearly half 
a century of patient work, can fail to recognise that the methods 
of metallurgists must, in some way or other, have been directed 
along more effective lines. That this has been practically 
admitted by petrologists may be seen by a reference to recent 
geological literature. 
What has occurred is that the metallurgist has pressed into 
his service every possible method of research. The main principles 
of physical chemistry formed the basis of his working hypothesis. 
The microscope aided him to study the structure of the substances 
he experimented with. By suitable thermal treatment he 
determined the particular structure to be developed. By 
delicate pyrometric measurement he could learn something of 
the evolution or absorption of heat consequent on the structural 
changes that were taking place during the heating or cooling of 
the mass. Chemical analyses and physical tests would be 
appealed to, and the result is seen in the brilliant results obtained 
^LS a result of the study of alloys — and in a lesser degree, slags 
— by an enthusiastic band of workers in this and other countries. 
In petrology it can hardly be said that any real progress has 
been made since Dr. Sorby, of Sheffield, read his, now classic, 
paper on the application of microscopical methods to the identi- 
fication of the mineral constitutents of rocks. With the exception 
of a few well-meaning attempts to discuss certain physico- 
chemical problems, petrology has remained a mere description 
of the microscopical features of the rocks. Its workers now find 
themselves in the position of having to admit that somehow 
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