Vol. 63.] CHANGES OF PHYSICAL CONSTANTS IN MINERALS, ETC. 145 



11. On Changes of Physical Constants which take place in certain 

 Minerals and Igneous Rocks, on the Passage from the 

 Crystalline to the Glassy State ; with a short Note on 

 Eutectic Mixtures. By James Archibald Douglas, B.A., 

 F.G.S., Burdett-Coutts Scholar in the University of Oxford. 

 (Read March 13th, 1907.) 



The effect of high temperatures on minerals and igneous rocks has 

 long been a subject of geological research. De Saussure, in 1779, 

 was the first to remark the fact that igneous rocks diminished in 

 density on fusion, and became converted into a glassy substance. 

 Further investigations as to these phenomena were made by 

 Dolomieu in 1788, Sir James Hall in 1790, Magnus in 1831, and 

 Bischof in 1841. The work of these early observers inspired 

 Delesse in 1846 to publish an important paper on the glasses 

 formed by the fusion of igneous rocks. This paper gives a good 

 idea of the methods employed, at a time when electrical heating 

 was an unknown factor in scientific research. With the improved 

 appliances of modern science, the number of investigators has so 

 greatly increased that it is only possible to mention the more 

 recent researches of Q Barus, Dcelter, Joly, Teall, Cusack, Bakhuis 

 B/Oozeboom, Yogt, Akerinan, Allen, and Day. 



At the present time, every well-appointed laboratory has its 

 electrical installation, and high temperatures may thereby be easily 

 attained. As I had a strong current at my disposal, Prof. Sollas 

 suggested that I should melt certain minerals and igneous rocks by 

 means of a simple platinum-resistance, and endeavour to redetermine 

 the change of specific gravity which accompanies their fusion. 



In the following paper I have given a brief account o£ some 

 experiments on the fusion of igneous rocks, and the determination 

 of their specific gravities in the glassy state ; I have also added a 

 few observations on the thermal properties of the felspars, and an 

 attempt to determine experimentally the existence of eutectic 

 mixtures of quartz and orthoclase, and orthoclase and albite. 



Before proceeding with an account of my apparatus, it may be 

 advisable to give a short summary of the methods employed by 

 Delesse (4) 1 in his memoir of 1846, as no important paper on the 

 glasses of igneous rocks seems to have been published since then. 



Somewhat less than a cubic decimetre of the rock to be examined 

 was crushed up and placed in a Hessian crucible, which was heated, 

 at first gradually, and finally with a sudden increase of temperature, 

 in an ordinary glass-furnace to a temperature about the melting- 

 point of orthoclase. After some time, the crucible was withdrawn 

 from the furnace and allowed to cool rapidly, the product of the 

 fusion being in the form of glass. This glass, after being removed 

 from the crucible, was crushed and its specific gravity was measured 



1 Numerals in parentheses throughout this paper refer to the Bibliographical 

 List, p. 160. 



