158 MR. J. A. DOUGLAS ON CHANGES OP PHYSICAL [May I907, 



Eutectic Mixtures. 1 



The relation which undoubtedly exists between saline solutions 

 and igneous magmas is one that bears strongly on all problems of 

 petrology. Akerman 2 , experimenting with silicate-slags similar in 

 composition to igneous rocks, has demonstrated that mixtures of 

 two given minerals show a distinct lowering of the melting-point, 

 the greatest lowering taking place when they are mixed in eutectic 

 proportions. This phenomenon is analogous to that observed in a 

 saline solution, such as a mixture of salt and water. 



In 1888, in his ' British Petrography,' pp. 401-402, Dr. Teall 

 suggested that a micropegmatitic intergrowth of quartz and felspar 

 probably represented an eutectic mixture, being the last product 

 of consolidation in the rock-magma. 



In dry melts, such as those obtained by Akerman, certain 

 minerals are typically absent ; these include quartz and orthoclase- 

 felspar. The eutectic proportions of these minerals cannot, then, 

 be determined by cooling a molten rock or silicate-slag in the 

 laboratory. The only method that appears possible is to deter- 

 mine the melting-points of certain mixtures of the two minerals, 

 and by this means endeavour to find the eutectic mixture, that is, 

 the one with the lowest melting-point. 



From the careful analyses of pegmatites from various localities, 

 which have been made by Dr. Teall, Prof. Vogt, and Prof. Lagorio, 

 the eutectic proportions have been calculated to be about 70 per 

 cent, of felspar with 30 per cent, of quartz. These proportions 

 have apparently never been determined experimentally, although 

 some years ago Prof. Joly 3 ascertained that the melting-point of 

 micropegmatite was slightly lower than that of orthoclase. 



The following is a brief account of an attempt to find experiment- 

 ally the eutectic proportions of quartz and orthoclase by determining 

 the melting-points of glasses corresponding in composition with the 

 formulas OrQ, 6 , OrQ, 3 , 0rQ 2 (eutectic), Or 2 Q 3 . These glasses were 

 obtained by fusing carefully-weighed proportions of chemically-pure 

 artificially-prepared silica and purified orthoclase in the platinum- 

 furnace. 



The eutectic proportion, so far as could be observed, showed no 

 lowering of the fusion-point ; in fact, there was a gradual rise in 

 temperature with the increase in the percentage of silica, showing 

 a certain resemblance to Type IV of Bakhuis Koozeboom (8). 



This result cannot, however, be taken as a proof telling against 

 the eutectic hypothesis, since the viscosity of the glasses is of so 

 high an order of magnitude that it is practically impossible, with the 

 comparatively-rapid heating necessary when using the meldometer, 

 to determine accurately the point at which melting commences. 



1 An excellent summary of recent work on the subject of eutectics is given by 

 Prof. H. A. Miers, in his Presidential Address to the Geological Section of the 

 British Association, at the Meeting in South Africa, 'Nature' vol. lxxii 

 (Aug. 24th, 1905) pp. 408 et segg. 



2 'Die zum Schmelzen von Schlacken erforderlichen Warmemengen ' Iern- 

 kontorets Annaler, 1886 ; also in ' Stahl & Eisen ' 1886. 



3 See J. J. H. Teall, Quart. Journ. Geol. Soc. vol. lvii (1901) p. lxxvi. 



