Constants of Igneous Rock. 175 



Table I. — -Density of Diabase before and after Fusion. 





Before 



Fusion 







After Fusion 





1 Sample 

 No. 



M. 



t. 



A. 



Sample 

 No. 



M. 



t. 



A'. 



A-A' 



A ' 



I. 



9 

 22-8950 



°C. 

 25 



30161 



*I. 



9 



69 9330 



°C. 



21 



2-7018 



•104 



II. 



45-3654 



21 



3-0181 



III. 



33-7659 



19 



2-7447 



•090 



III. 



54-7208 



21 



30136 



till. 



29-9777 



19 



2-7045 



•103 



IV. 



69-4940 



21 



30235 













The rock was fused both in clay and in platinum crucibles. 

 In the latter case the density of the known mass of metal had 

 been previously determined, and the glass was not removed 

 from the crucible. In the other case the clay was broken 

 away from the solid lump within, and its density then measured 

 directly. A few small bubbles were visible on the fresh 

 fracture of the glass, due, I presume, to the ejection of dis- 

 solved gases on solidification. At some other time I will 

 make vacuum-measurements of the density of powders both 

 of the rock and the glass, but I do not believe the data of 

 Table I. will be seriously changed by such a test. 



From Table I. it appears that the mean density of the 

 original rock is 3*0178 ; that of the glass after fusion is only 

 2*717, indicating a volume increment of 10 per cent, as the 

 effect of fusion. This remarkable behaviour is not isolated. 

 Niess (I. c. p. 47), quoting from ZirkePs Lehrbuch, adduces 

 even more remarkable volume-increments of the same nature t ; 

 viz., in garnet 22 per cent., in vesuvianite 14 per cent., in 

 orthoclase 12 per cent., in augite 13 per cent., &c. : but I 

 doubt whether the great importance of these facts has been 

 sufficiently emphasized. Suffice it to indicate here that it 

 makes an enormous difference into what product the magma 

 is to be conceived as being solidified ; and that throughout 

 this paper the molten rock solidifies into a homogeneous 

 obsidian. I am only determining, therefore, those volume- 



* Fused in a clay crucible. Glass detached wlieu cold. 

 f Fused in a platinum crucible. Glass not detached. 

 % Cf. Thoulet, Ztschr, f. Kryst. u. Mimralogie, v. p. 407 (1881) ; Bull. 

 Soc. Min, de France, iii. p. 34 (1880). 



