26 



Day, etc. — Determination of Mineral 



cold it formed a clear dark brown glass, with a refractive 

 index varying from 1*59 to 1*61. No bubbles were visible, 

 even under the microscope, and no crystalline material except 

 at a few points on the surface of the block. These were cut 

 away by grinding. 



It was to be expected that this fusion would not only drive 

 off the w T ater but would also cause some change in the state of 

 oxidation of the iron, so that the glass would not retain exactly 

 the composition of the original rock. The amount of moisture 

 was found to be very small (see Table IV). The amount of 

 change in the iron was determined analytically, and is shown 

 in Table IV. For the present approximate determinations of 

 the volume change, these small changes may be neglected, for 

 they affect but a small percentage of the rock. 



Table IV. 

 Changes in State of Oxidation of the Iron in Diabase. 



Original Eock 



Original Eock 



dried at 105° 



Glass, heated to 1346° 

 June, 1912 



Eecrystallized block 

 after volume deter- 

 minations under 

 tin. (From glass of 

 June, 1912)... 



Fe 2 3 

 % 



FeO 

 % 



Ferrous 

 Iron % 



Ferric 

 Iron % 



Total 

 Iron % 



1-51 



8'74 



6-79 



1-06 



7-85 



1-52 



8-79 



6-83 



1-07 



7-89 



4-54 



608 



4-73 



3-18 



7-91 



2-90 



7-59 



5-90 



2-03 



7-93 



H 2 0<£ 



0-61 



The volumes of the rock and its glass at room temperature 

 are given in Table V, in comparison with similar constants 

 determined by Barus for his sample of diabase (see p. 5). 



The experimental data are plotted in fig. 9. For the sake of 

 clearness the curves of rising and of falling temperature are 

 plotted separately. No satisfactory measurements were obtained 

 on the second block below 900°, as internal cracking in the 

 glass block caused its volume to appear considerably too large 

 until these cracks were opened by the shrinkage during crys- 

 tallization. 



It will be seen from the curves that the glass expands 

 slightly up to about 900°, when a sudden shrinkage takes place. 

 This is due to the crystallization of the glass, which is of 



