REPORT OF THE CHIEF ASTRONOMER 741 



SESSIONAL PAPER No. 25a 



some crystallization of the melt took place during the experiment. Such 

 crystallization was inevitable under the conditions of the experiment, in which 

 the cooling lasted several hours. Barus's curves do not, therefore, show directly 

 the volume changes suffered by pure diabase glass in passing from the molten 

 isotropic state to the rigid isotropic state at room temperature. Excluding the 

 'solidification' contraction, the glass loses but 3-5 per cent of its volume in 

 passing from the molten state at 1400° C. to room temperature; the loss of 

 volume through the same temperature interval was calculated in the first paper 

 as about 8 per cent. Barus found that the net decrease in specific gravity in 

 passing from rock at 20° C. to glass at 20° C. was 10 per cent. For his diabase 

 specimen, therefore, the decrease of specific gravity in passing Irom 20° C. to 

 molten condition at 1200° C. is possibly only about 13 per cent, instead of about 

 16 per cent, as noted in the first paper. 



Quite recently J. A. Douglas has made a number of very careful measure- 

 ments of the densities of typical igneous rocks and of their respective glasses, 

 all specific gravities being taken at room temperatures. Douglas's method 

 is reliable and his results accordant. For gabbro he found the decrease of specific 

 gravity, in passing from rock to glass, to be 5-07 per cent. Delesse had found 

 the decrease to be 11-46 per cent, as the average of measurements of two speci- 

 mens from different localities. Barus's determination, 10 per cent, is interme- 

 diate between the two. 



It seems probable, therefore, that a decrease of 6 per cent in specific gravity 

 (iock to glass at 20° C.) is close to the minimum for the average gabbroid 

 rock, and it is possible that Barus's 10 per cent decrease is too high for average 

 gabbro. For present purposes it is safer to use the minimum value of 6 per 

 cent. 



Among the most reliable of the older determinations are those due to 

 Delesse and Cossa. These are noted as follows (Table XLVLTL). 



For purposes of comparison the analogous results of the Carnegie Geophy- 

 sical Laboratory experiments with minerals are given in tabular form (Table 

 XLIX.) :— 



25a— vol. iii — 48J 



