60 THE FIGURE OF THE EARTH 



Professor Joly* and Dr. Carl Barus,! which provide us, 

 by direct observation, with the coefficients of expansion 

 of diabase and basalt up to and beyond the melting- 

 point. 



Mr. Fisher has taken the coefficient, supposed to be 

 constant as 0'0000071. 



The observations of Carl Barus show that between 

 C. and 855 C. the value of e is 0'000024, or a little 

 more than three times as great as has been supposed ; 

 between 855 and 914 C. it rises to 0*00003576, and 

 between 914 and 1,093 it is 0'0000345, a falling off, due 

 probably to errors of experiment ; between 1,093 and 

 1,190 C. a very rapid expansion takes place, the incre- 

 ment of volume being 0'0343, the original volume being 

 taken as unity ; this gives a coefficient of expansion of 

 0'000354. No doubt this sudden change indicates a 

 change of condition possibly from the crystalline to the 

 glassy state ; the rock now becomes viscous, and with 

 further rise of temperature expands like a liquid, the 

 average coefficient between 1,190 and 1,421 C. being 

 0-000047. 



Professor Joly's results are sufficiently concordant with 

 those of Dr. Barus, but since they w T ere not made on 

 crystalline rock, but on glass obtained from it by fusion, 

 they do not show so sudden change in volume just 

 below the fusion-point. It is just possible they would for 

 this reason be more applicable to the material forming 

 the interior of the earth, but we have very little evidence 

 as to the mode of behaviour of matter restrained from 

 fusion by excess of pressure. 



It is to be hoped that some one skilled in the higher 



:;: "On the Volume Change of Rocks and Minerals attending 

 Fusion," Sci. Proc. Trans. Eoy. Dublin Soc., vol ii., series ii., p. 283, 

 .1897. 



t Am. Journ. Sci., vol. xlii., 1891, p. 498; vol. xliii., 1892, p. 56. 



