and Rock Densities at High Temperatures. 5 



Forbes,* Mallet, f and Barns,:}: all of whom observed contraction 

 in passing; from liquid to solid. Joly used a novel method§ in 

 observing the expansion of spherical beads of different mineral 

 glasses with rising temperature, by projecting the magnified 

 image of a bead on a screen. All of these measurements have 

 been open to the serious criticism mentioned in a preceding par- 

 agraph, that the substances cooled partly or wholly to glass; 

 the measurements therefore did not include (or only partly in- 

 cluded) the change of state liquid-solid concerning which infor- 

 mation was sought. 



The measurements by Barus were probably the best up to 

 that time, and have been so frequently quoted that they deserve 

 further description. His method consisted in completely filling 

 a platinum tube with melted diabase, [j and following the down- 

 ward movement of the surface, as the melt cooled, by means 

 of a platinum contact point carried by a cathetometer outside 

 of the furnace. 



By preliminary fusion in the open air, Barus found that the 

 melted diabase cooled to a glass without crystallizing. He states, 

 therefore, that " throughout this paper the molten rock solid- 

 ifies into an obsidian. "^[ On the basis of his own statements 

 it has therefore been assumed that he really did not observe 

 the volume change accompanying the passage from liquid to 

 crystalline, and the irregularity which he undoubtedly did 

 observe has remained an unexplained anomaly. But, as we 

 shall see later, the rock really crystallized in part under the 

 conditions of slow cooling within his platinum tube, and the 

 sudden volume change which he observed at about 1095° repre- 

 sents a part of the difference in volume between crystalline and 

 glassy rock at that temperature. 



According to Barns' figures, the difference in specific volume 

 between diabase and diabase glass at 20° is 11 per cent of the 

 volume of the crystalline rock. At about 1100° he found a 

 contraction of 3 - 9 per cent in the glass.** 



Barus also observed that on reversing the measurements and 

 heating the rock until fluid, the volume change was more grad- 

 ual and came at a higher temperature than the contraction on 

 solidification. The reason for this will become clear from our 

 own experiments. 



*D. Forbes, Chem. News, p. 6, 1868. 



f E. Mallet, Phil. Trans., clxiii, 147-227, 1873. 



JC. Barns, U. S. Geol. Survey Bull., ciii, 25-44, 1893. 



§ J. Joly, Trans. Roy. Soc. Dublin (2), vi, 283-304, 1847. 



I The rock is described as " a typical diabase obtained from Mr. Clarence 

 King " (then Director of the U. S. Geological Survey). No information as to 

 its origin was published. It appears to have come from the Palisade diabase 

 along the Hudson Eiver. 



IT Loc. cit. (Bull, ciii), p. 26 ; also pp. 25, 36. 



**Loc. cit., p. 41. 



