Constants of Igneous Rock. 189 



19. Flotation. — Naturally I made a few tests on the flota- 

 tion of solid rock on the molten magma. Cf. § 3. To my 

 surprise such flotation usually occurs, notwithstanding the 

 fact that the original cold rock may be 8 per cent. -f 10 per 

 cent, more dense than the molten magma. The cause, how- 

 ever, is crudely mechanical, since the rock, in virtue of its 

 weight and temperature, hollows out a cavity and chills its 

 surface simultaneously, forming a little boat in which it floats 

 on the very viscous liquid below. This is indicated in fig. 9, 

 where a is the body of rock, AA the molten magma, and bb 

 the solidified skin. I also attempted to make Mess and 

 Winkelmann's " Fundamentalversuch " (Niess, I.e. p. 16), 

 by submerging the rock ; but here, both on account of the 

 intense white glare of the furnace and the tendency to chill 

 at the surface, I did not reach a definite point of view. 



Inferences. 



20. Hysteresis.— -In the above experiments I have only 

 studied the solidifying magma. It does not appear, there- 

 fore, whether solidification and fusion take place at identical 

 temperatures, or whether they will comprehend a volume-lag. 



In other work, in which the rock of the fusion-tube was 

 alternately fused and solidified, the loci are cyclic in marked 

 degree, covering considerably more than 50°. It so difficult, 

 however, to discriminate between true hysteresis and the 

 accompanying discrepancy due to insufficiently rapid heat 

 conduction as compared with the insufficiently slow change 

 of temperature, that experiments made with a tube which is 

 not at quite the same temperature throughout its length are 

 not unassailable. Having failed to perfect the experiments, 

 I omit the data altogether. 



21. Melting-point and Pressure. — Since the fusion of rocks 

 like diabase is thoroughly normal, it follows that melting- 

 point must increase with pressure. It is well to examine 

 tentatively into the nature of this relation, and for this pur- 

 pose I have constructed certain unpublished results of mine 

 for thymol, in the same scale as used for the rock, in fig. 11. 

 The curve m'n'o'p' shows the contraction of thymol at its 

 melting-point (somewhat below 50°), where the substance 

 is liquid along o'p' and solid along m { n'. The curve m n op 

 similarly applies at 0°C. It is seen, therefore, that here 

 solidification contraction and thermal expansion (solid or 

 liquid) decrease together. This is also true on passing from 

 thymol to the rock. Could liquid thymol be cooled down as 

 far as —25°, it would 

 traction as the silicate. 



