30 Day, etc. — Determination of Mineral 



some point during the cooling ; hence the sharp break in his 

 curves. With rising temperature the crystal aggregate then 

 fused at a higher temperature than that to which it had under- 

 cooled before crystallizing. 



That this conclusion offers the true explanation of Barus' 

 observations is, fortunately, susceptible of direct proof. The 

 original platinum tube and its charge of diabase, just as it was 

 left when the experiments of Barus were interrupted, came 

 into the possession of this laboratory some years ago and was 

 examined. The surface of the melt did indeed appear glassy, 

 but when broken out of the tube and examined under the 

 microscope the diabase charge was found to be about three- 

 fourths crystalline. Occasional small vacuoles were scattered 

 through the mass, due either to bubbles which had never 

 escaped from the glass, or to contraction after the surface had 

 become too stiff to yield. 



Since the data of Barus were calculated and stated in terms 

 of the final volume of the rock at room temperature, it became 

 possible to recalculate his volumes, approximately, on the basis 

 of the volume of the rock obtained from the tube. A number 

 of the largest fragments, weighing 35 grams in all, were se- 

 lected and their total volume determined under mercury. This 

 gives us the nearest possible approximation to Barus' basic vol- 

 ume, since the mercury does not fill the small cracks or vacu- 

 oles. The kilogram-volume was found to be 356, as compared 

 with 349'0 for our own recrystallized diabase. 



It was necessary also to revise the temperatures in Barus' 

 original publication. Assuming that these temperatures were 

 calculated in terms of the scale published* at about the same 

 time as his bulletin on rock expansion, we have revised the 

 figures to correspond with the high temperature scale in present 

 use.f The recalculated data of Barus' last three series (the first 

 two being stated by him to be much less reliable) are plotted 

 in fig. 10. 



It is plain from these curves that precisely the same phenom- 

 ena occurred in his diabase as in ours. The liquid cooled to a 

 temperature below that at which crystals would have separated 

 under equilibrium conditions, and then crystallized suddenly with 

 contraction in volume. With rising temperature, fusion began 

 at a temperature above that to which the glass had undercooled 

 before crystallizing. The progressive lowering of the curves is 

 due either to gas bubbles which had not yet escaped from the 

 liquid, or more probably to the sagging of the platinum tube, 

 causing an apparent contraction in total volume.;}: 



* " Die physikalische Behandlung und die Messung holier Temperaturen." 

 Leipzig, 1892. 



f Day and Sosnian, Carnegie Publication 157, 1911. 

 % Barus, Bull, ciii, p. 36. 



