56 C. Barus — Relation of Melting Point to Pressure. 



to the form of the oxide. The latter of the two processes, 

 which are identical so far as handling of the strontium is con- 

 cerned, yields the better results in the determination of the 

 calcium, doubtless because a trifling amount of calcium sulphate 

 remains in solution in the final filtrate in the former process, 

 while in the latter every trace should be recovered. The 

 deposition of minute traces of calcium in the precipitation of 

 the strontium nitrate, to which reference has been previously 

 made, is evidently a matter of no significance in comparison 

 with the ordinary errors of manipulation. 



Art. YII. — The Relation of Melting Point to Pressure in 

 Case of Igneous rock fusion ; by C. Barus. 



To determine this important constant for diabase, I made 

 use of the thermodynamic principle (Clausius, I, chap, vii, 

 § 2). From my last note* the fusion specific volumes, solid 

 and liquid, are known. Hence it is merely necessary to deter- 

 mine the latent heat of fusion. 



Two series of measurements of the thermal capacity of dia- 

 base, containing 27 independent measurements, were made 

 between 700° and 1400°. The first series gave me, in gram 

 calories, for the mean specific heat, solid, between 800° and 

 1100°, -304; for the mean specific heat, liquid, between 1200° 

 and 1400° -350; for the latent heat of fusion (1200°), 24; 

 and for the heat set free on solidification (1100°), 16. Simi- 

 larly the second series gave me, "290, *360, 24, and 16, respec- 

 tively. The last series is much the .more trustworthy and its 

 accuracy may be inferred from the following pairs of values of 

 temperature and thermal capacity : (solid) 781°, 180 ; 873°, 

 202; 948°, 227; 993°, 238; 1096°, 268; 1171° (incipient 

 fusion?), 302; (liquid) 1166°, 310; 1194°, 318; 1197°, 319; 

 1215°, 327; 1218° 330; 1248°, 339; 1251°, 340 and 342; etc. 

 Now whereas in my last note (1. a), solidification did not set 

 in above 1100°, evidences of fusion do not here show them- 

 selves until 1170° is reached. Rock fusion is therefore accom- 

 panied by hysteresis, f the lag being apparantly as much as 70°. 

 It is this property which makes it possible to obtain sharp 

 values of latent heat, for the rock can be operated on, either 

 solid or liquid, at nearly the same temperature. In view of 

 the great difficulty of obtaining the value of the melting point, 

 however, I shall in the following state the conditions for 1100° 



*This Journal, December, 1891. 

 fThis Journal, xlii, p. 140, 1891. 



