Constants of Igneous Rock. 305 



In both the tables, III. and IV., the solid points lie on lines 

 which, if reasonably curved, would be nicely tangent to an 

 initial specific heat of about 0*2 at °C. The grouping, in 

 other words, is so regular as to exclude the probability of 

 anomalous features, either in the observed or the unobserved 

 parts of the loci. The solid point near a (fig. 4, a similar point 

 occurs in Table III.) alone lies markedly above the curve ; but 

 inasmuch as in my volume work I found solidification to set in 

 at 1100°, it is altogether probable that the occurrence at 1170° 

 is incipient fusion (§ 13). 



The regularity of the liquid loci (Tables III. and IV.) is 

 slightly less favourable ; but the discrepancies which occur 

 are above 1300°, and obviously accidental (§ 10, end). 



12. Specific Heat. — As regards the mean specific heats be- 

 tween 800° and 1100° in Tables III. and IV., it will be seen 

 that the intermediate datum would satisfy both groups of 

 points about as well as the individual data given. A tracing- 

 made of the first group practically covers {}iq other. The 

 same remarks may be made for the liquid state. I have not 

 attempted any elaborate reductions, since the equations of 

 the necessarily curved loci would have to be arbitrarily 

 chosen, and since values for specific heat are of no immediate 

 bearing on the present inquiry. 



13. Hysteresis. — Recurring to the suggestion of the pre- 

 ceding paragraph, it appears that the fusion behaviour of rocks 

 must be accompanied by hysteresis* of the same nature as 

 that which I observed with naphthalene and other substances : 

 for, whereas in my volume work with diabase I was able to 

 cool the rock down to 1095° without solidifying it, evidences 

 of fusion (at a, figs. 4 and 5) do not occur in the present 

 work until 1170° is reached. The magnitude of the lag is 

 thus of the order of (say) 50°, and its pressure-equivalent 

 may be estimated as 500 atmospheres. 



14. Latent Heat. — In virtue of the fact that the (upper) end 

 of the solid locus (Tables III. and IV.) may be carried so 

 near the beginning of the liquid locus, the datum for latent 

 heat is determinable with some accuracy, in spite of its sur- 

 prisingly small (relative) value. Difficulties, however, present 

 themselves in the determination of the true melting-point, a 

 datum which can only be sharply defined when the tempera- 

 ture of the crucible is quite constant throughout. I have, 

 therefore, considered it preferable to state the conditions at 

 1200° and at 1100°, the former being nearer fusion and the 

 latter very near solidification. The latent heats for these 



* Am. Journal, xlii. p. 140 (1891) ; cf. ibid., xxxviii. p. 408 (1889). 



