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XXVIII. The Fusion- Constants of Igneous Rock. —Part III. 

 The Thermal Capacity of Igneous Rock, considered in its 

 Bearing on the Relation of Melting-point to Pressure. By 

 Carl Bakus*. 



[Plate VI.] 



1. TJSfTROBUCTORY.—The present experiments are in 

 series with the volume-measurements of my last paper, 

 and the same typical diabase was operated upon. Since it is 

 my chief purpose to study the fusion behaviour of silicates, 

 more particularly the relation of melting-point to pressure, 

 the observations are restricted to a temperature-interval 

 (700° to 1400°) of a few hundred degrees on both sides of 

 the region of fusion f (§ 11). 



2. Literature. — Experiments similar to the present, but 

 made with basalt, were published quite recently J by Profs. 

 Roberts- Austen and Rtickerjj. The irregularities obtained 

 by these gentlemen with different methods of treatment 

 (heating in an oxidizing or a reducing atmosphere, repeated 

 heating, sudden cooling), the anomalously large specific heat 

 between 750° and 880°, where basalt is certainly solid, and the 

 absence of true evidences of latent heat||, contrast strangely 

 with the uniformly normal behaviour occurring throughout 

 my own results. Basalt is chemically and lithologically so 

 near akin to diabase (particularly after melting) that I anti- 

 cipated a close physical similarity in the two cases. Unfor- 

 tunately the account given of the basalt work is meagre. 

 Detailed comparisons are therefore impossible. 



The elaborate measurements of Ehrhardt (1885) and of 

 Pionchon (1886-7) are less closely related to the present work. 



Apparatus. 



3. The Rock to be tested. — About 30 grammes of diabase 

 were fused in the small platinum crucible together with which 

 they were to be dropped into the calorimeter. Two such 

 charged crucibles were in hand, to be used alternately. The 

 molten magma, after sudden cooling, shows a smooth, appa- 

 rently unfissured surface, glossy and greenish black. After 



* Communicated by the Author. 



t The geological account of the present work is begun by Mr. Clarence 

 King, in the January number of the American Journal. 



% This was written some time ago. See American Journal, December 

 1891 and January 1892. A forthcoming Bulletin, No. 96, U.S. Geological 

 Survey, contains the work in full. 



§ Roberts-Austen and Riicker : this Magazine, xxxii. p. 355(1891). 



•| Supposing basalt to solidify (§ 13) below 1200°. 



