The Fusion Constants of Igneous Mock. 297 



drying and weighing, the mass is often found to have gained 

 5 per cent, in weight. I was at first inclined to believe that 

 this was attributable to water chemically absorbed by the 

 viscous magma; but the water is only mechanically retained, 

 for it passes off after 24 hours of exposure to the atmosphere, 

 or by drying at 200° C. for, say, 30 minutes. Hence I 

 weighed my crucibles at the beginning of each measurement, 

 having previously dried them at 200°. The solid glass, sud- 

 denly cooled from red heat, soon shows a rough and fissured 

 surface, and its colour changes from black to brown, possibly 

 from the oxidation of proto-to sesquisalt of iron, possibly from 

 mere changes in the optical character of the surface (§ 2). 



Throughout the course of the work the charge of the 

 crucibles was neither changed nor replenished. 



4. Thermal Capacity of Platinum. — Data sufficient for the 

 computation of the heat given out by the crucibles were 

 published in 1877 by Violle*, whose datum for the high 

 temperature (t) specific heat of platinum is '0317 + '000012 1. 

 Hence the increase of thermal capacity from zero Centigrade 

 to the same temperature is £(*0317 + *00000(>£), which is the 

 allowance to be made per gramme of platinum crucible. 



5. Furnace. — Inasmuch as heat is rapidly radiated from the 

 white-hot slag, it is necessary to transfer the crucible from 

 the furnace into the calorimeter swiftly. I discarded trap- 

 door, false bottom, and other arrangements for this purpose, 

 because the mechanism clogs the furnace, interferes with con- 

 stant temperature, and is too liable to get out of order. The 

 plan adopted is shown in figs. 1 and 2 (Plate VI.), in sectional 

 elevation and plan. The body of the furnace consists of two 

 similar but independent bottomed half-cylinders, A A and 

 B B, of fire-clay properly jacketed, which come apart along 

 the vertical plane c c c c. The lid, L L, however, is a 

 single piece, and fixed in position by an adjustable arm (not 

 shown). Each of the halves of the furnace is protected by a 

 thick coating of asbestos, C C, D D, and by a rigid case of 

 iron, E E, FF. Set screws, gggg, pass through the edges 

 of this in such a way as to hold the fire-clay and asbestos in 

 place. The horizontal base or plate of the casing E F is bent 

 partially around the two iron slides, G Gr, along which the 

 two halves of the furnace may therefore be moved at pleasure 

 while the lid is stationary ; as is also the blast-burner, H, 

 clamped on the outside (not shown), and entering the furnace 

 by a hole left for that purpose. 



* Violle's calorimetiic work will be found in C. R. lxxxv. p. 543 (1877), 

 lxxxvii. p. 981 (1878), lxxxix. p. 702 (1879) ; Phil. Mag. [4] p. 318 (1877). 



Phil. Mag. S. 5. Vol. 35. No. 214. March 1893. X 





