Constants of Igneous Rock. 179 



The products of combustion are carried off by the two 

 oblique tubes, E, E, in the lid LL (fig. 4). Note that the 

 water-screen bends around the vertical micrometer in such a 

 way that flames issuing from E do no injury, and a perforated 

 free plate, m m, closes the vertical hole in the lid L L. 



Two large size Fletcher-bellows, set like a duplex-pump 

 and actuated by a gas-engine, furnished the air-blast. 



In order to insure greater constancy of temperature, and 

 at the same time increase the high temperature efficiency of 

 the furnace, it is essential to jacket both the latter and the lid 

 heavily (1-2 centim.) with asbestos. 



7. Fusion-tube. — The platinum tube holding the molten 

 rock Z Z is 25 centim. long and about 1*5 centim. in dia- 

 meter, drawn as accurately cylindrical as possible and provided 

 with a flat bottom. To protect this tube from gases, to keep 

 it from bulging in consequence of the fluid-rock pressure 

 within, and to insure greater constancy and slower changes 

 of temperature, the platinum tube is surrounded by the fire- 

 clay tube F F (figs. 1 and 6) fitting loosely. Care must be 

 taken to allow for shrinkage of the clay, which in a fresh 

 tube, after some hours' exposure to 1500°, exceeds 3 percent, 

 or more, and is permanent. After this the tube expands nor- 

 mally. It rarely warps, and may therefore be fixed by fire- 

 clay arms, H (figs. 1, 2, and 7), suitably clamped on the 

 outside of the furnace. Near the bottom a perforated ring, 

 CC (figs. 1 and 5), embraces both the tube FF and a pro- 

 jection, G, in the furnace. 



8. Thermocouples. — In addition to figs. 1 and 2, figs. 6 and 

 8 give a full account of the adjustment. The tube F F is 

 laterally perforated with three pairs of fine holes, t, corre- 

 sponding to the two canals of the insulator*, T T. The wires 

 of the thermocouple are then threaded through t and the 

 insulator canals, in such a way that the respective junctions 

 lie in small cavities at t, immediately in contact with the out- 

 side of the platinum tube. Holes are left in F F and the 

 furnace for this operation. 



The cold junctions of the thermocouples terminate in three 

 pairs of mercury-troughs, insulated by hard rubber, and sub- 

 merged in a bath of petroleum. With these troughs the 

 terminals of the zero method are successively connected, and 

 the temperatures of the top, the middle, and the bottom of the 

 fusion-tube measured by § 4. 



* The method of making these is given in Ball. No 54, U. S. Geolog. 

 Survey, p. 95. 



