Constants of Igneous Rock. 9 



position, supported by the universal clamp mm, attached to 

 the vertical rod q q. A similar clamp n n, on the opposite side 

 of the furnace, supports the insulator of the thermocouple pp. 

 The ends of the wires of the latter appear at a and /3, and 

 pass thence to a petroleum bath at known temperature, where 

 they are suitably connected with the terminals of the measur- 

 ing apparatus (insulated mercury cups submerged in the 

 petroleum) . 



The figure shows that the thermocouples to be calibrated 

 may be easily inserted or withdrawn while the air-thermo- 

 meter remains in place. I will pass over the method of ad- 

 justing the latter, remarking only that the collar P has a slit 

 cut through it, from end to end, in the direction of a radial 

 plane, and sufficiently wide to admit the platinum capillary s 

 of the air-thermometer. This need not, therefore, be taken 

 apart to remove it from the muffle. 



9. Remarks on the Furnace. — The maximum temperature 

 attainable is indefinitely high, much higher than can be 

 measured by the porcelain air-thermometer. Measurements 

 may be made either while temperature is slowly increasing or 

 decreasing ; in the latter case the gas may be quite shut off 

 below 500°, for the furnace itself then cools sufficiently slowly. 

 My mean rate of rotation was about 50 per minute, but 

 smaller rates are necessary at very high temperatures when 

 the muffle begins to become viscous, and at low temperatures. 

 During the heating to white heat, the parts of the axles 

 within the furnace are liable to be hotter than the muffle. I 

 will omit the method of remedying this defect, since it is 

 quite absent on cooling, and since its effective value is almost 

 negligible. Its harmful tendency is chiefly to bend the stem 

 of the air-thermometer, an effect which can be counteracted 

 by rotating the latter 180° from time to time. Rotation, 

 moreover, keeps the muffle straight even when the clay is 

 approaching the viscous stage. 



Protected from direct flame by the revolving muffle, the 

 fragile porcelain bulbs are heated with great regularity, and 

 the liability of the thermometer to break is therefore nil. 

 The furnace has been so made that the zone of variable tem- 

 perature which surrounds the stem of the thermometer is as 

 narrow as possible. Indeed the correction is almost super- 

 fluous. Were it desirable to use a compensator (a duplicate 

 porcelain stem, closed at the bulb end, to gauge the stem 

 error), this instrument may either temporarily replace the 

 thermocouple, or be introduced through an independent 

 lateral tube, corresponding to the axles F F. 



In order to obtain the zero or fixed point of the bulb, the 



