6 Fusion Constants of Igneous Rock, 



8. Revolving Muffle. — In order that corresponding indica- 

 tions of thermocouple and air-thermometer maybe compared, 

 both must be placed in a space at practically uniform tem- 

 perature, which must be variable at pleasure from ordinary 

 temperatures to extreme white heat. I constructed a special 

 furnace for this purpose, in the way indicated in plan in the 

 diagram (fig. 4) . The body of the furnace is a thick cylin- 

 drical box, B B, surmounted by a hemispherical lid, suitably 

 Fig. 4. — Plan of the revolving muffle. Diagram. 



perforated. In this cylindrical enclosure a spherical muffle, 

 D, provided with hollow lateral arms or axles, E F, and 

 placed symmetrically with respect to the centre of figure, is 

 free to rotate around the horizontal axis of the arms. If the 

 rate of rotation be sufficient, this insures constancy of tempera- 

 ture within the muffle around the horizontal E F. Two blast- 

 burners, G and H, purposely placed tangentially so as to be 

 equivalent to a force-couple, blow a vortex of flame into this 

 furnace, equalizing temperature around the vertical. Vir- 

 tually, therefore, the muffle, regarded as a geometrical sphere, 

 has two rotations, one round E F, and the other round the 

 vertical through 0. To make this apparatus theoretically 

 perfect a third rotation, round a horizontal axis through 0, 

 and perpendicular to E F, would have to be supplied. This 

 third rotation is a mechanical impossibility, bearing always 

 in mind that cumbersome or complicated apparatus would 

 rather detract from the end to be obtained than add to it. 



In the spherical space of constant temperature thus obtained 

 is placed the bulb of the air-thermometer (not shown), with 

 its stem projecting into or through the axle F ; and the bulb 

 is held in position, free from the muffle, by a clamp attached 

 to the stem on the outside of the furnace. The thermo- 

 couple is introduced through the opposite arm E, and its 

 junction lies at the centre of figure of the bulb (fig. 2, 

 above). It is an essential part of the construction of the 

 furnace that during rotation the muffle touches neither the 

 air-thermometer nor the insulator of the thermocouple, the 

 latter being also supported by a clamp on the outside of the 

 furnace. 



The practical form of this furnace,, which after many trials 



