the Magnetism of Basalt. 47 
and furnace was made (see fig. 1). The furnace was con- 
structed by winding, on an unglazed porcelain tube. PP, 
37 ems. long and 27 cms, in diameter, 4() double coils of 
platinum wire, the coils covering 25 cms. length of the tube, 
and the w inding being closer at ; the ends than in the middle. 
The porcelain “tube, “the internal diameter of which was 
2°3 ems., was supported within a wide glass cylinder, and the 
platinum wire was kept in position by means of a packing, K, 
of kaolin clay which filled up the space between the olass and 
the porcelain, and the ends of the glass cylinder were stopped 
up with a wet paste of kaolin clay, which was afterwards 
hardened by heat. The porcelain tube projected a few centi- 
metres at one end, this projection being employed to carry 
a perforated disk, D, of biscuit ware to which the terminals of 
the heating-coil were attached. The glass cylinder fitted like 
a cartridge into the brass water-jacket on which the mag- 
netizing- coil, MM, was wound. Water, entering the water- 
jacket ‘at a low pressure by the tube A, was distributed by 
means of the small holes along the tube and drawn off thr ough 
the tube B. A bar of basalt could thus be placed within the 
furnace and heated to any required temperature in a magnetic 
field while the magnetizing-coil was kept cool by a slow 
current of water. The resistance of the furnace-wire when 
cold was 12 ohms, and an H.M.F. of 110 volts could raise 
the temperature inside the furnace to 800° C., the current 
then being 3°75 to 3°5 amperes. 
The furnace described above differs from that of Ledeboer* 
in having within the heating-coil a tube to hold the specimen, 
but, on the other band, it appears to be similar to one 
employed by Prof. Holborn -: 
It might be remarked here that in the furnaces of Holborn, 
Barton & Williams, S. W. Richardson, and others asbestos 
is employed as the heat-insulating material. This, in the 
ordinary commercial form, whether of sheet, twine, or wool, 
was found to be quite unsuitable for use in working wil 
basalt as it gave rise to considerable magnetic distuehances 
on account of the iron it contained. After testing various 
substances as heat-insulators, kaolin clay was substituted for 
the asbestos. 
The magnetizing-coil consisted of six layers of No. 16 silk- 
covered copper wire wound on the water-jacket, the total 
number of turns being 956, the length of coil 29:2 cms., and 
the field per ampere approximately 40 c.c.s. f, and the 
* Ledeboer, Journ. de Physique, p. 199 (1888). 
+ Nature, p. 350, Feb. 17, 1902. 
} Corrected for the end ‘effect. See Maxwell, Elec. and Mag. vol. ii. 
section 676; and J. S. Townsend, Phil. Trans. p. 533 (1896). 
