MR, A. E. TUTTON ON A COMPENSATE) D INTERFERENCE DILATOMETER. 323 



middle part of the spectrum, and of about 88 for the red hydrogen line C, and 92 

 for the greenish blue hydrogen line F. 



The heating apparatus consists of a double-chambered cylindrical air bath, 



mounted on a stout annular support resting by means of three levelling screws upon 



toe-plates also earned by the movable slab. During adjusting operations, and 



observations at the temperature of the room, the bath is removed from its stand. 



It is constructed throughout of brazed copper, and is covered outside with a thick 



coat of asbestos cloth. The inner chamber, y, is similar in shape to the outer, z, 



from which it is separated all round by the air space of about 4 centims. Within 



this inner bath the interference chamber of the dilatometer is to be lowered, by 



means of the rack and pinion, for observations at higher temperatures. The opening 



in the top of the bath is necessarily a little larger than the widest part of the 



interference chamber, the flange of the rigid base, k (fig. 3). The inner bath is 



supported within the outer one by the tubular wall of the opening, 3^ centims. deep, 



which connects the top of the outer with the top of the inner bath. The top of this 



tubular opening is closed, when the interference chamber has been lowered into 



position, with the two overlapping halves of an asbestos-lined and closely-fitting lid, 



which encircles the porcelain tube of the dilatometer, and is provided with suitable 



handles above and a deep rim beneath, which latter passes down into the tubular 



neck sufficiently far to obviate any appreciable diffusion of cold air from outside. 



Through a wide tubular opening at the top and near the side of the bath there is 



inserted into the outer bath, for the purpose of enabling the temperature within 



the bath to be regulated with constancy, a Muenke thermostat, provided with a 



regulator to control the size of the flame of the gas-burner when the mercury in the 



thermostat closes the orifice in the steel gas-supply tube. There are also two short 



tubes, on opposite sides of the centre, which pass air-tight through the tops of both 



baths into the inner one, for the passage of two thermometers. Both the thermostat 



and the thermometers are suspended free of the metal by non- conducting stoppers, 



and the thermometers are arranged so that the bulbs are on either side of and in 



close proximity to the interference tripod. A third thermometer, of such special 



construction that its bulb lies in actual contact with the tripod, passes through a 



hole in the front half of the lid of the bath. Details concerning the thermometers 



will be given at a later stage. A twenty-jet Fletcher ring gas-burner is employed 



as source of heat, the size of the jets being -controlled by the thermostat, which has 



been adjusted so as to obtain a constant upper temperature, as near the particular 



desired limit as possible. Of very great utility in attaining constant temperatures 



has been found a gas-tap provided with a long pointed lever arm, travelling over a 



graduated quadrant, divided to read directly to degrees. By its aid the temperature 



can be very nicely regulated, leaving only such little further regulation to the 



thermostat as is rendered necessary by the very slight variation in the pressure of 



the gas supplied from a meter furnished with a governor. 



2x2 



