130 THE ROYAL SOCIETY OF CANADA 



screws, and the metal exterior will insure absence of leaks which 

 would be very likely to occur in an ordinary wooden tub filled with oil. 

 In our apparatus the inside tub was 65 cms. in diameter and 40 cms. 

 deep. A space of about 4 cms. was left all round between the walls 

 of the inside and the outside tubs. A cover, preferably double, should 

 be constructed for the top, with holes for thje stirrer, thermometer 

 and necessary wires. Kerosene oil with a fîash point of at least 

 40°C, and 20° higher than the regulating temperature, constitutes a 

 suitable and safe medium for the bath. The slowly increasing electri- 

 cal conductivity of water when continually in contact with foreign 

 substances and subject to large evaporation, renders it an unsuitable 

 medium for most uses of the bath. 



The space between the two tubs is also filled, and this concentric 

 arrangement plays an important part in reducing the radiation 

 from the bath and thus making a finer regulation possible. 



(b) The stirrer should consist of a small two- or three-bladed 

 propeller screw mounted at the centre with its plane of rotation parallel 

 to and near the bottom of the bath. A brass rod with metal flanges 

 bent into shape is adequate, but it should be shaped and rotated in 

 such a direction that it produces a downward current at the centre 

 of the bath, spreading and rising at the sides. The use of a paddle 

 stirrer, or an upward current would steepen the temperature gradient 

 between the upper centre of the bath and the bottom or sides. The 

 stirrer can be run very conveniently by a pulley and belt attached to a 

 small electrical or water motor. It is advisable to enclose the belt 

 for the stirrer pulley with three or four small protective boards. 



(c) The heating coils used in the apparatus described, consisted 

 of bare manganin wire wound on two glass tubes each 35 cms. long. 

 The windings were held in position by means of a little hard shellac. 

 Each coil had a resistance of 150 ohms and being connected in parallel 

 thus gave an equivalent resistance of 75 ohms. Immersed electric 

 lamps are often used with success in thermostats, but their fragility 

 renders them a little dangerous for use with oil, and the radiating 

 surface presented, is not as good. The e.xterior electrical arrange- 

 ments are discussed in connection with the thermo-regulation, below. 



If cooling below room temperature is desired, ice-water or cold 

 brine may be passed through small lead or glass tubes placed within 

 or beside the heating coils. Heat is removed by them in excess, 

 and then regulation is obtained by the heating coils and thermoregu- 

 lator in the same manner as described. 



(d) The thermoregulator can be constructed easily by any amateur 

 glass-blower out of two three-way glass taps, glass tubing, platinum 

 wire, toluene and mercury. About four and a half metres of carefully 



