142 DR. E. H. GRIFFITHS AND MR. EZER GRIFFITHS ON THE 



When the values of the specific heat at C. were required, a special tank of 



15 gallons capacity and lagged with asphalt was used. Two screws protected by 

 strong metal cages caused a rapid circulation of water through the powdered ice. 

 For work at higher temperatures this tank was replaced by an oil or water bath, of 

 capacity about 20 gallons, the heat being supplied by immersed electric radiator 



lamps. 



Some difficulty was encountered in insulating the 200-volt leads of these lamps 

 when the tank-temperature was above 50 C., owing to the softening of the stretched 

 rubber tubing by continued exposure to hot water. The cement fastening the caps 

 of the lamps frequently broke down and entailed the loss of several experiments. 



The most satisfactory method of insulation hitherto tried was by clipping a 

 discarded motor tyre tube over the end of the " radiator " lamp, the leads being 

 separated within the tube by lengths of glass tubing. 



The " lux " lamps used for the purposes of fine adjustment, were insulated by 

 fixing glass tubing of slightly larger diameter over their ends, the joint being closed 

 by a short length of rubber tubing well covered with adhesive tape. 



JTiermostats. 



We tried a considerable variety of thermostats which proved defective from 

 one cause or another. The two most satisfactory ones may be briefly described. The 

 first was composed of thin solid-drawn copper piping f inch internal diameter and 



16 feet long. 



This tube was wound into an oval spiral, so as to surround the two brass cases in 

 the tank (see fig. 1). Two glass tubes were soldered into reduction pieces at each end, 

 one terminating in a tap, the other in a U-tube containing mercury. As the method 

 of constructing these soldered joints is both simple and effective, we have given in 

 Appendix III. a brief description of the process. 



Another form of thermostat used in a considerable number of determinations 

 consisted of a large branchwork of glass tubes fused together and so distributed as to 

 take the mean temperature of the tank. The thermostat was filled with toluol, 

 which however proved unsatisfactory at high temperatures, and was replaced by 

 commercial aniline, which in every way seems to be a suitable liquid for thermostats. 

 It has a high coefficient of expansion, low viscosity and a high boiling point (184 C.). 



We found it necessary to keep the tap closing the thermostat well greased, 

 otherwise slow leakage and consequent drift of temperature took place.* 



The motion of the mercury in the U-tube operated a relay, which in turn switched 

 on and off the lamps in the tank. Both the make and the break in the main circuit 



> We have some reasons for suspecting that, owing to neglect of this precaution, the temperature of the 

 bath was not maintained with its accustomed steadiness during some of the group of observations about 

 67' C. 



