CANE SUGAR. 137 



additional but uncontrolled heating surface whenever the temperature of 

 the air rose above that which it was sought to maintain in the bath. 



In Series III, the ice water was the cooling agent and the walls of 

 the bath were the heating surface. In this case the cooling agent was regu- 

 lated and the melting ice was the thermostat. The system was perfect 

 in principle, but failed because of the too slow circulation of the water 

 between the heating and refrigerating surfaces. 



In Series IV, the hydrant water was the cooling agent and the bath 

 walls were the heating surface. As in Series III, the cooling agent, 

 instead of the heating surface, was regulated. In Series III, the 

 thermostat was melting ice, while in Series IV, it was the valve between 

 the stand-pipe and the bath. 



Considered as a thermostat for one temperature only, nothing is 

 more perfect, of course, than melting ice, except a liquid of constant 

 boiling-point, while a valve regulating the flow of water of constant 

 temperature is obviously ineffective unless the external heat supply 

 is constant in quantity. The system of cooling employed in Series 

 IV was excellent. The failure to regulate satisfactorily the tempera- 

 ture of the bath was due to the fact that the thermostat (the valve) 

 was not sufficiently automatic in its action to overcome the inconstant 

 external temperature conditions. The remedy which suggested itself 

 and was immediately applied was the reinstallation in the bath of an 

 electric heating system controlled by a mercury thermostat. The 

 effect of this was, of course, to give the regulation of the bath to the 

 heating instead of the cooling system, which should always be done 

 unless the external temperature conditions are constant, or one can 

 employ melting ice or a boiling liquid. 



The valve did not become useless when it lost its character as a thermo- 

 stat, for it was still necessary as an economizer of water and heat, that 

 is, for the purpose of keeping the so-called "margin of under-cooling" as 

 small as practicable. 



The beneficial effect of the improvement in manipulation and appa- 

 ratus was immediate and large. In Series V, the loss in rotation was 

 small and was confined to the solutions of higher concentration, and 

 the fluctuations in bath temperature were less frequent and smaller 

 than in any previous series. 



The sum of the rotations of all the solutions in Series V was 1249.6°. 

 A loss of 2.50° amounts to 0.20 per cent. Expressed in osmotic pressure, 

 the dilution was equivalent to about 0.64 atmosphere. The corre- 

 sponding values in the preceding series were 1249.00°, 17.55°, 1.41 per 

 cent, and 4.30 atmospheres. The sum of all aberrations in bath temper- 

 ature was 1.40° in Series V and 10.30° in Series IV. There were no 

 upward displacements of the manometers. 



In Table 28 the results are corrected for dilution corresponding to 

 the observed losses in rotation. 



