553 



SCIENCE PROGRESS 



applied to the water, when working at the higher tempera- 

 tures, leaving only a small balance to be provided by the 

 regulated electrical heating. Ample provision was made for 

 driving enough water through the bath to keep the tempera- 

 ture uniform from end to end : usually a circulation of 400 litres 

 per minute was found to be ample. 



Fig. 5 shows the arrangement of the air-space above the 

 bath. This is provided with a system of pipes 7 through 

 which cold water can be circulated, a system of pipes 8 

 through which either hot or cold water can be circulated and 

 a series of four shaded lamps 3, 4, 5, 6, controlled by the 

 mercury thermostat 10. A fan 9 driven by a motor provides 



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FlG. 4. — Thermostat containing osmotic cells. 



a vigorous circulation of air and also serves to keep the 

 manometers continually agitated. 



Special apparatus has recently been introduced to secure a 

 steady temperature exactly at o° but this need not now be 

 described in detail. 



4. The Membranes. — The first membranes were formed in 

 the interior of the cell-walls but it became clear that with a 

 membrane so located it would not be possible to measure 

 osmotic pressures. In such a cell the minute pores between 

 the membrane and the inner wall would be choked with water, 

 which would require very long periods of time before it could 

 be displaced by the solution ; moreover, any temporary dilution 

 or concentration of the liquid in the pores, due to the displace- 



