132 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



osmotic pressure of the solutions, and a preference was expressed for 

 the third table, in which a compromise had been attempted. 



When, at a later period, it was proved that the whole loss in rotation 

 had been due to dilution, it was necessary wholly to discard the first 

 and third tables. 



Table 21 gives the results of Series III corrected for dilution only. It 

 is comparable with Table 17 for Series II. The corrected pressures 

 are probably somewhat more reliable in the former than in the latter. 



Series IV.* 



In Series IV it was attempted to maintain a bath temperature of 5°, 

 but the temperature varied as a rule between 4° and 5°. On two occa- 

 sions it exceeded 6° for a short time. Both compartments of the bath 

 were furnished with an extensive and continuous system of brass pipes for 

 the circulation of hydrant water. One-half of the pipes were immersed 

 in the water in the lower part of the bath, while the other half were sus- 

 pended from the top of the upper, or manometer, compartment. The 

 hydrant water entered at the bottom, and, after circulating through the 

 whole length of the pipes in the water, it ascended and traveled through 

 the whole length of the system in the air space before escaping from the 

 bath. It was fed to the bath system from the bottom of a standpipe, 

 4 meters in height, and its rate of flow was regulated by means of a valve 

 placed between the standpipe and the bath. In order that the pressure 

 upon the water circulating in the bath might remain constant, also that it 

 might have, at the time of entering, the temperature of the water in 

 the street mains, the standpipe was provided with an overflow at the 

 top, and the water was fed into it as directly as possible from the main 

 source of supply for the building, and at a comparatively high rate. The 

 water in the bath in which the lower half of the cooling system was 

 submerged was kept in constant circulation by means of a pump. 



The mean mid-winter temperature of the water in the street mains 

 is about 4°, and no difficulty was apprehended in maintaining a tem- 

 perature of 5° in the bath. But long before the series was completed, 

 the temperature of the hydrant water rose above 5°, and it was neces- 

 sary to insert a system of pipes, cooled by ice, between the standpipe 

 and the bath. 



The cooling system described above is essentially the same as that 

 now employed in all baths for temperatures above 0° and below the 

 highest temperature of the atmosphere, only it has been found better 

 to employ two independent systems — one for the lower part of the bath, 

 where the cells are located, and another for the air or manometer space. 



During the work upon Series IV, the cooling system was in the experi- 

 mental stage, and it failed to operate as satisfactorily as it afterwards 

 did when all its details had been perfected. This accounts, in part, for 



*Measurements by H. N. Morse, J. C. W. Frazer, and P. B. Dunbar. Am. Chem. Jour., 

 xxxvin, 175. 



