68 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The circulation of the water in the bath over the cooling and heating 

 surfaces is effected by means of the pump (11). It enters the pipes 

 (12 and 13), which end just inside the rear end of the bath, and passes, 

 in the direction of the arrows, into the large pipe (14), thence to the 

 pump and out again into the open bath. It will be observed that the 

 tendency is to draw the colder water upon the bottom of the bath very 

 rapidly into the pipes (12 and 13), but that, as it enters these, it is neces- 

 sarily mixed with water which has passed over the heating surfaces 

 (9 and 10). Many positions for the heating surfaces have been tried, 

 but that given in Figure 34 has been found most satisfactory. 



The rate of pumping depends upon what is found to be necessary in 

 order to secure identical temperatures at the two ends and the middle of 

 the bath. Ample provision is made for any rate which may be required. 

 A moderate rate for some of the larger baths is 400 liters per minute. 



The purpose of extending the pipes (12, 13, and 14) outside of the 

 bath, where, at their junction, the circulating water passes over a gas 

 stove, is obviously to economize electricity. The rule here, as in all 

 other baths, is to utilize gas for heating purposes to the utmost safe 

 limit, leaving for the electrical appliances only so much as is indispen- 

 sable for regulation. 



Five baths of Type II are in use, varying in size and equipment, but 

 all conforming in principle to that just described. Plate 3 presents 

 their appearance, also that of the baths of Type I. 



Type III. 



An example of one kind of bath in which osmotic pressure is measured 

 is shown in Figures 35 and 36. The first (Figure 35) represents the 

 lower part, which is filled with water and in which are located circulat- 

 ing systems similar to those described under Type II. In the second 

 (Figure 36) is seen the upper part of the bath, the so-called " air 

 space." Both divisions are lined with copper and are separated by a 

 vapor-tight brass plate (1, Figure 35 or 36), which is divided diagonally 

 across the bath into two parts which are reunited by the brass strip (2). 

 The brass plate (1) is screwed down upon the upper edge of the outer 

 wooden bath, but between the two, as also between 2 and 1, strips of 

 sheet rubber are placed to prevent the passage of water vapor from the 

 lower part of the bath into the "air space" above. The reason for keep- 

 ing the latter as dry as possible will appear later. Six lead-weighted 

 copper cans are suspended from the brass covering plate, the flange of 

 each resting upon a rubber collar; they serve as receptacles for the 

 cells. During a measurement of pressure, the space in the cans above 

 and around the cells is filled with wool. In two of the three baths of 

 Type III, the cans have been replaced by two long, narrow troughs, 

 whose depth is equal to that of the cans. The troughs have covers 

 which are divided into many readily removed sections. A bath so 

 arranged will easily accommodate 24 cells instead of 6. 



