30 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



The outlet is then immersed in a globule of pure mercury and the 

 screw is reversed. This manipulation brings a short calibrating thread 

 (h) into the manometer, which is separated from the main body of the 

 mercury by a cushion of air. The thread can 

 now be made to take any desired position in the 

 tube by simply turning the screw and simulta- 

 neously tapping the manometer. The electric 

 ' 'hammer" employed for the latter purpose will 

 be described in connection with the process for 

 the determination of capillary depression. 



The process of calibration consists in bringing 

 the lower end of the thread to the point at which 

 it is desired to begin, and then setting it exactly 

 end to end up the tube, determining each time the 

 length of the detached column. When the cali- 

 bration has been carried as far up as is desired, 

 the thread is run out and weighed. Subsequently 

 a long thread of mercury, one filling nearly the 

 whole length of the calibrated portion of the tube, 

 is drawn in and measured, and then run out and 

 weighed. It is evident that the weight of the short 

 thread, multiplied by the number of settings, will _.„,»*. 



' , p i i ,i .«„. ji Fig. 16. — First arrangement 



be less than that of the long thread tilling the same f or calibrating manometers. 



(a) Screw for setting cali- 

 brating thread; (b) hard- 

 rubber cup; (c) enlarge- 

 ment in glass tube (i) ; 

 (d) litharge-glycerine ce- 

 ment; (e) contracted end 

 of glass tube; (J) rubber 

 stopper; (#) manometer; 

 (h) calibrating thread sep- 

 arated from main column 

 of mercury by air; (?') glass 

 tube rilled with mercury. 



length of tube, by the weight of the mercury re- 

 quired to fill the double meniscus spaces. By 

 means of this relation, the correction for the vol- 

 ume of the double meniscus is readily calculated. 



SECOND METHOD. 



The later procedure differs from the earlier one 

 in manner rather than in principle. After etching 

 upon the glass the two lines previously men- 

 tioned, a small bulb is blown near each end of the tube outside the 

 portion to be calibrated. These serve to catch and preserve the cali- 

 brating thread in case of accident. For calibration, the tube is placed 

 in the horizontal position, over a ruled mirror, on the dividing engine, 

 the screw of which has been carefully compared with the graduated 

 meter scales employed in the measurement of osmotic pressure. 



The device employed for shifting the thread from one position to 

 another is shown in Figure 17. A is the manometer with its two bulbs 

 (a, a). The two lines of reference previously referred to as the 

 "scratches" are seen at b, b. The shifting arrangement (B) for the cali- 

 brating thread (c) consists of a steel ball (d), a large bicycle ball, which 

 is located in the center of a rubber tube (e). A and B are connected 

 through the glass tubes (/,/) and the rubber tubes (g, g). 



