48 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



formed can not recondense until the operation of closing is finished. 

 Otherwise the violent agitation of the mercury, due to rapid vaporiza- 

 tion and condensation, is apt to shatter the tube. When closed, no 

 bubble of air should be discernible at the top of the short mercury col- 

 umn. First attempts at closing usually fail in this respect, but after a 

 little practice, one is able to perform the operation with perfect success. 

 The short column of mercury in the upper end of the manometer has 

 a twofold purpose. It prevents, during the closing of the instrument, 

 any contamination of the nitrogen with air or with the combustion 

 products of the flame; and it fills up all that portion of the instrument 

 whose caliber may have been altered to an unknown extent by heating. 



DETERMINATION OF THE VOLUME OF THE NITROGEN. 



For this purpose, the manometer is placed in the steel block within 

 the bath (Figure 45), and the pressure upon the gas is regulated by the 

 device used in the determination of capillary depression, i. e., a glass 

 tube having an internal diameter of 35 millimeters, which is connected 

 with the steel block by means of a flexible tube. Formerly it was 

 attempted to use a stationary "side" tube. This consisted of a short 

 piece cut from the same tube as the manometer itself and, like the ma- 

 nometer, it was fixed rigidly in the block, the pressure being regulated by 

 the plungers. The practice was, however, based on the mistaken assump- 

 tion that in any given, fairly good tube the capillary depression is nearly 

 uniform throughout. It was discontinued when it was discovered that 

 the best tubes we could obtain were very uneven in this respect. 



The volume of the nitrogen is determined under a number of different 

 pressures, all of them, of course, quite near that of the atmosphere. To 

 determine it under high pressures, it is necessary to employ another 

 closed manometer — a so-called "standard manometer." There is, how- 

 ever, the same objection to the employment of standard manometers as 

 to the use of narrow side tubes in the determination of capillary depres- 

 sion and of gas volumes, the objection, namely, that all the errors of 

 both tubes — principally of capillary depression — are charged to the tube 

 under investigation. 



Sometimes, in order to increase the quantity of the gas in the ma- 

 nometer, more than the calibrated portion of the tube has been filled 

 with nitrogen. This was frequently done before the introduction of 

 manometers with large reservoirs of known capacity (Figure 19, etc.). 

 In such cases the use of a standard manometer could not be avoided. 



For the comparison of one manometer with another, the steel block 

 and also the "brass block" seen in Figure 27 are used. The latter 

 does not differ in construction from the former, except in the means 

 for fixing the tubes in their places. The arrangements employed for 

 that purpose are identical with those used in joining the cells and the 

 manometers. Some other liquid than mercury — either water or a 

 solution — is used in the brass block. 



