42 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



of osmotic pressure, the mercury has ample time to adjust itself, or 

 the adjustment is aided by means to be described hereafter; but in 

 operations connected with the determination of capillary depression, 

 and with the comparison and verification of these instruments, the "lag" 

 of the mercury must be overcome by jarring the tubes, and frequently 

 the tapping to which it is necessary to subject tubes of small bore is 

 severe and prolonged. This is notably the case in manometers in which 

 the glass is not perfectly clean or has been slightly roughened by the 

 reagents employed in cleansing it. 



The construction of the "tapper" is so obvious that it is only neces- 

 sary to notice two or three of its features. It is strongly inclined, 

 when in operation, to move away from the tube which the "hammer" 

 is striking. Hence the base is made of lead, for the sake of greater 

 weight, and is mounted upon three very sharp-pointed pegs, which 

 sink somewhat into the wood on which the instrument rests. The 

 hammer is covered with rubber or leather to prevent the possible 

 shattering effect of its blows. The tapper is connected, by means of a 

 flexible wire cord, through the battery, with a portable push-button 

 which is held in the hand of the observer behind the cathetometer, who 

 can therefore at any time hammer the tube without removing his eye 

 from the telescope. 



During the determination of capillary depression and other operations 

 which are connected with the preparation of manometers for use, the 

 instruments must be kept at constant temperature. Otherwise the all- 

 important meniscus is continually changing its form, to the great con- 

 fusion of the observer. The first effective device for the maintenance 

 of temperature was the so-called u manometer house" which is seen — 

 stripped of its coverings — in Figure 24 and Plate II. It was in this 

 that, for several years, all experiments on manometers, except calibra- 

 tion in the horizontal position, were carried out. The "house" con- 

 tains the "steel block," the "brass block" — to be described later — the 

 "tapper," a meter scale, a thermostat for the regulation of temperature, 

 electric heaters (lamps), and a fan motor, all of which will be recognized 

 in the figures. The shelf (Figure 24), on which rest the various instru- 

 ments, is supported by heavy steel brackets (not shown in the figure), 

 which are bolted to the heavy masonry wall behind, and afford a satis- 

 factory degree of stability. At each end of the shelf, a space 5 centi- 

 meters wide is left for the passage of air. Lamps are employed as the 

 source of heat, for the reason that they heat up and cool down more 

 quickly than other electric heating appliances. They are under the 

 control of the thermostat seen in the upper part of the house. The fan 

 is stationed before a hole of equal diameter in the partition 2. By 

 means of it, the air, heated by the lamps, is kept in continuous circu- 

 lation over all the instruments. The temperature which is maintained 

 in the compartment is always higher by a few degrees than the highest 

 temperature of the room in which it is located. 



