212 THE BLOOD. 



scale, starting from a zero at the same level, is engraved on 

 the measuring tube. The apparatus is filled with mercury by 

 raising the mercury-holder (t) to a sufficient height, the stop- 

 cock (/) remaining open ; in doing which the surface of the 

 mercury in t must not be more than a few millimetres higher 

 than the tap. As soon as the mercury appears at #, the stop- 

 cock is closed. The next step is to fill the laboratory tube. 

 Having inverted it in the trough, which has been previously 

 raised to the proper height, the operator draws out most of 

 the air by means of a bent tube, the point of which rises to 

 the top of the laboratory tube, and shuts the stopcock as soon 

 as the mercury rises. The removal of the air is completed by 

 joining g and g 1 so as to connect the laboratory tube with the 

 measuring apparatus, and then causing the air contained in 

 the former to pass over into the latter, b} r depressing t. The 

 stopcock h must now be closed and g and g' disconnected to 

 allow of the expulsion of the air from a. This having been 

 accomplished, g and g f are again brought together and care- 

 fully secured. The whole apparatus is now full of mercury ; 

 as soon as it has been ascertained that the joint is air-tight at 

 all pressures, it is ready for use. Before proceeding further, 

 however, the measuring tube, which, as already stated, is 

 graduated in millimetres measured from an arbitrary zero line 

 near the bottom, must be calibrated. In other words, it must 

 be ascertained as regards each principal mark of the gradu- 

 ation, what volume of air or water (as the case may be) the 

 tube contains, when the upper convex surface of the mercury 

 stands exactly level with it. For this purpose the orifice a, is 

 connected by means of an India-rubber tube with a reservoir 

 (a funnel) containing distilled water. The mercurial column 

 is then allowed to descend until it stands exactly at zero. A 

 weighed beaker having been then placed under a, water is ex- 

 pelled till the column stands at a height of fifty millimetres, 

 and the beaker again weighed. In a similar manner the out- 

 flow of water corresponding to a rise of the mercurial column 

 from fifty to one hundred millimetres is determined, until the 

 capacity which corresponds to each fifty millimetres of the 

 scale is ascertained. To insure accuracy, the process must be 

 repeated several times. If the results, after correction for 

 difference of temperature, are in close accordance, the means 

 may then be taken as expressing the capacities required. In 

 the upper part of the tube, calibration must be made at short- 

 er intervals. In calibrating, as in all subsequent measure- 

 ments, the height of the column must be read horizontally 

 through a telescope, so adjusted that its axis is at the same 

 height as the surface of the mercury. The temperature is 

 read by a thermometer suspended in the cylinder of water by 

 which the barometer and measuring tube are surrounded. 



