CHAP. IV.] THE BLOOD. METHODS OF RESEARCH. 209 



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 ensure 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 shorter intervals. In calibrating, as in all subsequent measurements, 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. 



"The measuring and laboratory tubes having been brought into connec- 

 tion in the manner described above, and both filled with mercury, the gas 

 to be analysed is introduced into the laboratory tube from the test- 

 tube in which it has been collected. It is then at once transferred to 

 the measuring tube by depressing t until the mercury rises in the laboratory 

 tube as far as the stop-cock g '. This done, the stop-cock g is closed, 

 and t raised or depressed till the column stands at one of the marks 

 of the graduation, in reference to which the capacity of the tube has been 

 determined. The temperature is then observed, and the pressure deter- 

 mined by adding the difference between the height of the column in 

 the measuring tube and that in the pressure tube, to the reading of 

 a barometer which stands by. A few drops of solution of caustic 

 potash having been introduced into the laboratory tube, the gas is returned 

 from the measuring tube. Absorption takes place rapidly. It is accelera- 

 ted by slightly agitating the trough, and by allowing the mercury to stream 

 into the laboratory tube after the gas has passed. The measurement of the 

 gas after absorption is performed in the same manner as before. About 

 half a centimetre of strong solution of pyrogallic acid is then introduced in 

 the same way as the potash, and the gas again returned. After absorption 

 of the oxygen, what remains is nitrogen. In analysis of blood gases, the 

 proportion of nitrogen is nearly constant, viz. about 2 '5 volumes in 

 100 volumes of blood. If a larger quantity is obtained, the fact indicates 

 that air has entered. Whatever method of analysis is employed, the 

 results must be reduced to temperature and 760 millimetres pressure 

 i.e. they must be expressed as if the measurements had been made 

 under those conditions. A further deduction must be made from 

 each measurement in respect of the aqueous vapour which the gas contains 

 (the measuring tube being always moist). This is accomplished by the 

 following well-known formula: 



v= V B'-f 



l + 0-00367* 760 ' 



V denotes the corrected volume; V the volume read; t the temperature; 

 H' the observed pressure; and f the tension of aqueous vapour at the 

 temperature t. The values of 1 + 1 0'00367 and/ are always obtained from 

 tables. For these and many other important practical details relating 

 to the performance of gas analysis, the reader is referred to Mr Button's 

 'Volumetrical Analysis,' whom I have to thank for two of the woodcuts 

 with which this section is illustrated. To illustrate the application 

 of the method to the analysis of the gases of the blood, I give the following 

 example : 



G. 14 



