CHAP. IV.] THE BLOOD. METHODS OF RESEARCH. 211 



"In the preceding example such variations of temperature and barometric 

 pressure as may occur during the analysis are disregarded. The readings 

 are taken immediately after the absorption of the carbonic acid gas ; as the 

 time occupied in the analysis up to this point is very short, the error 

 arising from the variations in question is inconsiderable. As regards the 

 absorption of oxygen, the error might be of more consequence, were it riot 

 that the residue of nitrogen is so small. As it is, it can be easily sheM'n 

 that it would require a difference of pressure amounting to three 

 millimetres, and a difference of a degree of temperature, to make an error 

 of one-hundredth of a percentage in the result as regards nitrogen or 

 oxygen. Within these limits, therefore, the errors arising from this source 

 may be regarded as trivial. 



Frankland's " Although determinations of oxygen made by absorption 



larger appa- with hydrate of potash and pyrogallio acid are not entirely 

 ratus for the f re e from objection on the score of accuracy, the results ob- 

 tained by the method above described are quite accurate 

 Som^trLTas enough for most of the purposes of physiological research, 

 well as ab- ^ or ^ e small errors are practically inappreciable, as compared 



sorptiometric with the variations in the proportion of oxygen contained in 

 methods. the blood to be analysed, produced by what might be regarded 



as very trifling differences in the mode of collecting it. If it is desired to 

 have recourse to explosion with hydrogen, the best methods for the purpose 

 are those of Dr W. Russell arid of Frankland and Ward. The following 

 short description of the latter will be readily understood from what has pre- 

 ceded. The apparatus (Fig. 46a} consists of two parts, corresponding to the 

 laboratory tube arid measuring tube of the instrument previously described. 

 The measuring tube (F, Fig. 46ct) communicates, as in that instrument, with 

 a second tube (ff, Fig. 46a), containing a column of mercury, by the height 

 of which the pressure to which the gas to be measured is subjected can be 

 estimated. The chief difference is that, whereas in the former more simple 

 instrument the pressure tube is open at the top, so that if air is contained in 

 the measuring tube, and the stop-cock by which it communicates with the 

 laboratory tube is closed, the difference between the heights of the two 

 columns indicates the difference between the tension of the gas in the measur- 

 ing tube and that of the atmosphere in the instrument now before us the 

 tube is closed and constitutes a barometer, so that the difference expresses the 

 actual tension of the gas in inches of mercury. In the horizontal channel, by 

 which the measuring tube and barometer communicate at the bottom, is a 

 three-way stop-cock (not shewn in the figure); by which they may be brought 

 into communication either with a vertical escape tube, the end of which 

 dips into a receptacle containing mercury several feet below, or with a tube 

 open at the top (G, the middle and longest in the figure), called the filling tube. 

 In this way the gas can be expanded or compressed at the will of the opera- 

 tor, and consequently can (in most analyses) be readily brought to the same 

 volume after each successive operation. The convenience of this is very 

 great, for obviously the tensions of different quantities of gas when expan- 

 ded to the same volume are proportional to the volumes they would 

 assume if they were all under the same pressure, so that the original 

 volume of gas to be analyzed being known, the relation between 

 that volume and the volume of the other quantities to be measured can 

 be readily calculated, the several volumes being proportional to the 



142 



