216 THE BLOOD. 



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 operator, and consequently can (in most analyses) be 

 readily brought to the same volume after each successive ope- 

 ration. The convenience of this is very great, for obviously 

 the tensions of different quantities of gas when expanded 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 rela- 

 tion between that volume and the volume of the other quanti- 

 ties to be measured can be readily calculated, the several vol- 

 umes being proportional to the corresponding readings of the 

 barometer. The original volume of gas to be analyzed is mea- 

 sured as before described, with this difference, that the absolute 

 pressure to which it is exposed is known without reference to 

 the barometric pressure outside at the time. The explosion is 

 effected in the eudiometer, into the upper end of which two 

 platinum wires are fixed for the purpose ; the arrangement of 

 these wires is the same as in Bunsen's eudiometer. As to the 

 mode of preparing and introducing pure hydrogen, and of ex- 

 ploding the mixture, the reader will find sufficient information 

 in Roscoe's translation of Bunsen's Gasometry. 



32. Bernard's Method of Determining the Propor- 

 tion of Oxygen combined with the Coloring Matter 

 of the Blood by Displacement with Carbonic Oxide. 

 As was before stated, the property which carbonic oxide pos- 

 sesses of displacing the oxygen combined with the coloring 

 matter of the blood, has been used by Bernard, as a substitute 

 for the vacuum, for the determination of the quantity of free 

 and combined oxygen contained in the blood. Bernard's 

 method consists in agitating the blood to be analyzed in a 

 tube half filled with carbonic oxide. The carbonic oxide to be 

 used must be perfectly pure. The tubulated retort into which 

 the oxalic and sulphuric acid are introduced must be cleared 

 of atmospheric air, by passing a stream of carbonic acid 

 through it, before heat is applied. The gas is best collected 

 in flasks, over water containing potash in solution. Two re- 

 sults are produced. In the first place, the oxygen of the hae- 

 moglobin is replaced by carbonic oxide ; and, secondly, the 

 atmosphere of carbonic oxide acts on the blood as if it were a 

 vacuum, the displaced oxygen and other gases passing out 

 into it until equilibrium is established. Inasmuch as the pro- 

 portion in which oxygen is absorbed is very small, as com- 

 pared with the quantity held in combination by haemoglobin, 

 nearly the whole is discharged, so that if the proportion of 

 that gas contained in the gaseous mixture which fills the place 

 originally occupied by the carbonic oxide be determined, it is 

 found to fall very little short of the proportion obtained from 



