314 RESPIRATION. 



Mode of determining the Strength of the Baryta Solution. 

 Thirty centimetres of baryta water having been introduced 

 into a small flask, the solution of oxalic acid is cautiously 

 added from a finely graduated burette. Between each addi- 

 tion, the flask is closed with the thumb and shaken. As an 

 indicator, Pettenkofer has found that turmeric paper gives 

 better results than litmus. The paper must be prepared by 

 digesting turmeric root in weak alcohol, and dipping strips of 

 Swedish filter paper into it, which must then be dried in a 

 dark place, and kept in the dark. When the liquid is so nearly 

 neutralized that it does not brown a strip of paper dipped 

 into it, a drop is placed with a rod on the strip. If there is 

 still a trace of alkaline reaction, a brown line appears at the 

 peripheiy. As soon as this is no longer the case, the point of 

 complete neutralization has been attained. This reaction is so 

 delicate, that it is sensibly affected by the presence of one- 

 tenth of a cubic centimetre of solution of oxalic acid, i. e., 

 one-tenth of a milligramme of carbonic acid gas, so that the 

 results of two determinations of the same liquid ought not to 

 differ from each other by more than the quantity named. It 

 is well, in order to save time, to make a first experiment with 

 a small quantity (say 5 cub. cent.). It is of great practical 

 importance to notice that the baryta solution must contain no 

 trace of caustic potash, or soda, the smallest quantities of 

 which render the determination impossible for the oxalate of 

 potash or soda formed in this case reacts on the carbonate of 

 baryta present, so as to produce oxalate of baryta and carbo- 

 nate of soda. Consequently, the liquid never loses its alkaline 

 reaction, for each renewed addition of oxalic acid re-converts 

 the alkaline carbonate into oxalate, which is again decomposed 

 by the carbonate of baryta as before. 



Mode of preparing and filling the Absorption Tubes. The 

 short arm of each tube is filled air-tight with an India-rubber 

 cork, pierced with a tube. The larger tube is connected at its 

 opposite end with the smaller, from which the air finally escapes 

 through an India-rubber tube, guarded by a screw-clip. By 

 adjusting this clip, the size of the bubbles is regulated, their 

 magnitude varying inversely as the resistance. To fill the 

 tubes, the required quantities of liquid are introduced into 

 flasks, fitted with air-tight corks, having necks sufficiently wide 

 for the introduction of a pipette. The strength of the solution 

 having been determined in thirty centimetres, as above de- 

 scribed, the stronger solution is to be delivered into the first 

 absorption tube in three quantities of 45 centimetres, and two 

 such quantities of the weaker into the second. [Tubes of the 

 size required for these quantities are made by Cetti & Co., 

 of Brobke Street, Holborn.] The tubes are then closed and 

 adjusted to the proper inclination (previously ascertained by 



