1891.] Formation of Carbonic Acid in Human Respiration. 61 



ceiver was so carefully counterpoised that the person under experi- 

 ment could not tell whether he was breathing into it or into the 

 external air. It was supplied with a scale divided into litres and 

 fractions of litres; an oil gauge showed the pressure of the air 

 within it, and a thermometer its temperature. A number of pre- 

 cautions were taken in connexion with the mode of collecting the air 

 expired, which cannot be entered into at present. I have satisfied 

 myself that breathing into these bell- jars is identical with natural 

 respiration : the volumes expired, say, per minute really correspond- 

 ing with the volume of air expired per minute while breathing natu- 

 rally into the open air. Each experiment lasted between 7 and 8 

 minutes. 



The determination of carbonic acid in the air expired was made 

 exactly in the way described in a previous paper to the Royal Society.* 

 The air expired was transferred from the be 11- jar to a cylinder, shaken 

 with baryta-water, and finally determined by titration, according to 

 Pettenkofer's method. 



The oxygen was determined in a eudiometer, constrncted on the 

 same principle as the eudiometer I have described in the * Proceedings 

 of the Royal Society, 'f but modified and improved. Instead of a 

 straight tube this instrument consists of a JJ-tiibe, with a neck and a 

 glass stop-cock at its bend. One limb is left open, and the other is 

 closed at the top by an iron cap, in which a three-way cock is fitted, 

 perfectly air-tight. Two short iron tubes project beyond the tap. 

 Both limbs of the \J -tube are graduated. The limb bearing the iron 

 cap is graduated into cubic centimetres, and the open limb is gradu- 

 ated into divisions corresponding exactly with those on the other 

 limb, so that, whatever be the level of the mercury in the [J -tube 

 under atmospheric pressure, the readings are identical in both limbs. 

 The scale at the back of the open limb is movable, so that it can be 

 brought easily into its proper position and fixed there. The closed 

 limb is surrounded with a water jacket, and the iron cap is partly 

 immersed in water, while the platinum wires are embedded in a 

 shellac cement, so as to be effectually protected from contact with 

 the water. 



The hydrogen for exploding the gas was prepared with every care 

 from zinc and sulphuric acid ; it was washed first through a strong 

 solution of potassium hydrate, and then through water. A volume of 

 hydrogen, at least nine or ten times that of the air-spaces in the 

 Woulffe's bottles, was passed through before collecting the gas ; it was 

 finally aspired into a glass bell-jar of a capacity of about a litre, and 

 movable up and down in a glass receiver holding water. 



* " A Chemical Enquiry into the Phenomena of Human Respiration," ' Phil. 

 Trans./ B, 1890. 



f " A new form of Eudiometer," ' Eoj. Soc. Proc.,' June, 1888. 



