Speedy Volumetric Determination of Carbonic Acid. 187 



easy to ascertain when the gauge is exactly under atmospheric pressure. 

 The fluid I have selected for the gauge is coloured petroleum, on 

 which the slightest difference of pressure will tell. 



Finally, there is an arrangement of brass tubes and stopcocks 

 through which the air, after having been driven from the left-hand 

 bell-jar into the other through the absorption-tube, can be trans- 

 ferred from the right to the left bell-jar without retracing its passage 

 through the absorption apparatus. 



In my early experiments, india-rubber tubing, pinch-cocks, and 

 glass T _ pi eces were nsed, but I substituted for these the present 

 brass tubes and stopcocks, and the air-tightness of all the apparatus 

 can be thoroughly relied upon. It is not possible to do away entirely 

 with india-rubber tubes, but those in use are now varnished with 

 several coats of copal varnish, and made thereby perfectly air-tight. 



The Analysis. — The air to be analysed, which, so far has been air 

 expired from the lungs or such air mixed with common air, is first of 

 all collected in an india-rubber bag, faced on both sides with oil-silk 

 in order to prevent any loss of carbonic acid by diffusion through its 

 substance. The bag is next connected by india-rubber tubing with a 

 gas-holder or counterpoised bell-jar, working in and out of a bath of 

 glycerine covered with a layer of almond oil. By weighting the coun- 

 terpoise of the bell-jar, the latter is slowly raised, aspiring the air 

 from the bag. A scale on the bell- jar and a pointer on the rim of the 

 tank enable the volume of air to be read off, but of course the extra 

 weight used for giving the receiver its ascending motion will have to 

 be removed and the holder placed under atmospheric pressure pre- 

 vious to taking the observation. I now find it more convenient to 

 read off the volume of air to be analysed from a smaller bell-jar 

 holding 11 litres of air. After collecting the air from the bag into 

 the larger receiver, which, when full, contains about 42 litres, it is 

 driven into the smaller bell- jar through a glass desiccator full of cal- 

 cium chloride, where it leaves its moisture without losing any of its 

 carbonic acid. In order to dry the gas thoroughly, it should pass 

 through the desiccator at the rate of no more than a litre per minute. 



The next step will be to bring this air in the small holder exactly 

 under atmospheric pressure, and a special contrivance had to be 

 adopted to effect this object. The instrument used is a clamp and a 

 screw in connexion with each other ; the screw, by means of a crank 

 movement, raises or depresses the clamp, while the fulcrum is the 

 rim of the tank to which this instrument is fixed. The cord holding 

 the counterpoise passes through the open jaws of the clamp, and 

 is free to move up and down ; but when the air in the receiver 

 has to be brought under atmospheric pressure, by turning a nut the 

 clamp is closed and the cord fixed : then, on moving the screw with 

 another nut either to one side or the other, the bell-jar is raised or 



