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Dr. W. Marcet. On the 



depressed. The adjustment is exceedingly fine, and by testing the 

 pressure of the air in the holder with a gauge supplied with coloured 

 petroleum, the air in the bell-jar can be brought under atmospheric 

 pressure with the greatest degree of accuracy. I am indebted to 

 Mr. W. Parkinson, Engineer, for the mechanism of this delicate 

 adjusting movement. 



The (smaller) air-holder is now connected with the left-hand bell- 

 jar of the analysing apparatus by varnished india-rubber tubing, in 

 the track of which is another tube of calcium chloride (omitted in 

 the plate), care having been taken to rinse out previously this con- 

 necting tube with the air to be analysed. The bell-jar is now raised 

 by turning the handle of the drum and lifting slowly the iron weight 

 which keeps the receiver immersed in the tank. When filled, the air 

 is brought under atmospheric pressure by the adjusting apparatus, and 

 a minute or two are allowed to elapse to make sure that the pressure 

 is constant ; the volume of air to be analysed is then read off: on the 

 scale of the air-holder. 



The temperature indicated by the thermometers in the two bell- 

 jars is observed by means of two little telescopes fitted into the 

 glazed shutters of the case, and which magnify considerably the 

 divisions of the instruments. By this means the temperature in the 

 bell-jars can be estimated to the fiftieth of a degree Fahrenheit, and 

 are recorded when steady ; the temperature in the absorption-tube is 

 also noted. After opening and closing the requisite stopcocks, the 

 air is driven slowly by means of the drum-handle through the 

 absorbing apparatus into the right-hand bell-jar, which is simul- 

 taneously raised. I find from experience that about 7 litres are 

 sufficient, and indeed the most convenient for the analysis of air 

 expired from the lungs ; and four minutes suffice for the absorp- 

 tion of the whole of the carbonic acid of that air run over the 

 alkaline solution. After the first operation, it is obvious that the 

 air filling the pipes of the left-hand bell-jar has been left behind. 

 By means of a combination of tubes and taps, the two bell-jars 

 are now brought into immediate communication, and the air is 

 returned into the left bell- jar up to the original mark on the scale 

 without passing through the absorption-tubes. Meanwhile, a con- 

 siderable suction has taken place in the apparatus from the absorption 

 of the carbonic acid, and this is shown by the pressure-gauge. The 

 small graduated glass receiver, originally full of common air under 

 atmospheric pressure, and which stood at 0, is depressed into the 

 mercury, and by this means atmospheric pressure is readily re- 

 established in the whole apparatus, as seen by the petroleum in the 

 gauge returning to exactly the same level in the two limbs ; the car- 

 bonic acid normally present in this atmospheric air is not taken into 

 account as too small in its proportion to affect the correctness of the 



