76 Composition of the Atmosphere 



bon dioxide absorbed. The volume of the air in the compensating pipette 

 is also adjusted to exactly the same pressure and temperature as that in 

 the measuring-vessel, and by measuring the apparent increase in volume 

 of the air in the compensating-pipette, a direct measure of the carbon 

 dioxide absorbed is obtained. The carbon-dioxide-free air is now passed 

 into the potassium pyrogallate to absorb the oxygen. When the air is 

 again returned to the measuring-pipette, the volume is adjusted so that 

 the tension of the residual gas (nitrogen and argon) is exactly the same as 

 the tension of the gas in the compensating-pipette. The amount of con- 

 traction in volume may then be directly read as the percentage of oxygen 

 in the air. 



DETAILED DESCRIPTION OF THE APPARATUS. 



A somewhat diagrammatic representation of the various parts of the 

 apparatus is given in fig. 1 ; the apparatus as actually in use is shown in 

 the frontispiece. The main features of this apparatus are two calibrated 

 measuring-pipettes A and B; two reagent containers C and D; a delicate 

 manometer M, and a series of glass stop-cocks permitting intercom- 

 munication between all parts. 



The whole apparatus is substantially mounted upon a heavy block of 

 marble. The pipettes and reagent containers are immersed in a glass 

 tank filled with water, which is supported at the bottom by a metal ring 

 support firmly fastened to two nickel-plated uprights (81 cm. high), 

 while a similar ring holds the upper part. In the bottom of the glass 

 tank are four holes, through which the ends of the various pipettes and 

 reagent containers pass, these passages being made water-tight by rubber 

 connections. The water in the tank is stirred by allowing a current of 

 air to bubble through it. Under these conditions, therefore, the tempera- 

 tures in the apparatus are uniform throughout, and while a thermometer 

 is suspended in the water-bath, the temperature readings made with it 

 are not essential. 



The two measuring-pipettes A and B, the two reagent reservoirs C 

 and D, and the manometer M are connected by capillary tubing and glass 

 stop-cocks, so that all five members are fused together into one whole. 

 Stop-cock a connects pipette B either with stop-cock b, which controls 

 the entrances to the two analytical reagent reservoirs, or with stop-cock 

 d, which, in turn, connects with either the outdoor air or with the right 

 side of the manometer M. Stop-cock c connects pipette A with either 

 the outside air or with the left side of the manometer. Stop-cocks / and 

 e at the bottom provide communication between the leveling reservoirs 

 F and E and their respective pipettes A and B. The best quality of stout- 

 walled rubber tubing should be used for connecting the leveling bulb with 

 the stop-cocks/ and e. This is important, since we find that many kinds 

 of rubber tubing contaminate the mercury, so that a thin coating of sul- 

 phide, which interferes seriously with the accurate reading of the meniscus 

 level, forms on the top. 



