308 Messrs. Haldane and Pembrey on an Improved Method 



seen below, the disadvantages of the chemical method, as 

 ordinarily used, are dependent on a want of attention to 

 details. It consists of a pair of test-tubes, containing pumice 

 soaked in sulphuric acid. The test-tubes are 4x1 inch, 

 and made of thin glass. Each tube is provided with a 

 double-bored cork about J inch thick, which is fitted with 

 glass tubing of about T 3 g- inch internal diameter, and in the 

 form shown in the figure*. The tubing must fit firmly. The 

 corks are covered with a layer of hard paraffin inside and out. 

 They are pushed down a very little below the tops of the 

 tubes, the end of each tube being first wiped inside free of 

 acid, and then warmed to soften the paraffin on the cork and 

 so facilitate its entrance. To enable the longer limb of the 

 tubing to be pushed down, a passage should first be cleared 

 in the pumice with a piece of glass rod. A layer of paraffin 

 is spread smoothly over the top of the cork until it is just 

 level with the edges of the tube. The apparatus must be 

 absolutely tight, and should allow air to pass perfectly freely 

 when suction is applied. It should finally be carefully 

 cleaned with a wet cloth to remove any traces of acid, and 

 then dried. 



The pumice is sifted through a wire sieve with about 7 

 meshes to the inch, and shaken in a fine one to remove the 

 powder. It is then heated to redness by playing on it 

 with a large blowpipe-flame, and thrown still hot into u pure 

 redistilled" sulphuric acid. The superfluous acid is then 

 poured off, and the pumice preserved for use in a stoppered 

 bottle. Before using the tubes for experiments we have 

 always taken the precaution of washing them out with air, 

 as the air passed through them at first usually tastes slightly 

 of sulphurous acid. 



Each pair of tubes when filled weighs about 80 grammes, 

 and can therefore be weighed on any ordinary balance. 



To diminish to a minimum errors arising from accidental 

 variations in weight of the tubes, we adopted the plan of 

 weighing against a counterpoise consisting of a similar 

 absorption-tube, and of about the same weight. This coun- 

 terpoise is always kept in the same place as the absorption- 

 tubes. Since during an experiment the absorption-tubes will 



* The longer piece of glass tubing in the first sulphuric acid absorption- 

 tube is made somewhat shorter than the corresponding tubing in the 

 other absorption-tube. The inconvenience arising from the collection of 

 water in the first tube is thus avoided. 



Since writing the above we have been supplied by Messrs. Gallenkamp 

 with pairs of tubes made in one piece. These are more brittle, but other- 

 wise very convenient. 



