of Determining Moisture and Carbonic Acid in Air. 329 



which had been washed with mineral acid and then allowed 

 to stand full of water for a day; but the result was not always 

 higher with this bottle. 



In the bottle experiment ~No. 6 (6), which gives the worst 

 result of all, a smaller bottle of about 3 litres capacity was 

 used. In Parkes's 'Hygiene/ 7th edition (1887), p. 711, it 

 is stated that the results by the bottle method vary with the 

 different sizes of bottle. It seems likely that as a rule the 

 results will be less correct the smaller the bottle used. 



When the results are too low this is as likely to be due to 

 insufficient time being given for the carbonate to settle out 

 as to incomplete absorption. If the attempt is made to titrate 

 too soon, the result may be much too low, and the end reaction 

 will be very indefinite. 



It is no conclusive test of the correctness of the bottle 

 method that two simultaneous experiments give the same 

 result. If the conditions are kept the same in the two 

 experiments, the results must be the same whether right or 

 wrong. In the above experiments we usually varied the 

 conditions somewhat as regards details supposed to be un- 

 essential. In experiments where the conditions were carefully 

 kept the same, results were practically identical. There are 

 doubtless several sources of error, which may neutralize one 

 another more or less. The subject is complicated by the fact 

 that nearly every observer introduces, often unconsciously, 

 some new modification of the method. 



We have come to the conclusion from the above experi- 

 ments that the bottle method in its ordinary form is of very 

 limited use for experiments on free air, though usually quite 

 accurate enough in the case of vitiated air. 



Blochmann * has recently described a modification of the 

 bottle method which enables him both to avoid all possibility 

 of contact of the baryta-water with air, and at the same time 

 to titrate with perfectly clear baryta- water. For want of the 

 special apparatus required we have not yet tested this method. 



We used Pettenkofer's tube method in some of our 

 earlier experiments for the purpose of testing the efficiency 

 of the absorption of carbonic acid by soda-lime. Before we 

 had overcome the difficulty caused by errors of weighing, we 

 believed that any carbonic acid which had escaped absorption 

 by the soda-lime would be indicated best by Pettenkofer 

 tubes filled with baryta-water. Accordingly in different 

 experiments the air was passed through one or more Petten- 

 kofer tubes after it had passed through the soda-lime. We 

 always found that the baryta-water had lost distinctly in alka- 

 * Liebig's Annalen, cexxxvii. p. 72 (1SS7). 



