220 Scientific Proceedings, Royal Dublin Society. 
The first thing necessary, therefore, was to decide what Petten- 
kofer’s process really is. 
In the Chemical Society’s Journal, there is an original paper,! in 
which Pettenkofer describes one modification of his process, and in the 
Journ. f. prakt. Chem.? another modification is described by him. 
There is a third method, which is usually termed the ‘ Aspira- 
tion’ method, in contradistinction to the ‘‘ Bottle’? method, which 
Pettenkofer employed originally in hig experiments on Respiration. 
In each of the two first processes, a standard solution of lime or 
baryta water is used as the absorbent, the excess of either of which 
is afterwards titrated with standard oxalic acid, turmeric paper 
being used as indicator. Similar absorbing vessels are employed in both 
of these methods, namely, glass bottles or flasks, closed by bungs or 
india-rubber capsules, the important point of difference between the 
two methods being that, whereas, in the first, the titration is performed 
within the absorbing vessel, in the second, it is made in a small 
bottle on an aliquot portion of the absorbent (after the latter has done 
its work) which has been pipetted off. As regards the third or 
‘aspiration’ method, it is not, I think (at least in Great Britain), 
closely associated with Pettenkofer’s name. The main principle is 
that a certain volume of air is aspirated slowly through a tube filled 
with the absorbent (lime or baryta water) placed at a slight angle, and 
an aliquot portion of the liquid subsequently titrated with oxalic acid, 
and turmeric paper as indicator. 
Of these three methods, I have adopted, in all the experiments I 
have performed, the second, for two reasons—first, because I believe it 
was the one which is chiefly associated with Pettenkofer’s name, and 
with which he did most of his work; and secondly, because the first 
method (from the description of the process) would appear to be more 
difficult of execution, and more subject to errors of manipulation. 
IT may now describe, in as few words as possible, the precautions I 
observed in carrying out the process. 
First of all, as regards the absorbing vessels, and the method of 
filling them with the air sample to be examined. 
Several series of experiments were performed. In the first of these, 
flasks of nearly three litres capacity were used, and in the latter series, 
similar vessels of about twice that capacity. 
1 Zoe. cit. 
* Journ. prakt. Chem. 85 [1862], p. 165. 
3 Liebig’s Annalen, [1862-1863], Supplb. 2, p. 23. 
