222 Scientific Proceedings, Royal Dublin Society. 
I have not been able to find the process by which Pettenkofer 
himself prepared his turmeric paper. In the preparation of the 
turmeric paper I have followed Sutton. The turmeric powder was 
first digested several times with hot water, and then transferred to a 
flask and digested with alcohol. The yellow solution was filtered into 
a large porcelain dish, through which strips of filter paper were drawn, 
and after draining for a moment or so were dried in the dark-room, 
and kept there until required for the several daily titrations. 
The whole process, therefore, consists of—(1) displacing the air in 
the flask by the air which is to be examined; (2) the addition of the 
absorbent, followed by (8) its titration, without the adoption of any 
extraordinary precautions to eliminate the errors arising from the 
transference of the absorbent, and the subsequent titration, durmg 
both of which operations the solution is exposed, not only to the air of 
the laboratory, but also, to a certain extent, to the breath of the operator, 
concerning which point Pettenkofer himself says :—‘‘It is merely the 
upper layer of the liquid that is affected by the carbonic anhydride 
in the air of the laboratory.” Hence, in transferring the solution to 
the titrating bottle, the pipette was allowed to reach almost to the 
bottom of the beaker and the lower layer of liquid only, used for 
titration. 
Next, as regards the process adopted by Professor Letts and Mr. 
Blake. The details of the method are described in the first part of 
their Paper, and in my experiments these details were adhered to. 
Certain points, however, require mentioning. The preparation of 
neutral distilled water for use in making the standard acid solutions 
was very interesting, and at Professor Letts’ suggestion the following 
experiment was tried :— 
To ordinary distilled water more than sufficient potassium hydrate 
was added to make it alkaline. It was then distilled, and portions of 
the distillate examined from time to time. They were at first alkaline, 
no doubt from ammonia, but then became acid, and eventually of a 
constant degree of acidity. This degree of acidity was found to 
correspond with the amount of carbonic anhydride in the laboratory in 
which the water was distilled. Thus the water, distilling over, satur- 
ated itself with carbonic anhydride at the tension due to that amount 
of carbonic anhydride in the atmosphere of the laboratory. A number 
of experiments showed that the amount of carbonic anhydride varied 
between 5 and -6 cubic centimetres per litre of water, whereas the 
amount calculated on the basis of four parts in 10,000 of air, and 
solubility coefficient 1:187 at 10° C. (the mean temperature of 
