Lerts & Burake—The Carbonic Anhydride of the Atmosphere. 219 
APPENDIX II. 
A Comparison! or THE Resutts oF DETERMINATIONS OF CARBONIG 
ANHYDRIDE BY PETTENKoFER’s OrtcinaL Process, AND THE MrruHop 
PROPOSED BY Proressor Letts anp Mr. Buaxe, anp on THE ERRORS 
INCIDENTAL TO PETTENKOFER’S Process. By Wm. Caupwett, B.A., 
Hon. Demonstrator of Chemistry in the Queen’s College, Belfast, 
and 1851 Exhibition Scholar. 
Prorsssor Letts and Mr. Blake have described a modification of 
Pettenkofer’s process, whereby results of very great accuracy may be 
- obtained ; and at Professor Letts’ suggestion, I have made a series of 
experiments in order to compare the results obtained by this method 
with Pettenkofer’s original process. 
It is difficult to decide as to what is really meant by Pettenkofer’s 
process, for Pettenkofer himself describes, at least, three modifications 
of it, and many of those who have employed his process have intro- 
duced their own. 
The question is further complicated by the fact that the principle 
involved did not originate with Pettenkofer at all, but with Dalton,’ 
who had his own disciples, namely, Watson and Emmett. 
Angus Smith says that the use of oxalic acid makes the process 
Pettenkofer’s ; but, against this, we have Pettenkofer’s own statement 
that ‘‘any dilute acid which is not volatile at ordinary temperatures 
_ may be used.’ 
1 We thought it would be more satisfactory if the comparison were entrusted to an 
independent observer, and was not made by ourselves, as we had acquired a good deal 
of experience of the working of our method, but none of Pettenkofer’s. There was 
therefore, we thought, a chance that the results we might obtain by the latter 
would not be so favourable by comparison as might be the case if we had equal 
experience of the two. 
For this reason we entrusted the work to Mr. Wm. Caldwell, in whose care and 
accuracy as an analyst we knew we might have full confidence. The subject was 
perfectly new to him, and he had had no experience whatever of these determinations. 
We requested him to carry out both processes exactly as described in the original 
memoirs and with all the precautions stated therein. He has pursued the work with 
zeal and ability, and the results he has obtained are very striking and of considerable 
importance. 
# Phil. Trans., [1826]. Part 2, p. 174. 
3 Chem. Soc. Journ., 10 [1858], p. 292. 
SCIENT. PROC. R.D.S., VOL. IX. PART II. R 
