36o Dr. Prout on the ultimate composition 
method is adopted, it obviously becomes necessary that no 
extraneous water be present ; but all pulverulent substances, 
and oxide of copper among the rest, are more or less hygro- 
metric, and rapidly attract moisture from the atmosphere. 
This circumstance seems to have struck the French chemists, 
and it occurred to me at a very early period. Dr. Ure, 
however, was the first who published a method of obviating 
this difficulty; and his method, if this were the only diffi- 
culty to contend with, is capable of considerable precision. 
But there is another, and perhaps still more troublesome 
property, possessed by the oxide of copper, in common with 
many other powders, namely, that of condensing air as well 
as water ;* and these two difficulties, taken together in con- 
junction with another mentioned in a note below, render great 
precision almost out of the question.-f To conquer these, 
* See Saussure’s paper on the absorption of the gases by different bodies. 
Annals of Philosophy, vi. 241. Also Gilbert’s Annalen der Physick, xlvii. 112. 
f As I am unwilling that so much labour should be lost, particularly as it may 
be of some use to other inquirers, I have thrown into the form of a note a few of 
the principal circumstances connected with the inquiry mentioned in the text. In 
my earlier experiments tubes of iron, copper, &c. were employed instead of glass, 
and charcoal instead of spirits, as the medium of heat ; and during this period 
most of the modifications of apparatus which have been since proposed as novelties 
or improvements, were tried and rejected. I first took the hint of employing a 
spirit lamp from Mr. Porrett, and was certainly among the first that did so 
employ it. Various forms of lamps were tried, but at length I was induced to 
relinquish the use of the horizontal apparatus for the vertical one;! and this, I 
have no hesitation in saying, is by far the best form of apparatus hitherto proposed 
for the substances to which it is adapted; nor would any other have been required 
by me, at least, had it not been for the properties of the oxide of copper alluded to 
in the text, which render this and all other forms of apparatus depending on the 
J Described in the Annals of Philosophy, xv. 190 (O. S.) and more completely in 
Dr. Henry’s Chemistry, ii. 167, ninth edition. 
