July 30, 1870.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
87 
ash, and then reading the contraction thereby pro¬ 
duced, is very unsatisfactory, inasmuch as the normal 
quantity of carbonic acid in the atmosphere is not 
more than twice as large as the experimental error 
involved in the two readings requisite for deter¬ 
mining on the amount of contraction sustained by 
the volume of air experimented on. This will be 
apparent from the following figures:—In 100 volumes 
of average air, there is from O’030 to O’040 of a 
volume of carbonic acid. The readings of “ original 
volume of air, before absorption by potash,” and 
“volume after absorption by potash,” cannot be de¬ 
pended upon as being accurate within less than the 
toImJo °f the total volume operated upon. In the in¬ 
stance, therefore, of a perfectly pure gas, absolutely 
devoid of carbonic acid, we should sometimes get 
0’02 for the percentage of carbonic acid. In other 
words, a variation of 0’02 per cent, is devoid of 
meaning when this method is employed. But 0’02 
per cent, is equal to more than one-lialf of the ave¬ 
rage percentage of carbonic acid in the atmosphere; 
consequently this method cannot be depended on for 
determining the actual variations which occur in 
the carbonic acid of the atmosphere, and this should 
be borne in mind in reference to results obtained in 
this way. Possibly the abnormal results obtained 
by Dr. Frankland in the examination of air from the 
summit of Mont Blanc, were due to this cause. 
Another method employed for this purpose, is to 
ascertain the gain in weight, which an apparatus 
charged with solution of caustic potash undergoes 
when a known volume of air is transmitted through 
it. This method is obviously unsatisfactory, for 
a gas, containing so little carbonic acid as the at¬ 
mosphere, cannot be deprived of its carbonic acid 
unless it be passed through solution of an alkali at a 
rate far slower than is practicable, and it is impos¬ 
sible to ensure that the immense volume of inflowing 
gas shall be in exactly the same state of dryness as 
the outflowing gas. The third method, which alone 
is satisfactory, is rather an old one. It consists in 
enclosing a known volume of air with a quantity of 
lime or baryta-water, and subsequently observing 
the quantity of carbonate formed. De Saussure, 
who was the first to operate in this manner, weighed 
the carbonate of lime; Dalton, and later, Pettenkofer 
accomplish the same end by means of titrations; the 
strength of the volume of lime or baryta-water being 
known beforehand, a titration of the lime or ba¬ 
ryta-water, after the absorption, furnishes the re¬ 
quisite data. As has been said, we never meet with 
so much carbonic acid in a badly-ventilated room as 
to be of any importance in itself; inasmuch, however, 
as the discharge of carbonic acid iuto the atmo¬ 
sphere never takes place without being accompanied 
by other contamination, the amount of carbonic acid 
in a given specimen of air may be useful as an index 
to the general purity of the specimen. From this 
point of view, accurate determinations of the carbonic 
acid in air acquire a value. They are, moreover, 
very easily made. 
Referring to Dr. Smith’s Report for 1809, let us 
consider the method of research followed in his in¬ 
quiries. This may be characterized as consisting in 
washing a known and generally enormous volume of 
air with a relatively small volume of water, and 
then examining the water. The results of the water- 
analysis are subsequently to be referred to the volume 
of air which has been submitted to the process of 
washing. 
The first question naturally arising is, how far 
washing with water is capable of removing micro¬ 
scopic quantities of impurity from air ? I have no 
hesitation in asserting that the operation of washing 
air, or other gas, with water is adequate for the 
entire transference of all finely divided solids, and 
all vapours from the former to the latter. A few 
years ago the author had a large quantity of 
gases, wherein was diffused the vapour of a liquid. 
That vapour was the product of a new reaction he 
was studying, and from the circumstances of the 
case, it could be obtained only as vapour mixed with 
large volumes of gases. How to extract the vapour 
was the question ? By reducing the temperature as 
far as possible nothing was to be expected. For, un¬ 
less at the lowest temperature reached, the tension 
of the vapour should fall so low that the volume of 
the gas would become supersaturated with vapour 
there could be no condensation at all. Even 
when condensation did occur, a certain quantity of 
vapour would necessarily remain, sufficient to satu¬ 
rate the gases at that temperature. Washing the 
gas with water was therefore tried, and it proved 
successful; the liquid sought for, which had existed 
in the form of vapour diffused through an immense 
volume of gas, was found in the wash-water. The 
ease with which such absorptions are effected, and 
the obvious completeness of the operation will com¬ 
mend themselves to all who make experiments of 
this kind. 
In order to ensure the complete absorption of every 
trace of vapour and finely divided solid, the method 
adopted by Dr. Ransome, in his recent experiments 
on the organic matter of respired air, will doubtless 
prove very advantageous. Dr. Ransome cooled the 
air until the aqueous vapour in it began to condense, 
and then shook up with water. A little considera¬ 
tion will show how admirably this method of pro¬ 
cedure is calculated to promote a thorough washing 
of every particle of the air under examination. 
In the new air-analysis, this extraction of the 
liquid and solid impurity by means of washing with 
water is a cardinal method, and it deserves a pro¬ 
minent place among the resources of modern chemi¬ 
cal analysis. 
The method of water-analysis, used by Dr. Angus 
Smith for the examination of the wash-water, was 
that invented by Wanklyn, Chapman, and Smith, 
viz. the ammonia-process. 
CHLORAL. 
BY C. A. MARTIUS AND P. MENDELSOHN-BARTHOLDY.* 
In the course of our experiments on the preparation 
of hydrate of chloral, it fell in our way to examine diffe¬ 
rent preparations found in commerce, the purity of 
which was guaranteed by crystallization from sulphuret 
of carbon and ether, and subsequent pressing out. Our 
attention was also directed to the varying statements 
respecting the points of fusion and ebullition of hydrate 
of chloral, and to the possibility of admixture of foreign 
substances, even after the renewal of free chlorine and 
hydrochloric acid, and notwithstanding its solubility and 
apparent homogeneity; and that to these foreign sub¬ 
stances might be due the discrepancies relative to phy¬ 
sical properties, and possibly also the physiological dis¬ 
crepancies found by different observers. We were espe¬ 
cially struck by the differences in boiling-point in diffe¬ 
rent preparations, and led to the preparation of a scries of 
* Buchner’s ‘ Repertoriiun fiir Pliarmacie,’ 1870. 
F 3 
