AJSD OTHEE PHENOMENA OF EESPIEATION. 
701 
This experiment also confirms Regnault’s observation, that the composition of the 
expired air is uniform during fasting. 
In reference to the variations of the day in the quantity of vapour exhaled, the high- 
est amount (2 '2 6 grs. per minute) was obtained in the two first hours of the inquiry, or 
those nearest to the last meal, and the lowest amount (1-74 gr.) was obtained after 8 p.m. 
On the following morning the quantity was increased to 2-2 grs. per minute, and it did 
not exceed that after I had taken the arrowroot and water. 
I do not find any observations on record with which I can compare these results. 
There yet remain for examination two other sources of variation in the respiratory 
phenomena, viz. those occurring at short intervals from day to day, and those extending 
over a lengthened period of months and associated with season. I shall now describe 
them. 
The experiments connected wfith both heads of the inquiry were made in the investi- 
gation of the influence of food, and under precisely the same external circumstances as 
to period of the day, food, exertion and excitement ; for they were all made between 
/ and 8 a.m., before breakfast, in the sitting posture, and in the absence of all exertion 
and mental excitement. They were commenced on March 30, and were continued with 
legulaiity bj/ Mi. Moul and myself until the middle of June, and then by myself alone 
until the present time. 
/3. Variations from day to day. 
Nearly all observers have found that the absolute quantities recorded on any day 
did not correspond with those of other days ; but as the inquiries were not made 
under precisely the same external circumstances, it was not possible, with certainty, 
to assign any cause to which the variation could be attributed. It would not 
pel naps be unieasonable to presume that, with identity of external circumstances 
on successive days, the system would not vary in any appreciable degree ; but a 
reference to Plate XXXIV. will show that whilst in not a few instances the quan- 
tities obtained were constant, in many others the luriation from day to day in the 
amount of carbonic acid amounted to half a grain, and in a few to even one grain 
per minute ; and that this was not due to any error in the inquiry, may be inferred 
from the fact that, when a food was under examination which did not vary the 
respiratory changes, the same numbers were obtained in several successive experi- 
ments within from one to two hours. Hence we must search within the system for 
the explanation, and we shall find reason to believe that the nature of the night’s 
rest, and the amount of nutriment remaining in the system, cause variations in the ' 
amount of carbonic acid expired in the early morning, before food has been taken. 
On manj occasions I noted these conditions at the time that I made the experiment. 
Thus I passed a restless night on April 7, June 1, 16, 18, 23 and 24; had short night’s 
sleep on April 24 and 28 ; was not well on April 15, 16, May 8, 18, June 14, and 
