

AMOUNT OF CARBONIC ACID EXHALED. 383 



gen of the fatty matter, to form part of the water which is exhaled from 

 the lungs. 



690. This interchange would seem to depend upon the tendency 

 which all gases have to mutual admixture, when they are separated 

 by a porous septum. According to the law discovered by Prof. Gra- 

 ham, the relative volumes of the gases which will thus replace each 

 other, are inversely as the square-roots of their specific gravities ; thus, 

 the specific gravity of oxygen being to that of hydrogen as 16 to 1, the 

 replacing volume of oxygen is to that of hydrogen as 1 to 4. The 

 same holds good, when one of the gases is absorbed by a liquid ; pro- 

 vided the replacing gas be also capable of being absorbed to the same 

 extent. On this principle, the replacing volume of oxygen is to that 

 of carbonic acid as 1174 to 1000 ; but as the actual amounts inter- 

 changed do not constantly follow this ratio, it is obvious that they are 

 liable to be modified by other conditions, these being chiefly (it seems 

 probable) the relative quantities of the two gases already present in the 

 blood, and the relative facility with which they are absorbed into it or 

 extricated from it. 



691. It is difficult to form an exact estimate of the actual quantity 

 of Carbon, thrown off from the lungs in the form of Carbonic Acid 

 during any lengthened period : since the amount disengaged during 

 experiments carried on for a limited time, cannot, for many reasons, be 

 taken as affording a fair average. Thus the quantity will vary with 

 the external temperature, with the state of previous rest or activity, 

 with the length of time that has elapsed since a meal, and particularly 

 with the general development of the body. The amount of carbonic 

 acid exhaled is greatly increased by external cold ; as is shown in the 

 results of such experiments as the following. Small Birds and Mam- 

 mals having been enclosed in a limited quantity of air, for the space of 

 an hour, at ordinary temperatures, the quantity of carbonic acid they 

 produced was noted. The experiment was then repeated at a tempera- 

 ture nearly approaching that of the body ; and was performed a third 

 time at a temperature of about 32. The following are the comparative, 

 amounts. 



Temp. 59 68. Temp. 86 106. Temp, about 32. 



Grammes. Grammes. Grammes. 



A Canary, 0-250 0-129 0-325 



A Turtle-dove, 0-684 0-366 0-974 



Two Mice, 0-498 0-268 0-531 



A Guinea-pig, 2-080 1-453 3-006 



Thus it would appear that the quantity of carbonic acid exhaled between 

 86 and 106 is not much more than half of that which is exhaled be- 

 tween 59 and 68 ; and is only about two-fifths of that which is given 

 off at 32. 



692. The quantity of Carbonic Acid exhaled during exercise, and for 

 a certain time after it, and also after a full meal, is considerably in- 

 creased ; whilst on the other hand, it is greatly diminished during sleep. 

 Thus a person who was excreting 145 grains of carbon per hour, whilst 

 fasting and at rest, excreted 165 after dinner, and 190 after breakfast 

 and a walk ; whilst he only excreted 100 during sleep. The variation 



