292 ABSORPTION OF VAPOUR—POISONOUS GASES. 
this channel—Of the water of the blood, from which this 
exhalation is given-off, a small part is most probably formed 
by the direct union of the hydrogen contained in the food 
(especially when this is one of its predominating components, 
§ 153) with the oxygen absorbed. For it has been found by 
careful experiment, that the proportion of inspired oxygen 
which disappears (not being contained in the carbonic acid 
expired, § 305), is much greater in animals that are fed on a 
flesh diet, than in those living on farinaceous food. Another 
portion of such oxygen probably unites with the sulphur and 
phosphorus of the food and tissues, to form sulphuric and 
phosphoric acids, which are excreted through the kidneys in 
combination with alkaline bases (§ 367). 
344. Certain gases act as violent poisons, even when respired 
in very small proportion. Thus, a Bird is speedily killed by 
breathing air which contains no more than 1-1500th part of 
sulphuretted hydrogen; and a Dog will not live long in an 
atmosphere containing l-800th part of this gas. The effects 
of carburetted hydrogen are similar ; but a larger proportion 
is required to destroy life. Both these gases are given-off by 
decomposing animal and vegetable matter; the neighbourhood 
of which is consequently very injurious to health. Several 
cases of arsenical poisoning have occurred, from the accidental 
inhalation of a small quantity of arseniuretted hydrogen, the 
amount of arsenic contained in which must have been so 
minute as to be scarcely appreciable. 
CHAPTER VII. 
OF EXCRETION AND SECRETION. 
General Purposes of the Excreting Processes. 
345. We have seen that the Blood, in the course of its 
circulation, not only deposits the materials that are converted 
into the several fabrics of which the body is composed, but 
also takes-up into itself the products of the decomposition 
which is continually going-on in its various parts; and it is 
to replace this, that the constant Nutrition of the tissues is 
required. In order that the blood may retain its fitness foi 
