DET AND MOIST AIK. 181 



poisonous gases is extremely different in different animalg. 

 Exhaustive experiments on this subject are not before us; 

 hence the only general conclusion, as applicable to all animals, 

 that we can draw from the experiments made on certain 

 animals by physiologists, is, that every gas contained in the air 

 affects the animals that breathe it according to its relative 

 proportion, and to the peculiarities of each individual animal. 

 Thus, for instance, carbonic acid, which is highly poisonous to 

 man, ceases to be injurious when it is contained in the atmo- 

 sphere in a proportion of only 1 to 2,000 (of volume); but even 

 then it is not directly advantageous to animal life, unless, 

 indeed, its stimulating action may perhaps be recognised as a 

 not unimportant factor in the life of animals. At present, how- 

 ever, we know little on this point. On the other hand, we 

 know positively that no air-breathing animal is capable of de- 

 composing and assimilating carbonic acid as plants do ; '' nay, 

 it may be doubted, as we have seen in a former chapter, 

 whether even those aquatic animals — Sponges, Infusoria, 

 Worms, &c. — which are said to have true chlorophyll in their 

 tissues, do in fact make use of this constituent as an organ for 

 the decomposition of carbonic acid ; nor do we know whether 

 the maximum of carbonic acid which can be endured by the few 

 animals experimented on — which is perhaps even advantageous 

 to some — is equally endurable by all air-breathing animals, or 

 whether, for many of them, it may not lie, even higher. Pro- 

 bably in this respect the various species, and perhaps even 

 different individuals of the same species, may behave quite 

 differently.'^ 



What is of more importance to our enquiry, at any rate in 

 this place, than the admixture of different gases in air, is the 

 proportion of water contained in any given volume of air at a 

 given time. Our personal experience teaches us that a dry or a 

 damp wind has a totally different effect on different indivi- 

 duals; phthisical patients are sent in North America to the 

 driest mountain regions of the Union, as Colorado, while in 

 Europe they are frequently sent to very damp places, as 

 Madeira, &c. Moisture in the air frequently induces rheuma- 

 tism, but in this respect also different individuals are differently 



