RESPIRATION. 229 



Fish and Crustacea purify their blood by the air contained in the water which 

 they draw over their gills. They perish if the water is deprived of air ; and in this 

 case, as well as when the water is aerated but limited in quantity, and whether it 

 is exposed to the air or in close vessels, they perish sooner as the temperature is 

 higher. (Dr. Edwards, 1. c. P. ii. ch. 2.) And the younger and smaller they 

 are, when there is too little air in the water, the more they come to breathe at the 

 surface, and the sooner die if prevented, (p. 118.) Fish die in the air by drying 

 and wasting, (p. 126.) The syren lacertina sindproteus anguina have both gills and 

 lungs. Insects have no lungs, but openings on the surface of the body leading to 

 air-vessels which are distributed in the interior. Dr. M. Hall has shown that, 

 in the lungs of at least the toad, frog, and salamander, the blood-vessels sub- 

 divide into capillaries suddenly, so as to subdivide as much blood as possible, 

 and cause it to present the largest possible surface. (1. c. p. 36. sqq.) All 

 the experiments of naturalists made it appear that no animal could live without 

 oxygen, but M. Biot has asserted that what are called blaps and tenebrions remain 

 in as good a vacuum as can be formed for any length of time without apparent 

 inconvenience. Animals found in many parts of the bodies of others can hardly 

 be thought to have access to gaseous oxygen. In regard to the frequency of 

 respiration in cold-blooded animals, Dr. Stevens incidentally mentions that he 

 observed it no more than three or four times in a minute in an alligator, which 

 he once lield in his hand, and in which it was probably quick from the animal 

 being young and agitated. (1. c. p. 35.) 



In the light, vegetables produce changes in the air opposite to those produced 

 by animals. They decompose carbonic acid, retain the carbon, and leave the 

 oxygen. It is the green substance of the living leaf which effects the decom- 

 position. In the dark, the leaves absorb oxygen ; a tendency which, indeed, the 

 flowers, roots, and other parts, always have. This oxygen unites with the carbon 

 of the sap ; and, although some of the carbonic acid formed is said to be exhaled, 

 the greater portion combines with the fluids of the sap, and parts with its oxygen 

 again in the leaves when daylight comes. Carbon obtained in the state in which 

 it exists at the moment of its separation from carbonic acid appears the object. 

 While animals, therefore, increase the carbonic acid of the atmosphere and lessen 

 its oxygen, vegetables increase its oxygen and lessen its carbonic acid, at least 

 during the light ; and the functions of vegetables are the most active at that 

 period of the year when the days are much longer than the nights. 



