578 GENERAL REVIEW OF THE NUTRITIVE PROCESSES. 



is a condition necessary for the production of Heat, is fully proved by the fact, 

 that no caloric is evolved when the flowers are excluded from the contact of 

 Oxygen ; whilst the substitution of pure oxygen for atmospheric air occasions 

 the elevation of temperature to be more rapid and considerable than usual.* 

 The same may be said of the heat liberated by seeds in the act of Germina- 

 tion : a large amount of oxygen is absorbed, and of carbonic acid given out 

 during this process ; and the evolution of Heat may be easily shown to be as 

 dependent upon this change as in the instance just quoted. It is the opinion 

 of some physiologists, that the production of heat under these circumstances 

 is dependent upon a real process of Combustion ; the carbon of the Plant 

 uniting with the oxygen of the air, and thereby giving out caloric, as it does 

 in the ordinary burning of charcoal. Perhaps this account is rather too 

 simple. It is well known that most chemical changes, especially those in 

 which there is an alteration in the form of the agents concerned, are attended 

 with a change of temperature ; and it is not unreasonable to suppose, that, of 

 those Molecular alterations which have been shown to be so continually oc- 

 curring in the living system, some may be connected with the disengagement 

 of heat peculiar to it. For the continuance of these alterations, however, the 

 absorption of Oxygen and the extrication of Carbonic Acid are necessary condi- 

 tions ; if these be suspended, therefore, the temperature soon falls. 



725. When the phenomena of Calorification in Animals are carefully ex- 

 amined, they are found to harmonize completely with this view. Throughout 

 the whole kingdom, an exact conformity may be perceived between the amount 

 of Oxygen consumed and of Carbonic acid given off, and the degree of Heat 

 liberated. In the cold-blooded animals, whose temperature is almost entirely 

 dependent upon that of the surrounding medium, the respiration is feeble, 

 being carried on, for the most part, through the medium of water. In the 

 warm-blooded Vertebrata, however, which have the power of keeping up the 

 heat of their bodies to an elevated standard, even when that of the surround- 

 ing air is far beneath it, the quantity of oxygen consumed is very large ; and 

 that required by Birds is more, in proportion to their size, than that employed 

 by Mammalia; as we should expect from the more elevated temperature of 

 the former. In the class of Insects we have a very remarkable illustration of 

 the same general fact. It appears, from the researches of Mr. Newport, that 

 Insects, during their larva and pupa states, and even in their perfect condition 

 when at rest, are to be regarded as truly cold-blooded animals ; their tempera- 

 ture rising and falling with that of the surrounding medium, and being at no 

 time more than a degree or two above it. In a state of activity, however, the 

 temperature of the body attains a considerable elevation, frequently as much 

 as 10 or 15 above that of the air. It must be remembered that, owing to 

 their larger extent of surface in proportion to their bulk, small animals are 

 cooled much more rapidly than large ones ; and the temperature of Insects 

 would probably rise much higher, if it were not for the loss they are thus 

 continually experiencing, which is greatly increased by the action of the 

 wings. In one of Mr. N.'s experiments, a single Humble-bee, in a state of 

 violent exckement, communicated to three cubic inches of air as much as 4 

 of heat within five minutes ; its own temperature being raised 7 in the same 

 time. When several individuals in a state of excitement, however, are clus- 

 tered together, so that the loss of heat is prevented, the elevation of tempera- 

 ture is much more considerable ; thus, a thermometer introduced among seven 

 " Nursing-Bees" stood at 92^, whilst the external air was only 70; and the 

 temperature of a hive was raised by disturbing it, during winter, from 48! 



* See the very interesting experiments of MM. Vrolik and Vriese, in the Ann. des Sci. 

 Nat., N. S. Botan., torn. xi. p. 551. 



