ANIMAL HEAT. 257 



the day, in the green stems, the leaves, the buds, and even the 

 roots and fruit. The maximum temperature of these parts, above 

 that of the surrounding atmosphere, was sometimes a little over 

 one-half a degree Fahrenheit; though it was often considerably 

 less than this. 



The different parts of the vegetable fabric, therefore, generate 

 different quantities of caloric. In the same manner, the heat- 

 producing power is not equally active in different species of ani- 

 mals; but its existence is nevertheless common to both animals 

 and vegetables. 



With regard to the mode of generation of this internal or vital 

 heat, \ve may start with the assertion that its production depends 

 upon changes of a chemical nature, and is so far to be regarded as 

 a chemical phenomenon. The sources of heat which we meet with 

 in external nature are of various kinds. Sometimes the heat is of 

 a physical origin ; as, for example, that derived from the rays of 

 the sun, the friction of solid substances, or the passage of electric 

 currents. In other instances it is produced by chemical changes ; 

 and the most abundant and useful source of artificial heat is the 

 oxidation, or combustion, of carbon and carbonaceous compounds. 

 Wood and coal, substances rich in carbon, are mostly used for this 

 purpose ; and charcoal, which is nearly pure carbon, is frequently 

 employed by itself. These substances, when burnt, or oxidized, 

 evolve a large amount of heat ; and produce, as the result of their 

 oxidation, carbonic acid. In order that the process may go on, it 

 is of course necessary that oxygen, or atmospheric air, should have 

 free access to the burning body; otherwise the combustion and 

 evolution of heat cease, for want of a necessary agent in the chemi- 

 cal combination. In all these instances, the quantity of heat gene- 

 rated is in direct proportion to the amount of oxidation ; and may 

 be measured, either by the quantity of carbon consumed, or by that 

 of carbonic acid produced. It may be made to go on, also, either 

 rapidly or slowly, according to the abundance and purity in which 

 oxygen is supplied to the carbonaceous substance. Thus, if char- 

 coal be ignited in an atmosphere of pure oxygen, it burns rapidly 

 and violently, raises the temperature to a high point, and is soon 

 entirely consumed. On the other hand, if it be shut up in a close 

 stove, to which the air is admitted but slowly, it produces only a 

 slight elevation of temperature, and may require a much longer 

 time for its complete disappearance. Nevertheless, for the same 

 quantity of carbon consumed, the amount of heat generated, and 

 17 



