20 FAMILIAR LETTERS ON CHEMISTRY. 



respirations ; it is, therefore, obvious, that in the same animal the quantity of 

 nourishment required must vary with the force and number of respirations. A 

 child breathes quicker than an adult, and, consequently, requires food more fre- 

 quently, and proportionably in larger quantity, and bears hunger less easily. A 

 bird, deprived of food, dies on the third day, while a serpent, confined under a bell, 

 respires so slowly, that the quantity of carbonic acid generated in an hour can 

 scarcely be observed, and it will live three months, or longer, without food. The 

 number of respirations is fewer in a state of rest than during labor or exercise ; the 

 quantity of food necessary in both cases must be in the same ratio. An excess of 

 food, a want of a due amount of respired oxygen, or of exercise, as also of great 

 exercise, (which obliges us to take an increased supply of food,) together with weak 

 organs of 'digestion, are incompatible with health. 



But the quantity of oxygen received by an animal through the lungs, not only 

 depends upon the number of respirations, but also upon the temperature of the 

 respired air. The size of the thorax of an animal is unchangeable ; we may, there- 

 fore, regard the volume of air which enters at every inspiration as uniform. But its 

 weight, and consequently the amount of oxygen it contains, is not constant. Air is 

 expanded by heat, and contracted by cold an equal volume of hot and cold air 

 contains, therefore, an unequal amount of oxygen. In summer, atmospheric air 

 contains water in the form of vapor ; it is nearly deprived of it in winter. The 

 volume of oxygen in the same volume of air is smaller in summer than in winter. 

 In summer and winter, at the pole and at the equator, we inspire an equal volume 

 of air ; the cold air is warmed during respiration, and acquires the temperature of 

 the body. In order, therefore, to introduce into the lungs a given amount of oxygen, 

 less expenditure of force is necessary in winter than in summer, and for the same 

 expenditure of force more oxygen is inspired in winter. It is also obvious that, in 

 an equal number of respirations, we consume more oxygen at the level of the sea 

 than on a mountain. 



The oxygen, taken into the system, is given out again in the same form, both in 

 summer and winter; we expire more carbon at a low than at a high temperature, 

 and require more or less carbon in our food in the same proportion ; and conse- 

 quently more is respired in Sweden than in Sicily, and in our own country an 

 eighth more in winter than in summer. Even if an equal weight of food is con- 

 sumed in hot and cold climates, Infinite Wisdom has ordained that very unequal 

 proportions of carbon shall be taken in it. The food prepared for the inhabitants 

 of southern climes does not contain in a fresh state more than twelve per cent, of 

 carbon, while the blubber and train, oil, which feed the inhabitants of polar regions, 

 contain sixty-six fco eighty per cent of that element. 



From the same cause it is comparatively easy to be temperate in warm climates, 

 or to bear hunger for a long time under the equator; but cold and hunger united 

 very soon produce exhaustion. 



The oxygen of the atmosphere received into the blood in the lungs, and circulated 

 throughout every part of the animal body, acting upon the elements of the food, is 

 the source of animal heat. 



LETTER VII. 



MY DEAR SIR: 



The source of animal heat, its laws, and the influence it exerts upon the 

 functions of the animal body, constitute a curious and highly interesting subject, to 

 which I would now direct your attention. 



All living creatures, whose existence depends upon the absorption of oxygen, 

 possess within themselves a source of heat, independent of surrounding objects. 



This general truth applies to all animals, and extends to the seed of plants in 

 the act of germination, to flower-buds when developing, and fruits during their 

 maturation. 



In the animal body, heat is produced only in those parts to which arterial blood, 

 and with it the oxygen absorbed in respiration, is conveyed. Hair, wool, and 

 feathers, receive no arterial blood, and therefore, in them no heat is developed. The 

 combination of a combustible substance with oxygen is, under all circumstances, the 

 only source of animal heat. In whatever way carbon may combine with oxygen, 



