There are two circumstances which at first sight may appear to interfere with the explanation above given of this very beautiful phenomenon. First, the lungs are found to be but very little warmer than any other part of the body, although, as we have stated, the animal heat is produced in them: and, secondly, the quantity of carbon produced by respiration is very small compared with the genial heat produced by its conversion into carbonic acid. With regard to the heat of the lungs, a series of experiments instituted for the purpose of ascertaining how they were kept at so moderate a degree of temperature, led to the discovery of an extraordinary change which takes place in the vital fluid after being purified, which satisfactorily explains the circumstance. The pure blood is found to have a greater capacity for heat than impure blood: it will absorb more; and in consequence, all the heat produced by its purification is immediately absorbed by it, and carried away as fast as it is generated, to be distributed over the body. As the blood becomes impure in its progress, it gradually loses its power of retaining the heat it had so imbibed; and the heat therefore is distributed during the circulation of the blood, and every part receives a due supply. This change in the power of the vital fluid to absorb heat, according as it is more or less pure, is a fact that was not established in the time of Paley, or he would have been able to add another proof of design to his unequalled argument. The quantity of fuel (if we may use the expression) required for generating the heat of the animal frame, is certainly less than we might anticipate. All animal and vegetable food contains a considerable portion of carbon, which of course, after being digested, becomes a part of the vital fluid, and in this way it is supplied for the process. It is well known also that in cold climates, where a greater quantity of animal fuel is required, the inhabitants are extremely fond of fat and oily matters, which contain more carbon than any other kind of food; yet it would hardly be imagined that so small a quantity as the eighth part of an ounce of carbon per hour would be sufficient to maintain the heat of the body at an uniform temperature of 98 degrees. We are assured by the best chemists, however, that the average quantity of carbonic acid generated by a person in health in twenty-four hours is about 40,000 cubic inches, and this contains only about 11½ ounces of pure carbon. Rather less than half an ounce is therefore used per hour in preserving the body at its usual temperature.