Suppose an animal to be full grown. Take a full grown man. All that he eats as food is intended merely to renovate or replenish his system, to restore that which is daily removed from every part of his body by natural causes. _In the full grown state, every thing that enters the body must come out of the body_ in one form or another. The first part of the food that escapes is that portion of its carbon that passes off from the lungs during respiration. This quantity varies in different individuals—chiefly according to the quantity of exercise they take. From 5 to 9 ounces a day is the average quantity, though in periods of violent bodily exertion 13 to 15 ounces of carbon are breathed out in the form of carbonic acid. Suppose a man to eat a pound and a half of bread and a pound of beef in 24 hours, and that he gives off by respiration 8 ounces of carbon (3500 grains) during the same time. Then he has Carbon. Nitrogen. Taken, in his food, about 4500 grains, and 500 grs. while He has given off in } respiration, } 3500 and little or no nitrogen, Leaving to be converted } into food, or to } 1000 grs. and 500 grs. be rejected, } Our two conclusions, therefore, are clear. The vegetable food, by respiration, is freed from a large portion of its carbon, which is discharged into the air,—nearly the whole of the nitrogen remaining behind. In the food consumed the carbon was to the nitrogen as 9 to 1; in that which remains, after respiration has done its work, the carbon is to the nitrogen in the proportion of only 2 to 1. It is out of this residue, rich in nitrogen, that the several parts of animal bodies are built up. Hence the reason why they can be formed from food poor in nitrogen, and yet be themselves rich in the same element. It is this same residue also which, after it has performed its functions within the body, is discharged again in the form of solid and liquid excretions. Hence the greater richness in nitrogen,—the greater fertilizing power of the dung of animals than of the food on which they live. Two other remarks I shall add for the benefit of the practical man. 1. The manure of the cow, taking it mixed, is not so rich in nitrogen as that of man,—because the cow in the stall, large though it be, and great the bulk of food it consumes, does not give off much more carbon by respiration than an active full grown man. Hence the proportion of carbon in the excretions of this animal is greater than in those of man. The dry manure is richer than the dry food, weight for weight, but not in the same proportion as if the cow respired a quantity of carbon more nearly corresponding to its bulk, when compared with the weight of carbon thrown off from the lungs of man.