Nuns in their cloistered convents, prisoners in jails often grow fat in spite of their wretched food, because the air they breathe being deficient in oxygen, withdraws but a small portion of the carbon from the system, the remainder going to the formation of fat. It is when the human body has attained its full growth, and especially in the decline of life, that fat in excess begins to be developed. I am of opinion that want of exercise is one of its principal causes. With increasing age the step becomes more guarded, and a repugnance is felt for all bodily exertion. In this way the quality of the air, and the quantity of oxygen it contains have much to do with the formation of fat. By virtue of that happy distribution and balance of forces to be met with throughout the universe, the expired carbonic acid gas of men and animals is destined to the nutrition of plants, which assimilate the carbon and set free oxygen gas. Plants being thus chiefly composed of carbon, are, when taken as food, rich in the chief constituent of fat; and fat itself is frequently a vegetable production. Mutton fat resembles that of the cacao bean, and human fat is similar to olive oil. It is therefore clearly established that the immediate and direct cause of the development of fat in the case of men and animals is to be sought in the nature of the aliment, giving, at the same time due weight to the several general conditions which have a tendency to favor the development of obesity. All food which is not flesh--all food rich in carbon and hydrogen must have a tendency to produce fat. Upon these principles only can any rational treatment for the cure of obesity satisfactorily rest. CHAPTER V. ON THE TREATMENT OF OBESITY. It can scarcely be necessary that I should record all the several methods which have been proposed and adopted for the reduction of obesity; yet, lest I should be charged with ignorance, some mention must be made of the several useless and contradictory opinions and methods which have been adopted, frequently to the serious injury of the general health of the patient.