On the other hand, we learn also from chemistry, that one of the principal constituents of meat is nitrogen, an element which does not enter into the composition of fat. Food consisting chiefly of meat must be less productive of fat than food mainly composed of carbon and hydrogen, such as vegetables, &c. Distinguished chemists have endeavoured to shew in what manner the development of fat takes place in the animal economy. A paper was read by me before the Academy of Sciences, at Paris, on the 15th December, 1851, from which the following extract is made: "Three different opinions are entertained by distinguished chemists, who have given attention to this subject. The first, that of Dumas, maintains that the fatty matter of the body is derived solely from substances analogous to fat in composition, which pre-exist in the food. The second opinion, that of Liebig, is to the effect that the formation of fat is due to a modification of those ternary compounds which constitute so large a proportion of the food of animals. The third opinion suggests that fat may arise in consequence of some special fermentation taking place in the stomach. "Numerous experiments have been made, in order to determine which of these opinions is correct; but it may be safely said that no satisfactory conclusion has been arrived at. "In the first place, the experiments have never been conducted under circumstances favourable to the formation of a correct opinion. It is obviously of the first importance, when conducting experiments of this nature, that the food should be supplied so as not to interfere with the tone of the general health, considered morally as well as physically. We can conceive that the deprivation of liberty, in the case of an animal usually in the enjoyment of freedom, may render the experiment of dubious import. Although man is omnivorous, it is impossible that any one can submit, for a great length of time, to live upon one kind of food only, without suffering a sense of loathing. "What inference can be drawn from those experiments, made for the purpose of ascertaining whether sugar is capable of producing fat, when they were made upon pigeons and doves, which were fed solely upon this substance; at one time being deprived altogether of water, and at another time allowing them as much as they chose to drink?