Towards the close of the eighteenth century oxygen was discovered by Priestley, which explained the process of combustion, which he believed to take place in the animal organism as well as outside the body. Liebig made valuable advances in the study of metabolism, and later investigations have verified the truth of his statements. In 1840 he published a dietary study which, from the standpoint of modern work, on account of its incompleteness, is of but little value. Still it was a pioneer publication which gave much assistance to many of his followers.
From 1850 to 1870 many experiments were made along these lines with animals, including cattle, dogs, and sheep. In 1865 and 1866 Voigt and Pettenkofer published the results of many experiments which they had made on man. To Voigt and his follows should be given the credit of the most valuable work of recent years.
Thorough work of a very high order has been done in Russia from which much accurate knowledge has been gained. The name of Van Noorden stands out prominently on account of his work on metabolism. Japan, Italy, Sweden, and England have all furnished students who have aided science along this line. As Americans we are especially proud of that which has been done in our own country. Chittenden and Flint have been earnest workers, and more accurate results are being obtained each year at the expense of our own government, under the able direction of Professor Atwater.
In the latest experiments account is taken of all food consumed, the excretory products, and the total energy manifested during the experiment, as heat or muscular work. The ideal has not as yet been reached, as no account has been made of body gain or loss, or the energy stored or transformed during the experiment.
