Whilst the microscope on the one hand has gone on developing now structures, the chemist has demonstrated that these structures exhibit life but in obedience to chemical laws. Numerous treatises have been written on the chemistry of animal life, and all bear more or less the impress of the genius of Liebig.
If the first work excited controversy, it could hardly fail to be pro duced by the second. Mulder accused Liebig of appropriating his discoveries without acknowledgment, especially his great discovery of protein. To this question Liebig, who, in the meantime had some doubts with regard to the real nature of this substance, replied " Will Mulder say what is protein ?" Whether this substance exists or not, the discovery is undoubtedly due to Mulder of the identity in animals and plants of the substances known as fibrine, albumen, and caseiue, and that the animal is dependent on the vegetable kingdom for its supply of them, in one form or the other. The importance of this discovery can hardly be overrated, whether protein lies at the foundation of those nitrogenous matters or not. Many of Liebig's physiological views have met with very decided opposition, and many of his opinions have been shown to be incorrect. But his great glory will always be the method he pursued. By this method he has pat the physiologist in the right direction to attain the great aim and ends of his science. These views are of the highest interest for mankind, as they involve no Ices questions than the very existence of man, and the best possible means of enjoying that existence.
However complete the first outlines of his theories might appear to be, Liebig never ceased working at correcting and perfecting them. Between the period of the publication of the editions of his works on Agricultural and Animal Chemistry, his Annalen ' and the conti nental journals teem with his papers on various points which had been canvassed in his books; and in all directions, in his own laboratory and iu other places, we find men working under his advice and direc tion. It was thus that, from the time the subject of food occupied his attention at all, he prosecuted new researches on the nature of the food, and of those changes in the animal body by which it becomes the source of life, and ultimately the material rejected from the system. In 1849 another work was prepared for the English press,
and translated by Dr. Gregory. This wee entitled 'Researches ou the Chemistry of Food.' In this work he gave an account of his experi ments on the changes which the tissues of the body undergo, and which result in the conversion of fibrin° and albumen into gelatine, and eventually urea. In these experiments he operated on large quantities of animal flesh, and succeeded in demonstrating the uni versal presence of kreatinc, a compound first described by Chevreul, also of kreatinine, lactic acid, phosphoric acid, and inosiuio acid, in the flesh of animals. In this work he also drew attention to the exist ence of phosphate of soda iu the blood, and its power of 'absorbing carbonic acid, as having an iuteresting relation with the function of respiration. He has also shown in this work that the proper cooking of food can only be carried on upon fixed chemical laws, and that much improvement in the economical and sanitary relations of this art may bo expected from a larger knowledge of the changes undergone by food in its preparation.
In all his labours Liebig has ever striven to avoid being one-sided. No one seems to have felt from time to time more acutely than himself the fact that, after all, the organic body is not an apparatus of glees tubes and porcelain dishes. He ever tried to penetrate into the nature of those properties and laws which, acting upon the textures of the human body, seemed to interfere with an anticipated necessary chemical result.. It is in this spirit that we find him prosecuting researches upon the physical properties of the tissues, and inquiring into the nature of those laws of the diffusion of matter which had been known under the name of endosmose and exoamose. The results of his researches and inquiries on this subject were again communi cated to the English public through Professor Gregory, who trans lated the work on The Motions of the Juices in the Animal Body,' which was published in 1848.