stance (urea) dates our modern organic chemistry. Liebig representing the school of Gay-Lussac and Wöhler that of Berzelius, one at Giessen and the other at Göttingen, serve as an interesting example of scientific cooperation to develop a new science.
Liebig's work led directly to those activities which we now group under the term physiological or biological chemistry, but physiology was at this time making rapid strides along another line of attack—the application of the principles of mechanics and physics. The part of physics in medicine from Galileo to Roentgen is one of the most fascinating phases of the history of medicine; in principle and practise, in theory and science, its influence has been one of fundamental importance and in its application to methods of clinical diagnosis it shares equally with pathological anatomy in the awakening of modern clinical medicine. The first widely reaching application was in Harvey's interpretation of the circulation of the blood and the action of the heart, but it was not until organized physiological laboratories had been instituted that the application of the principle of physics bore abundant fruit. To recall the state of physics at that time it is only necessary to state that the work of Galvani and Volta was completed and that Ampere and Ohm, Faraday and Wheatstone, were still active. Charles Bell had already (1811) given to England the second of two great discoveries in physiology, the differentiation of sensory and motor nerves. Haller, as we have seen, had in the preceding century presented and discussed the irritability of muscle. The time was at hand for the study of the general physics of muscle and nerve and the special senses. Ernst Weber announced the principles of his psycho-physics in 1825 and Johannes Müller those of his physical chemistry in 1826; Purkinjé had already established the first university laboratory of physiology in 1824 at Breslau; in 1838 the celebrated physiological institute at Berlin was formed under the direction of Müller and in 1840 Ernst Weber was made professor of physiology at Leipzig. From these two centers, Berlin and Leipzig, from Johannes Müller and Ernst PL Weber, came a great volume of minute investigations based on exact methods of inquiry. Both schools were largely busied with studies of the mechanism of the perceptions of the senses, that of Weber tending to include mental phenomena, thus anticipating the modern school of psychologists, that of Müller including not only the methods of physics, but also those of general biology. Müller (1801-1858) was indeed the last of a school which attempted to embrace all of the territory of biology in its broad sense; a territory which now has its separate and distinct fields of morphology, physiology and chemistry. He may, however, be regarded as responsible for some of the divisions into which the older biology has been split, and for the impulse to new lines of study, for he was the teacher of the masters who came in time to occupy high places in biol-
