1783 he and Laplace reported the results of their investigations on the subject of life to the Paris Academy of Sciences, but had little to say about the nature of life itself. In 1839 the British Association requested Liebig to study the subject of biology, as it then existed, and report at a subsequent meeting all that was known about it. The report was thorough, valuable and gave great satisfaction, but shed no light on the origin and nature of life. Though a vitalist, Liebig gave less attention to the defence of this belief than to the applications of the principles of chemistry to economics, especially as related to agriculture. Here his efforts were epoch-making. John Müller, of Berlin, and the Weber brothers, of Leipzig, investigated the physical and mechanical processes of life, so far as they could trace them by careful experiment in the laboratory. Du Bois Beymond and Helmholtz were trained by Müller and were among his most distinguished pupils.
It was while seeking answers to questions which investigations as to the origin and nature of life were constantly presenting that Helmholtz and Meyer, in entire independence of each other, discovered the principle of the conservation of force or energy. In 1847 Helmholtz proved, as he thought beyond any reasonable doubt, that living forces are manifestations of a certain quantity of power to do work. The outcome of these studies and the publication of theories based upon them brought about a change, in Germany especially, in the opinions hitherto held as to the nature of the vital force or vital principle. Not a few were content to reduce life to a mechanical process and to deny any distinction between life and matter. Others sought to discover the life process and to make its development clearer by many different theories. The "potential energy" of Helmholtz, the cell formation theory of Schwann, set forth in 1839, and the theory of Max Verworn, of Jena, that life consists in the "metabolism of proteids" do not require the supposition of any principle or force apart from matter itself to account for life in any of the forms in which it has appeared. And yet these theories do not deny the possibility of the existence of a life principle. Neither does acceptance of the "physiological unit," as suggested by Mr. Herbert Spencer, prevent us from believing that life may be something quite different from this unit and independent of it even if it manifest itself in and through it. Accurately as these life processes, as they are termed, have been traced by the most capable experimenters in the world, the product of these experiments, however exactly it imitate that of nature differs from it in toto. The laboratory product may contain the same elements, and so far as can be seen, arranged in the same proportion, and yet be entirely unlike the product of nature. No chemist has yet learned how to arrange atoms of matter in a living organism and adapt that organism to an environment in any such way as to compete with nature or indeed to give to the product of the laboratory anything worthy of being called life.