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 368 CHEMISTRY searches fall more properly within the prov- ince of physics, they are nevertheless of the greatest interest in their chemical bearings. Preeminent in this respect are his works upon the liquefaction of gases, and upon certain com- pounds of hydrogen and carbon, which last was one of the starting points of the doctrine of isomerism, also upon the compounds of carbon and chlorine, and of ammonia and metallic chlorides. Working like Faraday in the do- main of both physics and chemistry, Mitscher- lich of Berlin (1794-1863) exerted a great in- fluence on the present condition of the science by his discovery of the law of isomorphism, in accordance with which certain groups of sub- stances exist, any one member of which can be replaced by any other member or set of members in equivalent proportion in its com- pounds, without changing the crystalline form of the latter to any material extent ; and of dimorphism, the power possessed by some sub- stances of crystallizing in two distinct systems not reducible to the same primary form. Fol- lowing as did these discoveries upon those of Faraday, and that of the French physicists, Dulong and Petit, of the relation between the specific heats and equivalent weights of sub- stances, it tended greatly to call the attention of chemists to the physical relations of bodies. A physico-chemical school has thus been found- ed, to which several of the leading chemists of the present day belong. The discoveries of Mitscherlich, the details of which he worked out by a series of most laborious researches, were soon brought to bear with advantage upon the classification of the elements, while that of minerals underwent an entire reform. The theory of compound radicals proposed by Ber- zelius was again made specially prominent by the publication (in 1832) of a memoir, the joint production of Liebig (1803-'73) and Woliler (born 1800), upon the ben zoyl series. Although Berzelius refused to admit the generalization made by these chemists, and disbelieved in the existence of radicals which like benzoyl con- tain oxygen, an interest was nevertheless ex- cited, which greatly contributed to the advance- ment of organic chemistry. The ethyl theory quickly followed, and was adopted by most German and English chemists. These results mark an era in the history of modern chemistry. A throng of pupils immediately gathered about Liebig. No other chemist has ever had con- trol over an amount of talent equal to that of the students who for years crowded his labora- tory at Giessen. Through them he exerted an incalculable influence upon the present position of the science, while by his popular writings he did much to diffuse chemical knowledge among the masses. Of the special labors of Liebig may be mentioned his efforts to determine what substances should be re- garded as radicals, and to classify all known organic bodies in accordance with them ; also his important improvements in the methods of analyzing organic substances. Besides the investigation of the oil of bitter almonds, which led to the discovery of the existence of ben- zoyl, his most important researches were made in conjunction with Wohler; for exam- ple, those upon cyanogen compounds and the derivatives of uric acid, by which an immense number of new compounds were discovered. Wohler has also independently brought to light many new facts, not only in organic but also in inorganic chemistry; for example, his investigation of the compounds of tungsten, the preparation of aluminum and of glucinum, and, in conjunction with H. Deville, of silicon and boron. His method of preparing urea (1828) from inorganic substances was the first and for years the only example of the power of chemists to form organic compounds from their elements. All organic substances previ- ously obtained had been either derived directly from plants or animals, or had been products of the decomposition of substances thus obtained ; it had indeed been doubted if any others could ever be prepared. The labors of the recent French chemists have aided perhaps more than any others in elevating the science to its pres- ent position. Although seemingly standing for years in direct opposition to those of the Ger- man school, the systems of both have at length been harmoniously combined. Dumas of Paris first discovered, by his research upon the ac- tion of chlorine on acetic acid, that the three equivalents of hydrogen contained in the latter can be replaced by as many equivalents of chlorine, while the acetic acid retains most of its characteristic properties. Upon this and similar observations he founded his theory of substitutions, according to which hydrogen and some other ingredients of compounds may be replaced, equivalent for equivalent, by some other element or group of elements, while the properties of the original substance are usually not essentially changed. Few theories have been more bitterly opposed than this, but the facts are now universally admitted. The in- vestigation of the substitutions which can be produced in organic compounds has been for several years a favorite study with chemists. Of late the means of bringing them about, and of reproducing the primary substance by re- placing the elements originally removed, have been greatly increased and have led to many fine discoveries. Numberless researches upon organic substances have been conducted by Dumas with very important results. His deter- minations of the specific gravity of many va- pors were of great value, the ingenious appa- ratus which he devised for this purpose having since been almost universally used. Determi- nations of the specific gravity of gases made in conjunction with Boussingault were also valua- ble. Among his most important investigations are those upon the amidogen compounds, ethers, volatile oils, and especially upon wood spirit, made in conjunction with Peligot, by which its resemblance to alcohol was shown an analogy carried yet further by Balard's discovery of