Nevertheless, the significance of the generalisation discovered by Dulong and Petit, in spite of its limitations, was quickly appreciated, as it was perceived that a knowledge of the specific heat of an element might be of great value in determining its atomic weight. The immediate effect was that a certain number of the atomic weights fixed by Berzelius mainly on chemical considerations were required to be halved. Although subsequent experience has proved that the law of Dulong and Petit is not capable of the simple mathematical expression they gave it, it has shown itself to be of great value in fixing doubtful atomic weights. =Pierre Louis Dulong= was born in 1785 at Rouen, and, after studying chemistry and physics at the Polytechnic School at Paris, became its Professor of Chemistry and subsequently its Professor of Physics. In 1830 he was made its Director of Studies; and in 1832 he became permanent Secretary of the Academy of Sciences. As a young man he worked with Berzelius, with whom he made the first approximately accurate determination of the gravimetric composition of water. In 1811 he discovered the highly explosive _nitrogen chloride_, in the investigation of which he was severely injured, losing an eye and several fingers. He died in 1838. His collaborator, =Alexis Therese Petit=, was born in 1791 at Vesoul, and died, when holding the position of Professor of Physics at the Lycée Bonaparte, in 1820. The attempt made by Neumann to extend Dulong and Petit’s “law” to compound substances was only partially successful. Nor has any important generalisation followed from our knowledge of the specific heat of liquids. Almost simultaneously with the publication of Dulong and Petit’s “law,” Mitscherlich made known the fact that similarity in chemical constitution is frequently accompanied by identity of crystalline form. Boyle, as far back as the middle of the seventeenth century, had insisted upon the importance of the forms of crystals in throwing light upon the internal structure of bodies. Romé de l’Isle and Hauy had remarked that many different substances had the same crystalline form. It had been observed that a crystal of potash alum would continue to grow and preserve its shape in a solution of ammonia alum; and similar observations had been shown to occur in the case of vitriols. The invention of the reflecting goniometer by Wollaston greatly facilitated the investigation of such phenomena. Mitscherlich showed that the phosphates and arseniates of analogous composition had the same crystalline shape, or, in other words, were isomorphous. The same fact was observed to occur in the case of the analogously constituted sulphates and selenates, and in that of the oxides of magnesium and zinc, etc. The value of isomorphous relations in determining the group-relationships of the elements and in deducing the composition of salts was at once recognised by Berzelius, who