A third method of arriving at the correct atomic weight of an element was suggested in 1819 by Billiard Mitscherlicll, then Professor in Berlin. When two substances crystallise in the same crystalline form, they are said to be isomorphous " with each other. It is often the case that such compounds are similar chemically ; that is, they may contain the same number of atoms, and may also closely resemble each other physically. Thus, there is a large class of compounds, named " alums," which are sulphates of two metals. Ordinary alum is a sulphate of aluminium and potassium ; it crystallises in eight-sided regular figures, termed "octahedra." When the rare metal gallium was discovered, it was found to form an "alum ; " it gave a sulphate of gallium and potassium, crystallising in octahedra, and similar in properties to ordinary alum. Now, Mitscherlich's statement was, that when one element takes the place of another in an isomorphous crystal of the same chemical character, the substitution occurs so that one atom of the one replaces one atom of the other, Hence, if the atomic weight of the one element is known, the weight of the other element which replaces it will be proportional to its atomic weight. In the case above mentioned, it was found that 27.1 parts by weight of aluminium were replaced by 69.9 parts of gallium ; and as it was known from experiments such as those previously described that the atomic weight of aluminium is 27.1, it follows that 69.9 is the atomic weight of gallium. But care is necessary in using this indication of the atomic weight ; for it may happen that two compounds may contain the same number of elements in the same proportions, and have a similar crystalline form ; and yet Mitscherlich's law may not be applicable.