Tables and diagrams illustrative of chemistry and pharmacy / by John Murray.

Date:
1835
Catalogue details

Licence: Public Domain Mark

Credit: Tables and diagrams illustrative of chemistry and pharmacy / by John Murray. Source: Wellcome Collection.

Provider: This material has been provided by the Royal College of Physicians of Edinburgh. The original may be consulted at the Royal College of Physicians of Edinburgh.

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    identical, the measurement of the angles sometimes differing to an extent of two degrees. But this difficulty may be in a great degree obviated by reference to some facts lately established by Mitscherlich, and by Breithaupt. They have as- certained that crystals of the same substance occasionally diflFer in the measure- ment of their angles to an amount of from half a degree to a degree, and that even the symmetric angles of the same crystal are liable to vary to an amount of three degrees. Another important fact is, that many crystals expand unequally, from heat increasing more in one of their dimensions than in another : thus, when calcareous spar is heated from 32 to 212, it becomes less rhombic, or approaches to the cube. It is, therefore, conceivable, that there may be a certain zero point of temperature at which plesiomorphous groupes are truly isomorphous, and that it is unequal expansion from the heat present in them, at ordinary temperature, which in some produces change of form. Mitscherlich is of opinion, that iso- morphism is also occasionally modified by chemical affinity. Isomorphous bodies, besides their conformity in figure, are commonly allied by resemblance of their other properties ; thus a close resemblance exists between phosphorus and arsenic, between sulphur and selenium. The principle of isomorphism is of essential importance in mineralogy. It shews that crystallographic measurement cannot indicate chemical composition ; it also warns the analyst of the substances which may replace each other in a mi- neral, and which he might fail to discriminate ; and it shews, that in the che- mical classification which must ultimately prevail in mineralogy, a mineral species must not be too strictly limited as to individual ingredients, but must be repre- sented by a formula that will admit different isomorphous bodies, a kind of framework, the compartment of which may be filled up with equivalent sub- stances, as these may happen to replace one another. What the influence of this doctrine may be on chemistry we can scarcely yet judge, but it seems likely to unfold some very important relations ; and the speculation may be indulged, that by the aid of the atomic theory, which teaches us the relative magnitude of atoms, of isomorphism, that aims at the discovery of their form and arrangements, and of isomerism, which shews the effect of the ar- rangement on properties, we may at length erect a perfect theory of combination ; so that the chemist, when the problem is proposed to him, of the union of given elements, shall be able to state what will be their relative proportions, and what the structure and properties of the compound,—a splendid result, if it shall ever be attained! Printed by James Walker. 6. James's Court.