phuret of cyanogen with hydrogen, is composed also of equal volumes of azote, of hydrogen, of carbon, and of sulphur. M. Berzelius admits, that the phenomena presented by the cyanurets and the sulpho-cyanurets, can only be explained by a theory analogous to that which Sir Humphry Davy and M. Gay Lussac have applied to the muriates, with which M. Berzelius finds that the sulpho-cyanurets have a striking analogy. This analogy allows us to presume, also, on an analogy between chlorine or the oxymuriatic gas and the sulphuret of cyanogen. M. Berzelius has attempted to obtain this last substance in an insulated state, for the purpose of studying its properties; but he has hitherto tried it in vain.

Selenium gives with the cyanuret of iron and of potassium phenomena analogous to those produced by sulphur. The selenio-cyanuret of potassium resembles perfectly the sulpho-cyanuret; but if we mix it with any acid, the Selenium is immediately precipitated in red flowers.

Tellurium allows itself to mix with the double cyanuret of iron and potassium, if they are melted together; but water separates them; the cyanuret dissolves without alteration, and the Tellurium remains in the form of a metallic powder.

2. Account of M. Mitscherlich's Experiments on the Forms of artificially crystallised Salts.

M. Mitscherlich, a young chemist from Berlin, has been much occupied in determining the form of artificially crystallised salts. In the course of this inquiry, he has arrived at many results of very high importance respecting the relation which exists between the composition and the form of these crystals. Having studied during the last year under M. Berzelius, he has repeated before him a great number of his experiments, which were found to be perfectly exact. M. Mitscherlich has discovered that several substances, simple as well as compound, may replace one another in compound bodies, without any change of form taking place in the latter, provided that the other constituent principles remain the same, and in the same proportions. He has found for example, that Phosphorus and Arsenic replace one another in such a manner, that the Phos

20 M. Mitscherlich on the Forms of Artificially Crystallized Salts. phates crystallise in exactly the same manner as the Arseniates of the same bases, when they are at the same point of saturation, and contain the same number of atoms of water of crystallisation, which is generally the case. The protoxides of the five following metals, viz. iron, zinc, cobalt, nickel, and manganese; the deutoxide of copper, and also lime and magnesia, replace one another mutually, provided always, that in the combinations which are examined the number of atoms of water be the same. Alumine, the deutoxide of iron, and also that of manganese, may also be substituted for one another, without any change of form. Barytes, strontian, and the oxide of lead, are in the same predicament, and also chlorine and iodine, and sulphur and selenium, &c. To these different groups, M. Mitscherlich has given the name of Isomorphous Bodies.

This ingenious chemist is at present occupied in determining how many of such isomorphous groups exist among simple bodies, and among their different degrees of oxidation; and also in determining to what isomorphous group each of them belongs.

The discoveries of M. Mitscherlich throw great light upon mineralogy, and will give a key to an explanation of the contradictions of chemical analysis, and of the geometrical measurements of crystals; because, in a mineral species whose form has been determined with the greatest certainty, one or more elements may vary, provided that they belong to the same isomorphous class, and that the other elements remain the same. Hence, it is for this reason that lime, magnesia, the protoxide of iron, and the protoxide of manganese, are substituted for one another in the Amphiboles and the Pyroxenes.

M. Mitscherlich has found also, that when several combinations, isomorphous salts, for example, are mixed in the same liquid, and when this liquid is afterwards evaporated, the isomorphous salts crystallise together, forming a part of the same crystal, and their relative proportion is then determined only by the relative quantity of each which the liquid has had to abandon at the moment of crystallisation. The crystal, in short, is, as it were, built of isomorphous molecules, without any chemical affinity having a share in it, and without our being able to perceive fixed and determinate proportions. This experiment is

one of high importance, as it explains the objections which the results of the analyses of certain minerals form to the Theory of Definite Proportions.

3. Account of the Analyses of the Pyroxenes and Amphiboles, by MM. Rose, Nordenskold, and Bohnsdorff.

MM. Rose, Nordenskold, and Bohnsdorff, three young chemists, who are at present working in the laboratory of M. Berzelius, have undertaken to verify by analyses the application of the ideas of M. Mitscherlich to mineralogy. With this view, they have begun a series of analyses of the Pyroxenes and the Amphiboles. It results from this inquiry, which is still far from being finished, that the mineral called Pyroxene, when it is uncoloured, is a double bisilicate of lime and magnesia, containing an atom of each; but that when it is coloured, it then consists of a mixture of bisilicate of lime, of bisilicate of magnesia, of bisilicate of the protoxide of iron, and, less frequently, of the bisilicate of the protoxide of manganese; without these bases being combined in proportions conformable to the theory of definite proportions. The only thing constant is, that all the bases belong to the same isomorphous class, and that they are all in the form of bisilicates.

One of these pyroxenes, analysed by Mr Rose, was found to be a double bisilicate of lime, and of protoxide of iron, containing an atom of each of these bases. This pyroxene is the one which has been called Hedenbergite, and which has been considered, after the analysis of M. Hedenberg, as a bisilicate of the protoxide of iron. Another has been found to be composed almost entirely of the bisilicate of the protoxide of manganese, with a very little of the bisilicate of lime.

4. Account of M. Rose's Analyses of several Species of Mica, containing Fluoric Acid.

M. Rose has lately analysed several species of mica, in which he has discovered Fluoric Acid in considerable quantity. The Mica of granites contains more of it than that of primitive carbonate of lime, which contains only traces of it. We may easi ly discover if any species of Mica is more or less rich in fluoric

acid, by exposing it to the fire. That which contains about a per cent. of it, loses its lustre and becomes matted, while that which contains only traces of it assumes a metallic lustre. The following are the results of M. Rose's analyses of three kinds of

ART. III.-On Isothermal Lines, and the Distribution of Heat Over the Globe. By Baron ALEXANDER DE HUMBOLDT. (Continued from Vol. III. p. 274.)

AFTER what has already been stated respecting the limits be

tween which the annual heat divides itself on the same isothermal curve, it will be seen how far we are authorised to say, that the Coffee-tree, the Olive, and the Vine, in order to be productive, require mean temperatures of 64°.4; 60°.8, and 53°.6 Fahr. These expressions are true only of the same system of climate, for example, of the part of the Old World which stretches to the west of the meridian of Mont Blanc; because in a zone of small extent in longitude, while we fix the annual temperatures, we determine also the nature of the summers and the winters. It is known likewise, that the olive, the vine, the varieties of grain, and the fruittrees, require entirely different constitutions of the atmosphere. Among our cultivated plants, some, slightly sensible of the rigours of winter, require very warm but not long summers; others require summers rather long than warm; while others, again, indifferent to the temperature of summer, cannot resist the great colds of winter. Hence, it follows, that, in reference to the culture of useful vegetables, we must discuss three things for each climate,—the mean temperature of the entire summer,—that of the warmest month,-and that of the coldest month. I have published the numerical results of this discussion in my Prolegomena de Distributione Geographica Plantarum, secundum Cali Temperiem; and I shall confine myself at present to the limits of culture of the olive and the vine. The olive is cultivated in our continent between the parallels of 36° and 44°, wherever the annual temperature is from 62°.6 to 58°.1, where the mean temperature of the coldest month is not below from 41°.0 to 42°.8, and that of the whole summer from 71°.6 to 73°.4 *. In the New World, the division of heat between the seasons is such, that on the isothermal line of 58°.1, the coldest month is

In cases like the present, we have not used the round numbers of Fahrenheit, as is done in the original with the Centigrade scale, but have given the real value of the degrees used by the author, that his exact numbers may always be ascertained.-ED.