Another element occurring in allotropic forms is sulphur, of which many forms have been described.
When perfectly pure phosphorus is a white, transparent, waxy solid, but as usually prepared it is yellowish owing to the presence of the allotropic " red phosphorus," J.
Properties.-Sulphur exists in several allotropic modifications, but before considering these systematically we will deal with the properties of ordinary (or rhombic) sulphur.
Allotropic Modifications.-Sulphur assumes crystalline, amorphous and (possibly) colloidal forms. Historically the most important are the rhombic (Sa) and monoclinic (So) forms, discussed by E.
N"Ecv, to smell) observed in 1785 by Martin van Marum to be formed by the action of a silent electric discharge on the oxygen of the air; he showed it to be an allotropic modification of oxygen, a view subsequently confirmed by Marignac, Andrews and Soret.
The preparation of crystalline boron in 1856 by Wohler and Sainte Claire Deville showed that this element also existed in allotropic forms, amorphous boron having been obtained simultaneously and independently in 1809 by Gay Lussac and Davy.
Wohler, he investigated the allotropic forms of silicon and boron.
In these processes the electric current is used solely to generate heat, either to induce chemical reactions between admixed substances, or to produce a physical (allotropic) modification of a given substance.
Hittorf thinks that two allotropic forms of chromium exist (Zeit.
Osmond showed that the wonderful changes which thermal treatment andthe presence of certain foreign elements cause were due to allotropy, and from these and like teachings have come a rapid growth of the use of the so-called " alloy steels " in which, thanks to special composition and treatment, the iron exists in one or more of its remarkable allotropic states.
But carbon affects the properties of iron not only by giving rise to varying proportions of cementite, but also both by itself shifting from one molecular state to another, and by enabling us to hold the iron itself in its unmagnetic allotropic forms, 0and 7-iron, as will be explained below.
Thus, sudden cooling from a red heat leaves the carbon not in definite combination as cementite, but actually dissolved in (3and 7-allotropic iron, in the conditions known as martensite and austenite, not granitic but glass-like bodies, of which the " hardened " and " tempered " steel of our cutting tools in large part consists.
Austenite, gamma ('y) iron.-Austenite is the name of the solid solution of an iron carbide in allotropic y-iron of which the metal normally consists when in region 4.
Beta (13) iron, an unmagnetic, intensely hard and brittle allotropic form of iron, though normal and stable only in the little triangle GHM, is yet a state through which the metal seems always to pass when the austenite of region 4 changes into the ferrite and cementite of regions 6 and 8.
Of free iron of the /3 allotropic modification, which surrounds the kernels or grains of the residual still undecomposed part of the austenite.
That to which the metal, in which by suitable thermal treatment the iron molecules have been brought to the allotropic -y or 1 3 state or a mixture of both, can be heated without losing its hardness through the escape of that iron into the a state.
In short, these elements seem to impede the allotropic change of the iron itself.
The monovalent iodine, for instance, is transformed by heating into an allotropic form, corresponding to the formula I, whereas ordinary iodine answers to I 2.
No Doubt There Must Be Approximate Relations Between The Atomic And Molecular Heats Of Similar Elements And Compounds, But Considering The Great Variations Of Specific Heat With Temperature And Physical State, In Alloys, Mixtures Or Solutions, And In Allotropic Or Other Modifications, It Would Be Idle To Expect That The Specific Heat Of A Compound Could Be Accurately Deduced By Any Simple Additive Process From That Of Its Constituents.
Several allotropic forms of selenium have been described, but the work of A.
Several allotropic forms of phosphorus have been described, and in recent years much work has been done towards settling their identities.
(4) Allotropic modifications.