obtained boils at about 700°, being converted into a dark yellow vapor, the specific gravity of which has been found to be 82.3. The atomic weight of selenium has been determined to be 79.5. This discrepancy between the vapor-density and the atomic weight is to be ascribed to the imperfection of the experimental determinations. Of itself, selenium has neither taste nor odor. When heated in the flame of a lamp, it burns with a beautiful blue flame and exhales a peculiarly offensive odor, like that of putrid horseradish, selenious acid, SeO,, being the chief product of the reaction. Selenium combines with most of the elements, usually in the same way as sulphur, though not always, since it is a weaker chemical agent than sulphur; its compounds are as a rule somewhat less stable than the corresponding sulphur compounds. With oxygen it forms selenious acid, SeO2, and selenic acid, SeO,,-analogous to sulphurous and sulphuric acids respectively. Besides these, there is a lower oxide, SeO (?); it is a colorless gas, having the strong and disagreeable odor like horseradish before mentioned. 252. Both selenious and selenic acids form numerous salts, which closely resemble the corresponding sulphites and sulphates in composition and in many of their properties. Normal seleniate of potassium, for example, K,SeO,, cannot be distinguished, by its external appearance, from sulphate of potassium, K,SO,,-the crystalline form of the two bodies, as well as their texture, color, and lustre, being identical. If solutions of these two salts be mixed, neither of the salts can subsequently be crystallized out by itself when the solution is evaporated; the crystals obtained will be composed of sulphate of potassium and seleniate of potassium mixed in the most varied proportions. Bodies which are thus capable of crystallizing together in all proportions, without alteration of the crystalline form, are said to be isomorphous (like-formed). The formulæ of the two isomorphous salts, just mentioned, differ only in this-that the one contains the atom Se, where the other contains the atom S. It is therefore possible to replace 32 parts by weight of sulphur by 79.5 parts of selenium, or 79.5 of selenium by 32 of sulphur, without changing the crystalline form of the salts; it follows that 32 parts by ATOMIC VOLUME-TELLURIUM. 32 1.91 201 = 16.75 cubic 79.5 оссиру 4-51 =16.53 weight of solid sulphur, and 79.5 parts of solid selenium, occupy the same space. That this is actually the case may be shown by comparing the quotients obtained by dividing the atomic weights of the two elements by their specific gravities; these quotients will be found to be equal, or as nearly equal as the limits of error of the physical determinations involved will permit. The specific gravity of prismatic sulphur is 1.91, or, in other words, one cubic centimetre of solid sulphur weighs 1.91 gramme; the specific gravity of crystalline selenium is 4.81, or one cubic centimetre of selenium weighs 4.81 grammes; 32 grammes of sulphur will, therefore, occupy centimetres; 79.5 grammes of selenium will cubic centimetres. What is true of grammes, is true of any parts by weight, and ultimately of the atoms. This quotient, obtained by dividing the atomic weight of an element by its specific gravity, is called the atomic volume of the element; it must be borne in mind that the standard of specific gravity for liquids and solids is water, for gases hydrogen, and, therefore, that the atomic volume of a solid or liquid must not be directly compared with that of a gas. Two elements whose atomic volume is the same can be exchanged in their compounds without alteration of crystalline form, precisely as a brick or stone taken out of a wall can be replaced by another of the same size and shape without changing the form of the wall. 253. With chlorine, selenium forms two compounds, SeCl and SeCl, the first of which is analogous to dichloride of sulphur. With hydrogen it forms a compound, H,Se, called selenhydric acid, or seleniuretted hydrogen, which is perfectly analogous to sulphuretted hydrogen, HS, but possesses a still more disagreeable odor. In its action upon solutions of the metallic salts, upon metals and metallic oxides, selenhydric acid behaves like sulphydric acid, a selenide of the metal being always formed. 254. Tellurium (Te) occurs in nature even more rarely than selenium. Sometimes it is found in the free state, but more generally in combination with the heavy metals, such as gold, silver, lead, copper, and bismuth. It is one of the few elements with regard to which chemists have, at times, been in doubt whether or not they should be classed as metals. Many of its physical properties are similar to those of the metals, and it particularly resembles the metal antimony; but it is so intimately related to sulphur and selenium in its chemical properties, its crystalline form, and mode of occurrence in nature, that it is now almost always studied as a member of the sulphur group. 255. Tellurium is of a silver-white color and glittering metallic lustre. It is hard and brittle, and crystallizes very easily in rhombohedrons. It is a bad conductor of heat and electricity. Its specific gravity is 6-2; its specific heat is 0·04737, and its atomic weight 128. It melts at a temperature somewhat above the melting-point of lead, and is volatile at a full red heat, the vapor being of a yellow colour, like that of selenium. When heated in the air, it takes fire, and burns with a greenish-blue flame, copious fumes of tellurous acid, TeO,, being at the same time evolved. 256. The compounds of tellurium and oxygen (tellurous acid, TeO,, and telluric acid, TeO,) are analogous to sulphurous and sulphuric acids. By uniting with metallic oxides, they form numerous salts, analogous to, and isomorphous with, the corresponding compounds of sulphur and selenium. So, too, the hydrogen compound, H,Te, is analogous to sulphuretted and seleniuretted hydrogen, in composition and properties. With the metals it unites directly to form tellurides. There are chlorine compounds also, TeCl and TeCl. 257. The close relationship which subsists between sulphur and oxygen has been already alluded to, as well as the many points of resemblance between sulphur, selenium, and tellurium; the student is therefore now prepared to recognize the fact that in oxygen, sulphur, selenium, and tellurium we have another group or family of elements, as intimately and naturally related to each other as are the members of the chlorine group. (See § 152.) It will be seen at a glance that in passing from oxygen, at one end of the series, to tellurium, at the other, we meet with the same progression of physical and chemical properties that was so noticeable in passing from chlorine to iodine. The properties of the various compounds formed by the union of the members of the sulphur group with other elements exhibit the same kind COMBINATION BY VOLUME. 203 of progression; that these compounds are of analogous composition has been shown in the preceding paragraphs. With hydrogen the members of the sulphur group unite in the proportion of two atoms of hydrogen to one atom of the other element; thus, H2O, H2S, H2Se, H,Te. This peculiar relation to hydrogen is an important characteristic of the group. In this sulphur group, precisely as in the chlorine group, the relative chemical power of each element in the family is great in proportion as its atomic weight is low (§ 153); oxygen is, as a rule, stronger than sulphur, sulphur than selenium, and selenium than tellurium, their atomic weights being respectively: 0 = 16, S = 32, Se = 79-5(80?), Te = 128. CHAPTER XV. COMBINATION BY VOLUME. 258. A comparison of the formulæ representing the volumetric composition of all the well-defined compound gases and vapors which have been thus far studied, will bring into clear view some of the general facts relating to combination by volume. It has been established, by experiment, that the following compounds are formed by the chemical union, without condensation, of equal volumes of the two elements which enter into each compound : It has further been demonstrated that the following compounds of two elements contain two volumes of one element and one 204 COMBINATION BY VOLUME. volume of the other, but that these three volumes are condensed, during the act of combination, into two volumes : Lastly, still a third mode of combination by volume with condensation of four volumes to two has been thoroughly studied in the case of ammonia, and has been further illustrated in the composition of anhydrous sulphuric acid: Selenious Acid or 2 vols. Tellurous Acid or 2 vols. 3 S O + = 32 48 80 Throughout these tables the unit-volume is, of course, the same for every element and compound. What the absolute bulk of this unit-volume may be, is not an essential point; for the relations remain the same, whatever the unit of measure. Some chemists have thought that an advantage was gained by using the bulk of one gramme of. hydrogen at the ordinary pressure |