476 CHEMISTRY [ELEMENTS stance. When such substances are heated in a confined space, not only does decomposition take place, but a certain proportion of the decomposition products recombine, and for each degree of temperature a definite relation exists between the number of molecules broken up and the number of molecules reconstituted ; an equilibrium is thus maintained, and the tension remains constant. But as the temperature rises, a larger number of molecules are de composed and a proportionally smaller number reformed than at the lower temperature, so that the tension rises with the temperature. When, however, a substance like calcium carbonate is heated in an open vessel, the carbon dioxide escapes as it is formed, and therefore it is impos sible for recomposition to occur, and ultimately the whole of the carbonate is decomposed. Apparently whenever solid substances undergo dissociation and furnish a solid and a gaseous product, the tension of dissociation is always inde pendent of the amount of the solid decomposition product present. The Elements and their Compounds. The elements are usually divided into two great classes, the metallic and the non-metallic elements, the following being classed as non-metals, and the remainder as metals; Selenium. Tellurium Hydrogen. Chlorine. Bromine, Iodine Fluorine. Oxygen Sulphur. Nitrogen Boron. Carbon Silicon. Phosphorus Iodine. Oxygen. Sulphur. Selenium. Tellurium. The metals are mostly bodies of high specific gravity ; they exhibit, when polished, a peculiar brilliancy or metallic lustre, and they are good conductors of heat and electricity; whereas the non-metals are mostly bodies of low specific gravity, and bad conductors of heat and electricity, and do not exhibit metallic lustre. The non-metallic elements are also sometimes termed metalloids, but this appellation, which signifies metal-like substances, strictly belongs to cer tain elements which do not possess the properties of the true metals although they more closely resemble them than the non-metals in many respects ; thus, selenium and tellurium, which are closely allied to sulphur in their chemical proper ties, although bad conductors of heat and electricity, exhibit metallic lustre and have relatively high specific gravities. But when the properties of the elements are carefully contrasted together it is obvious that no strict line of demarcation can be drawn dividing them into two classes ; and if they are arranged in a series, those which are most closely allied in properties being placed next to each other, it is evident that there is a more or less regular altera tion in properties from term to term in the series. When binary compounds, or compounds of two elements, are decomposed by an electric current, the two elements make their appearance at opposite poles. Those elements which are disengaged at the negative pole are termed electro positive, or positive, or basylous elements, whilst those disengaged at the positive pole are termed electro-negative, or negative, or chlorous elements. But the difference between these two classes of elements is one of degree only, and they gradually merge into each other; moreover the electric relations of elements are not absolute, but vary according to the state of combination in which they exist, so that it is just as impossible to divide the elements into two classes according to this property as it is to separate them into two distinct classes of metals and non-metals. The following, however, are negative towards the remain ing elements which are more or less positive : Fluorine. Chlorine. Bromine. Elements which readily enter into reaction with each other, and which develop a large amount of heat on com bination, are said to have a powerful affinity for each other. The tendency of positive elements to unite with positive elements, or of negative elements to unite with negative elements, is much less than that of positive elements to unite with negative elements, and the greater the differ ence in properties between two elements the more power ful is their affinity for each other. Thus, the affinity of hydrogen and oxygen for each other is extremely powerful, much heat being developed by the combination of these two elements ; when binary compounds of oxygen are decomposed by the electric current, the oxygen invariably appears at the positive pole, being negative to all other elements, but the hydrogen of hydrogen compounds is always disengaged at the negative pole. Hydrogen and oxygen are, therefore, of very opposite natures, and this is well illustrated by the circumstance that oxygen combines, with very few exceptions, with all the remaining elements, whilst compounds of only a very limited number with hydrogen have been obtained. Hydrides. The only elements which are known to form binary compounds with hydrogen are fluorine, chlorine, bromine, iodine, oxygen, sulphur, selenium, tellurium, carbon, silicon, nitrogen, phosphorus, arsenic, antimony, and copper. Palladium has the property of absorbing a large volume of hydrogen gas, and it is supposed forms a definite compound with it ; sodium and potassium, when heated to 350 to 400 C., also absorb hydrogen and appear to form definite compounds. With the exception of car bon, which furnishes an enormous number of compounds with hydrogen, the above-named elements unite with hydrogen only in one proportion, or at most in two or three different proportions. Excepting oxygen, fluorine, chlorine, and bromine, they do not readily enter into reac tion with hydrogen, so that in most cases, in order to obtain their compounds with hydrogen, it is necessary to resort to indirect methods of preparation. The compounds of the elements with hydrogen may all be included under the general title of hydrides, but usually they possess spe cific names ; thus, the hydride of oxygen is known as water, and the hydride of nitrogen as ammonia. Some of the hydrides of carbon are gases, but the majority are either liquids or solids ; the two known hydrides of oxygen, water and the so-called peroxide of hydrogen, are liquids ; a gaseous and a liquid hydride of sulphur are known ; three hydrides of phosphorus are said to exist, one of which is liquid, one solid, and one gaseous ; hydride of copper is a solid ; and the hydrides of the remaining ele ments are gases. Excepting the hydride of copper, all are colourless substances. The following is a list of the known hydrides, including, however, only the first or simplest hydride of carbon : Name. Formula. Hydrogen hydride, or hydrogen ....................... H 2 fluoride, or hydrofluoric acid ............. HF chloride, or hydrochloric acid ............ H Cl bromide, or hydrobr omic acid ............ H Br iodide, or hydriodic acid .................. HI oxide, or water .............................. H 2 dioxide ........................................ H 2 sulphide, or sulphuretted hydrogen.... H 2 S persulplude ................................... H 2 S Selenium hydride ......................................... H 2 S Tellurium hydride ................ . ....................... H 2 Te Nitrogen hydride, or ammonia ........................ H 3 N Phosphorus hydride, or phosphine .................. H 3 P (?) gas. liquid. , , gas. liquid. gas liquid. solid. gas. ................... H 2 P 4 (?) Arsenic hydride, or arsine .............................. H 3 Ar Antimony hydride, or stibine ......................... H 3 Sb Carbon hydride, methane, or marsh -gas ............ H 4 C , , Silicon hydride ........................................... H 4 Si ,,
Copper hydride .......................................... H a Cu, (?) solid.