THEIR COMPOUNDS.] CHEMISTRY 477 Oxides. The element oxygen is known to form com pounds with all the elements excepting fluorine, but the affinity of the various elements for oxygen, and con sequently the stability of the compounds resulting from their union with oxygen, varies within very wide limits ; the oxides of many elements which have but a weak affinity for oxygen can only be obtained by indirect means. Although many of the elements unite with oxygen in several proportions, none are known to furnish more than five distinct oxides, and these belong to a certain limited number of forms, since they contain one, two, three, four, or five atoms of oxygen in the molecule. But as the same number of atoms of oxygen may unite with a given element in more than one proportion, each of these five classes in cludes sub-classes. The composition of the oxides may be best illustrated and their behaviour explained by regarding them as formed on the type of the two oxides of hydrogen, hydrogen monoxide or water, OH , and hydrogen dioxide, O 2 H 2 . The monoxides are formed on the type of a single mole cule of water, that is to say, they may be regarded as derived from a single molecule of water by the displace ment of the two atoms of hydrogen by the equivalent amount of other elements. If two atoms of a monad element, or single atom of a dyad element, or of a polyad element which functions as a dyad, displace the two atoms of hydrogen, normal monoxides result, examples of which are potassium monoxide, K 2 O, and copper oxide,
- Cu"O. But monoxides are known in which two atoms of
a dyad or it may be polyad element displace the two atoms of hydrogen in water ; it is supposed that in this case the two atoms of polyad metal are united together and there fore function as a dyad compound radicle. Monoxides of this kind are usually termed suboxides ; the suboxide of copper, (Cu 2 )"0, and the suboxide of palladium, (Pd 2 )"O, may be cited as examples. The supposed constitution of the three classes of monoxides is represented by the follow ing graphic formulas : O A K O K Cu=0 Cu Cu . Potassium oxide. Copper oxide. Copper suboxide. Two classes of dioxides may be distinguished : the one formed from two molecules of water by the displacement of the four atoms of hydrogen by a single tetrad atom, such as carbon dioxide, C IV O 2 , and tin dioxide, Su"0 2 ; the other formed from a single molecule of hydrogen dioxide by the displacement of the two atoms of hydrogen either by two monad atoms, or by a single dyad atom, such as potassium dioxide, K/ 2 O 2 , barium dioxide, Ba"0 2 , and manganese dioxide, Mn"O 2 . These two kinds of dioxide differ enormously in chemical properties ; their supposed constitution is represented by the following graphic formulas : K I Ba<| 0=0=0 K O Carbon dioxide. Potassium dioxide. Barium dioxide. The dioxides derived from hydrogen dioxide are usually termed peroxides. The trioxides are divisible into three classes, but all may be regarded as derived from three molecules of water. In the one class the six atoms of hydrogen are displaced by a single hexad atom, as in sulphur trioxide, S TI 3 , and tungsten trioxide, W"0 3 ; in the second class the six atoms of hydrogen are displaced by two polyad (tetrad 1) elements, which together function as a hexad compound radicle, as in ferric oxide, (Fe 2 ) TI 3 , and chromic oxide, (Cr 2 ) TI O 3 . A third class may be regarded as formed by the displacement of the six atoms of hydrogen by two triad atoms, and includes boron trioxide, B " 2 O 3 , and bismuth trioxide, Bi " 2 3 . There are considerable differences in properties between these three classes of trioxides ; their graphic formulae are as follows : O II o=s=o Sulphur trioxide. o< Ferric oxide. Bi=0 )< Bi=0 Bismuth trioxide. As examples of tetroxides, osmium tetroxide, Os0 4 , anti mony tetroxide, Sb 2 4 , the so-called magnetic oxide of iron, Fe 3 4 , and lead tetroxide, Pb 3 O 4 , may be quoted. These oxides undoubtedly belong to different classes, but too little is known of them for their relations to each other to be defined. Lastly, the few pentoxides which are known may, with scarcely an exception, be regarded as derived from five molecules of water, the ten atoms of hydrogen being dis placed by two pentad atoms. This is illustrated by tha following graphic formulae of phosphorus and arsenic pent- oxides : O O O
P-0 P I! 11 O Phosphorus pentoxide. As As O
Arsenic pentoxide. Several of the oxides of chlorine and of nitrogen, and one of the oxides of sulphur, are gases ; the oxides of hydrogen, and the oxides of chlorine and of nitrogen which are not gaseous, are liquid ; and the remaining oxides are solid bodies. Acids Bases Salts. Many oxides have an extremely powerful affinity for water, and readily combine with it ; but the compounds formed in this manner by different oxides differ remarkably in properties. For example, when sulphur trioxide, S0 3 , is added to water, a solution is obtained which has the property of changing the colour of blue vegetable colouring matters, such as litmus, to red, and possesses a sharp sour taste ; but when the oxide of a highly positive metal, such as barium oxide, is added to water, a solution capable of restoring the blue colour to reddened litmus is obtained. In both cases the water combines with the oxide, the sulphur trioxide being con verted into sulphuric acid: and the barium oxide into barium hydroxide : = Ba0 2 H 2 . Sulphuric acid and barium hydroxide are representatives of two most important classes of compounds, the acids and bases ; the oxides which furnish acids when combined with water are conveniently termed acid oxides, whilst those which furnish bases may be termed basic oxides. The acid oxides, however, unite with the basic oxides to form a third class of compounds called salts, which are usually neutral bodies, that is to say, they have no action either on red or on blue litmus ; thus, sulphur trioxide and barium oxide unite to form the salt barium sulphate: The term acid is applied by some chemists to what are here called acid oxides, and what we have called basic oxides are frequently spoken of as bases, the acids being regarded as salts of hydrogen ; this view of the constitu tion of acids is adopted because they are formed by the union of oxides of negative elements with the oxide of the
positive element hydrogen, just as what are ordinarily