5. According to the periodic system every element occupies a certain position, determined by the group (indicated in Roman numerals) and series (Arabic numerals) in which it occurs. These indicate the atomic weight, the analogues, properties, and type of the higher oxide, and of the hydrogen and other compounds--in a word, all the chief quantitative and qualitative features of an element, although there yet remain a whole series of further details and peculiarities whose cause should perhaps be looked for in small differences of the atomic weights. If in a certain group there occur elements, R_{1}, R_{2}, R_{3}, and if in that series which contains one of these elements, for instance R_{2}, an element Q_{2} precedes it and an element T_{2} succeeds it, then the properties of R_{2} are determined by the properties of R_{1}, R_{3}, Q_{2}, and T_{2}. Thus, for instance, the atomic weight of R_{2} = 1/4(R_{1} + R_{3} + Q_{2} + T_{2}). For example, selenium occurs in the same group as sulphur, S = 32, and tellurium, Te = 125, and, in the 7th series As = 75 stands before it and Br = 80 after it. Hence the atomic weight of selenium should be 1/4(32 + 125 + 75 + 80) = 78, which is near to the truth. Other properties of selenium may also be determined in this manner. For example, arsenic forms H_{3}As, bromine gives HBr, and it is evident that selenium, which stands between them, should form H_{2}Se, with properties intermediate between those of H_{3}As and HBr. Even the physical properties of selenium and its compounds, not to speak of their composition, being determined by the group in which it occurs, may be foreseen with a close approach to reality from the properties of sulphur, tellurium, arsenic, and bromine. _In this manner it is possible to foretell the properties of still unknown elements._ For instance in the position IV, 5--that is, in the IVth group and 5th series--an element is still wanting. These unknown elements may be named after the preceding known element of the same group by adding to the first syllable the prefix _eka_-, which means _one_ in Sanskrit. The element IV, 5, follows after IV, 3, and this latter position being occupied by silicon, we call the unknown element ekasilicon and its symbol Es. The following are the properties which this element should have on the basis of the known properties of silicon, tin, zinc, and arsenic. Its atomic weight is nearly 72, higher oxide EsO_{2}, lower oxide EsO, compounds of the general form EsX_{4}, and chemically unstable lower compounds of the form EsX_{2}. Es gives volatile organo-metallic compounds--for instance, Es(CH_{3})_{4}, Es(CH_{3})_{3}Cl, and Es(C_{2}H_{5})_{4}, which boil at about 160°, &c.; also a volatile and liquid chloride, EsCl_{4}, boiling at about 90° and of specific gravity about 1·9. EsO_{2} will be the anhydride of a feeble colloidal acid, metallic Es will be rather easily obtainable from the oxides and from K_{2}EsF_{6} by reduction, EsS_{2}