24 TABLE OF ELEMENTARY BODIES, [13. Table of Elementary Bodies, with their Symbols and Atomic Weights. Compounds with Oxygen, Chlorine, and other Elements. Al,O, alumina ; Al,Cl, aluminic chloride Sb. O, antimonic anhydride; SbCl,, antimonious chloride BigO, bismuth oxide; BiCl,, chloride CdO, cadmic oxide; CdCl2, cadmic chloride CaO, lime; CaCl2, calcic chloride 2 CO,, carbonic anhydride; CH1, marsh gas HCl, hydrochloric acid Cr2O,, green oxide; Cr,Cl, violet chloride COO, cobaltous oxide; CoCl2, cobaltous chloride CuO, black oxide; CuCl2, cupric chloride DO, oxide 13.] WITH SYMBOLS AND ATOMIC WEIGHTS. 25 MoO,, molybdic anhydride NIO, nickelous oxide; NiCl,, chloride Pd(NO), palladious nitrate; Pdo, palladious oxide HP, phosphuretted hydrogen; P,O,, phosphoric anhydride PtCl, platinic chloride KHO, caustic potash NaCl, RoCl, double chloride RbHО, caustic rubidia; RbCl, rubidic chloride SeO,, selenious anhydride SiO, silica; SiCl,, silicic chloride Ag,O, oxide; AgCl, argentic chloride NaCl, common salt; NaHO, caustic soda Sro, strontia; SrCl, strontic chloride H.SO,, oil of vitriol; H,S, sulphuretted hydrogen Ta,O,, tantalic anhydride; TaF,, tantalic fluoride H,Te, telluretted hydrogen TIO, oxide; TICI, thallious chloride; TICI,, thallic chloride ThO, thorina; ThCl2, thorinic chloride SnO2, tin stone; SnCl,, stannic chloride TiO, rutile; TiCl, liquid chloride WO, tungstic anhydride VO, vanadic anhydride; VCI,, vanadic chloride (vanadic anhydride, VO; oxychloride, VOCI, Roscoe) YO, yttria; YCl2, yttric chloride ZnO, zincic oxide; ZnCl2, zincic chloride ZrO2, zirconia, ZrF4, zirconic fluoride In the foregoing table, Berzelius, to whom we are indebted for the fundamental determinations of the atomic weights of the elements, referred them all to 100 parts of oxygen. for convenience of comparison, a column is added coutaining the corresponding numbers on the oxygen scale, when this standard is followed, which, however, is now generally abandoned. The names of the most important of the elements, 22 in number, are printed in capitals; those of small importance or rare occurrence, 27 in number, in italics. The numbers representing the atomic weights are those actually obtained by experiments conducted with all the reinements which the most scrupulous exactness could suggest. The researches of Stas in particular furnish a model for investigations of this nature. For practical purposes many of these numbers may be represented by others which are usually multiples by an integer of the atomic weight of hydrogen. Prout advanced the hypothesis that all the elementary substances had atomic weights which were multiples by a whole number of that of hydrogen, and the researches made in this direction within the 1 st twenty years have had chiefly in view the verification or disproof of this hypothesis. This subject is considered more fully in the concluding chapter of the second volume. The numbers used in this volume are given in the table facing page 1. 26 SYMBOLIC NOTATION. [13. of chemical changes, since it greatly abridges the labour of description, and after a little practice, enables the student to trace at a glance, reactions even of a complicated character. Its use has, in fact, become indispensable both to the teacher and to the pupil. Every elementary substance is represented by a symbol, consisting of the first letter of its Latin name; in cases where more than one element has the same initial, a second distinguishing letter is added. Any symbol, when it stands alone, always represents one atom of the body which it indicates. For instance, the symbol O stands for one atom of oxygen; H, for one atom of hydrogen; C, for one atom of carbon, and so on. The symbols appropriated to the various elements are placed opposite to them in the column headed Symbols in the preceding table. It must be borne in mind that the notation employed by chemists is not a true algebraic notation, although it resembles it in appearance. For example, the juxtaposition of two chemical symbols constitutes a chemical formula; such juxtaposition, however, indicates chemical combination, not multiplication; so that a compound body is represented by writing the symbols of its constituent atoms side by side; for example, HCl indicates one molecule of hydrochloric acid, CaO one molecule of lime, the quantities included in each formula always indicating one molecule of the compound. If it be necessary to express that more than one atom of a body enters into the formation of a molecule, the object is attained by writing a small figure to the right of the symbol below the line;-H, would indicate a molecule of hydrogen; H,O,, a molecule of hydric peroxide, composed of 2 atoms of hydrogen and 2 of oxygen; CO,, one molecule of carbonic anhydride, composed of 1 atom of carbon and 2 atoms of oxygen, and so on. Some authors place the small figures above instead of below the line, and write 2 atoms of hydrogen, for example, as H2. Secondary compounds, such as salts, are expressed in an analogous way, the metal being usually placed first, CaCO, representing one molecule of calcic carbonate. When a comma is used to separate two members of a formula, these two members are represented as united chemically, and a more intimate union is supposed than when the sign of a period is used to separate them. For instance, in the formula for crystallized sulphate of magnesium and potassium (MgSO,,K,SO. 6H,C), the compound MgSO, is supposed to be more intimately united with K,SO, than the 6H2O, which may be readily expelled by heat. Where it is necessary 14.] NOTATION-LAW OF VOLUMES. 2 27 to indicate more than one molecule of a compound, the whole formula of that compound is preceded by the indicating number. If, for example, H be 1 atom of hydrogen, H, its molecule, 3H, will indicate 3 molecules of hydrogen. If brackets be used, the figure prefixed or subjoined, multiplies nothing beyond the symbols included within the brackets, as for example, 3 (MgSO, 7 H2O), three molecules of crystallized magnesic sulphate; (H,N), four molecules of ammonia. The use of brackets is ofter. neglected, and then the figure prefixed multiplies all the symbols included between it and the next comma or period, or sign of addition or of equality. The sign should never be used to connect together the constituents of the same compound, but should be employed only to indicate cases of true addition, in which two different bodies are actually mixed with each other, although many chemists neglect, with manifest inconvenience, to attend to this rule. The sign does not indicate identity or absolute equality, but is usually employed in the sense of the word 'yields;' and when placed between the two members of an equation, it indicates that if the compounds which precede it are mixed with due precaution, the result of the chemical changes which occur will be such as is represented by the arrangement of the symbols placed after the sign =. A little practice will make these various modifications familiar to the mind. To expedite the acquisition of this knowledge, the student will find it advantageous to exercise himself in the expression of chemical changes by symbols, whenever the opportunity occurs, until he is thoroughly acquainted with their signification and use. (14) Law of Volumes.-When bodies are capable of assuming the form of gas or vapour, a very simple relation exists between the volumes of any two gases which combine together, and the volume of the gaseous compound formed by their union. This important observation is due to Gay-Lussac. It has been found, for example, that two gases unite together either in the proportion of equal volumes, or else that two volumes of a given gas which may be distinguished as A, combine with one volume of a second. gas which may be called B, or that three volumes of A unite with one of B, or sometimes that three volumes of A unite with two of B. Some simple ratio of this kind is always observed between the volumes of two gases which enter into combination. The cause of this uniformity depends upon the fact that if quantities of each element be compared in the ratio of their atomic 28 LAW OF GASEOUS VOLUMES. [14. weights, when converted into vapour (under similar conditions of temperature and pressure) they will all yield the same volume of vapour, except in the cases of mercury, cadmium, and zinc, which give double the volume, and phosphorus and arsenic, which yield one half of the volume of the corresponding quantity of hydrogen. For example, in the following table we take a number of grammes of hydrogen, of nitrogen, of oxygen, and of chlorine, corresponding with the atomic weight of each element respectively, and the result is in all cases a volume of 1116 litres: : In other words, gaseous nitrogen is 14 times as dense as hydrogen, oxygen 16 times as dense, and chlorine 35'5 times as dense as hydrogen, when compared under equal pressures, and at the same temperatures. In order to facilitate the calculation of the weight of a volume of gas, Dr. Hofmann has proposed the use of a unit which he calls the crith (from кpɩ0ǹ, a barley-corn). This is the weight of a litre of hydrogen at o° C. and 760 millim. Bar. or o‘0896 gramme. One litre of nitrogen will therefore be 14 criths or 0'0896 × 14 = 12544 gramme. Combination by volume, therefore, is to be carefully distinguished from combination by weight; because when the volumes are the same the weights are different, and when the weights are the same the volumes are different. After the union of the gases with each other, the volume of the compound, though it is often less than the joint volume of the two separate gases, yet bears a simple relation to it. It may happen that two gases unite without undergoing any change of volume; this only occurs when the constituent gases combine in equal volumes; in other cases three volumes of the gases may become condensed into two; or three volumes may occupy one volume; or, again, two volumes may be condensed into one volume. In no case do the combined gases occupy a larger volume than they did when separate. The mode of combination of hydrogen with chlorine and with oxygen may be taken as an illustration of some of these points. |