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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, HC1 indicates one molecule of hydrochloric acid, CaO one molecule of lime, the quantities included in each formula always indicating one molecule of the compbund.
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;—H2 would indicate a molecule of hydrogen; H202, a molecule of hydric peroxide, composed of 2 atoms of hydrogen and 2 of oxygen; C02, one molecule of carbonic anhydride, composed of i 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, CaCOg 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 aud potassium (MgS04,K2S04. 6H20), the compound MgS04 is supposed to be more intimately united with K3S04 than the 6H20, which may be readily expelled by heat. Where it is necessary
1+] NOTATION LAW OP VOLUMES. 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 i atom of hydrogen, H2 its molecule, 3H2 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 (MgSOv 7 H;0).. three molecules of crystallized magnesic sulphate; (H3N)4, four molecules of ammonia. The use of brackets is often 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 mauy 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 13, 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 quantitiea of each element be compared in the ratio of their atomic i gramme of hydrogen, at
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 in6 litres:—
Litres. 32°F.and29-922inchesBar.) _ _ _ (or o° C. & 760 millim.) j-11'10
14 grammes of nitrogen = irj6
16 grammes of oxygen =iri6
35-5 grammes of chlorine =11*16
and so on.
In other words, gaseous nitrogen is 14 times as dense as hydrogen, oxygen 16 times as dense, and chlorine 355 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 npiOri, a barley-corn). This is the weight of a litre of hydrogen at o° C. and 760 millim. Bar. or o-o8c;6 gramme. One litre of nitrogen will therefore be 14 criths or 0-0896 x 14 = t'2544 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.