The complicated bodies of organic chemistry when they admit of being vaporized, obey this law of vapour volume as strictly as the simplest combinations of inorganic nature. A molecule of hydrochloric acid (HCl), for instance, yields the same volume of vapour as a molecule of alcohol (C,H,O), or as a molecule of the still more complex body aniline (CHN). Consequently, if the weight of a given bulk of hydrogen be taken as the unit of comparison, the vapour densities of compound bodies are represented by half their atomic or molecular weight; for example:

(16) Symbolic Notation.-Before proceeding further, it will be advantageous to describe the principles of notation, as applied in the construction of chemical formulæ. This notation constitutes a kind of short-hand, which materially facilitates the representation of chemical changes, since it greatly abridges the labour of description, and with a little practice, enables the student to trace at a glance reactions even of a complicated character. Its employment 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. These symbols, when used singly, always represent one atom of the body which they indicate. The symbol O, therefore stands for one atom of oxygen; H, for one atom of hydrogen; C, for one atom of carbon, and so on.

A compound body is represented by writing the symbols of its constituent atoms side by side; thus HCl indicates one molecule of hydrochloric acid, CaO one molecule of lime, the quantities included in each formula always indicating 1 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 letter below the line :H, would indicate a molecule of hydrogen; H,O,, a molecule of peroxide of hydrogen, composed of 2 atoms of hydrogen and 2 of oxygen; CO2, one molecule of carbonic anhydride, composed of 1 atom of carbon and 2 oxygen.

2 2

Secondary compounds, such as salts, are expressed in an analogous way, the metal being usually placed first, CaCO, representing one molecule of carbonate of calcium. When a comma is used to separate two compounds, a more intimate union is supposed than when the sign + is used. Thus, in the formula for crystallized sulphate of magnesium and potassium (MgSO,, K,SO,+6H2O), the compound MgSO, is supposed to be more intimately united with K2SO, than the 6H2O, which may be readily expelled by heat. Where it is necessary to indicate more than one molecule of a compound, the whole formula of that compound is preceded by the indicating number. Thus if H be 1 atom of hydrogen, H, its I molecule, 3H, will indicate 3 molecules of hydrogen. If brackets be used, the figure prefixed multiplies nothing beyond the symbols included within the brackets, as for example, 3(MgSO,+7H2O), 3 atoms of crystallized sulphate of magnesium. Frequently the employment of brackets is neglected, and then the figure prefixed multiplies all the symbols included between it and the next comma, or sign of addition.

A very 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. The reaction between nitrate of silver and chloride of sodium (14), might be expressed by symbols in a single line, which, if the combining numbers of the elements concerned were fixed in the memory, would convey all the information of a minute description, thus

WEIGHTS AND MEASURES-SPECIFIC GRAVITY.

(17) Weights and Measures.-The foundation of all accuracy in experimental science consists in the possibility of determining with exactness the quantity and the bulk of those substances which are submitted to examination. In the force of gravity we possess an unvarying standard of comparison. A pound weight, for example, at the same spot of the earth's surface, is invariably at

32

ENGLISH WEIGHTS AND MEASURES.

tracted towards the earth with the same force, so that its weight is uniformly the same at that spot.

The force of gravity diminishes slowly from the pole to the equator. A mass of matter which would compress a spring with a force equal to that of 194 lb. at the equator, would act upon it with a force of 195 lb. at the poles. This difference would not, of course, be perceived in the ordinary mode of weighing by the balance, as both the weights and the body weighed would be similarly and equally affected.

The common process of weighing consists in estimating the force with which any given mass is attracted towards the earth, by comparison with other known quantities of matter, arbitrarily selected for the purpose; consequently, the weight of a body is the expression in terms of the standard so selected, of the exact amount of force which is required to prevent the body under examination from falling to the ground.

The standard of weight used in this country is the avoirdupois pound, which is subdivided into 7000 grains.

The system of weights is connected with the measures of capacity in use in this country, through the medium of the Imperial gallon; which is defined by an Act of Parliament of the year 1824 to be a measure containing 10 lb. avoirdupois of distilled water, weighed in air at a temperature of 62° F., the barometer standing at 30 inches. The gallon of distilled water, therefore, contains

70,000 grains.

These measures of capacity are related to those of length by the determination that a gallon contains 277 276 cubic inches. A cubic inch of distilled water weighs, in air at 62°, with the barometer at 30 inches, 252'456 grains; in vacuo (23) it weighs 252.722 grains. The standard of length is the yard measure, and is subdivided into 36 inches.*

(18) French System of Weights and Measures.-The French system of weights and measures is connected together in a manner far more philosophical than the foregoing; and, as it is the one generally adopted by scientific men abroad, and is gradually being introduced into the writings of men of science in this country, it is essential that the principles upon which it is based should be understood.

The standard of reference is a measurement of one of the great

* In order further to connect the measures of length with those of weight, Captain Kater determined the length of a seconds pendulum, the oscillations of which are produced by the action of the force of gravity. The length of a pendulum, which beats seconds at the level of the sea in vacuo, and in the latitude of Greenwich, he found to be 39′13929 inches.

FRENCH STANDARDS OF WEIGHT AND MEASURE.

FIG. 3.

33

Metre Inch

0.8

circles encompassing the earth itself. The ten-millionth part of a quadrant of the meridian constitutes the unit of the system. This quadrantal arc was fixed at 6213 miles and 1450 yards English measure; consequently the ten-millionth part of this, the metre, is equivalent to 39'37079 English inches, nearly 3 inches more than our standard yard, or a fraction of an inch longer than the seconds pendulum. This metre is subdivided into tenths, called decimetres; hundredths, or centimetres; and thousandths, or millimetres. A millimetre amounts very nearly to 'th of an English inch, and a centimetre to nearly ths of an inch. A kilometre, or thousand metres, nearly of an English mile, is employed in many parts of France as the ordinary road measure. Fig. 3 represents a decimetre subdivided into centimetres, one of which is subdivided into millimetres, compared with English inches. The measures of capacity are connected with those of length by making the unit of capacity in this series a cube of a decimetre, or 3'937 English inches in the side; this, which is termed a litre, is equal to 1.765 Imperial pints, or rather more than 1 English pints. The litre is again subdivided into tenths, or decilitres, and hundredths, or centilitres; and finally, the system of weights is connected with both the preceding, by taking as its unit the weight of a cubic centimetre of distilled water, at the temperature of 39°2 F.; it weighs 15'432 English grains. The gramme, as this quantity is called, is further subdivided into tenths, or decigrammes; hundredths, or centigrammes; and thousandths, or milligrammes ; and its higher multiple, 1000 grammes, forms the kilogramme. The kilogramme is the commercial unit of weight, and is something less than 24 lb. avoirdupois, being 15432.3 English grains. The litre, as it consists of 1000 cubic centimetres of water, at 39°2, contains exactly a kilogramme of water, and is 0.005 equivalent, at 39°2, to 61024 cubic inches English.

0.05

0.01.

0.5

(19) The Balance.-The familiar operation of weighing is for the most part effected by means of the balance.

This instrument consists essentially of an inflexible bar, delicately suspended at a point exactly midway between its extremities, from which depend the scale-pans; in one of these the weights, in the other the objects to be weighed, are placed. When the balance is in equilibrio, the arms of the beam assume a direction

D

34

THE BALANCE-SPECIFIC GRAVITY.

perfectly horizontal. The main points requiring attention are1st, Equality in the lengths of the arms of the beam; 2nd, suspension of the lever just above its centre of gravity; and 3rd, care that the friction at the points of suspension both of the beam and of the scale-pans be reduced to a minimum. The points of support in delicate balances are usually made of fine edges of hardened steel, which bear against flat polished plates of agate. Provided that the suspensions be sufficiently delicate, it is easy, by the process of double weighing, to obtain exact weighings by means of a balance the arms of which are not equal. For this purpose, the material to be weighed is accurately balanced with shot, sand, or any other convenient substance; it is then removed from the pan, and weights substituted, until the sand or shot remaining in the other pan is again accurately counterpoised: the number of weights needed will show the weight of the substance under experiment. In all delicate experiments the balance must be screened from currents of air, and the bodies weighed must have sensibly the same temperature as that of the surrounding atmosphere, otherwise currents of air, ascending or descending within the case, will be produced, and they will impair the accuracy of the observation. A good balance will indicate a difference of weight equal to about 1,000 of what it will carry in each pan.

Specific Gravity.

(20) If equal bulks of matter of different kinds be compared together, they will be found to differ very greatly in weight.

Platinum, the heaviest body with which we are acquainted, is upwards of 200,000 times as heavy, bulk for bulk, as hydrogen, which is the lightest material known.

The comparison of the weights of equal bulks of different bodies, when referred to a uniform standard, constitutes their specific gravity, or relative weight, i.e., the weight which is specific or peculiar to each kind of matter. The specific gravity of a body forms one of its most important and distinguishing physical characters. The mineral iron pyrites, for instance, is in colour almost exactly like gold; but it is at once distinguished from the precious metal by the difference in specific gravity, an equal bulk of gold