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34 ATOMIC CLASSIFICATION OF ELEMENTS. [l$.

with equal ease, inasmuch as 108 mgrms. of silver require for conversion into nitrate 62 mgrms. of nitrion, and therefore represent 170 mgrms. of argentic nitrate; consequently 1435 mgrms. of argentic chloride indicate the presence of 170 mgrms. of argentic nitrate in the liquid under examination.

(16) Equivalency of the Elements.—From what has been already stated (p. 23) it is clear that the atom of one element is by no means necessarily equivalent in chemical power to the atom of another element. The most recent investigations, in fact, show that the elementary bodies may be arranged in groups, according to their chemical equivalency in relation to hydrogen, as follows—viz.:—

1. Monads, or Uniequivalent Elements, in which one atom of each in combination is usually equivalent to H, or one atom of hydrogen. In these the atomic and equivalent numbers are identical.

2. Dyads, or Biequivalent Elements, in each of which one atom, in combining with other bodies, is generally equivalent to H^ or two atoms of hydrogen. In these the atomic number is double the equivalent number.

3. Triads, or Terequivalent Elements, in each of which one atom, in entering into combination with other bodies, is generally equivalent to H3, or three atoms of hydrogen.

4. Tetrads, or Quadrequivalent Elements, each of which in combining represents H^ or four atoms of hydrogen.

5. Pentads, or Elements, each of which in combining represents 5 atoms of hydrogen.

6. Hexads, or Elements, each atom of which in combining may represent 6 atoms of hydrogen.

Occasionally it will be found convenient to indicate the equivalent power of an element by affixing dashes or Roman numerals to the symbols. Ca", for example, would indicate the biequivalent power of the proportion of calcium represented by its symbol; P"' would indicate the terequivalent power of phosphorus; SnlT the quadrequivalent power of tin, and so on.

In the following table the principal elements are arranged into six groups upon the principle just indicated:—

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(16 a) Atomicity of the Elements.—The elements of these six groups have also different combining or atom-fixing powers. Thus one atom of a monad, can only be united to one atom of another monad in a compound, as in the case of hydrochloric acid, where an atom of chlorine is united with one of hydrogen. The dyads are capable of fixing two monad elements, as in water, consisting of one atom of oxygen combined with two of hydrogen. Again, triad nitrogen in ammonia is united with three atoms of hydrogen; and in ammonic chloride the same element has pentad functions, being combined with four atoms of hydrogen and one of chlorine. Thus, if we have one molecule of a compound containing one atom of an element (of which the atom-fixing power or atomicity is unknown) united with hydrogen or other monads, we can usually measure the atomicity of the element by the number of mouads present; for, since we have no evidence that these monads can unite with more than one other element at one time, we can only explain the existence of such compounds as ammonia, NHS, or marsh gas, CH4, by supposing that the three atoms of hydrogen in the former and the four of hydrogen in the latter are directly united with the nitrogen and carbon respectively. The atom of nitrogen thus appears to have three combining powers or points of attachment, or bonds which enable it to unite with three monad elements at the same time; similarly, carbon has four such bonds; oxygen two, while hydrogen and the other monads hare only one. The greatest precaution must be taken against

36 ATOMICITY OF THE ELEMENTS. [l6 ffl.

allowing the mind to imagine that there is any material connecting link between atoms in chemical compounds; all that the term bond is intended to imply is a connexion similar to that existing between the members of the solar system; and without the supposition of the existence of some such force it is difficult to account for the stability of chemical substances.

To assist in comprehending the constitution of compounds, chemists have adopted a form of graphic representation of the bonds of the elements. The symbols of the elements in a compound are joined to one another by lines, the symbol of a monad element having one line, a dyad two, and so on. Thus, H—

I I

—O— —N— —C— The compounds hydrochloric acid,

I water, ammonia and marsh gas, would be represented by

H H

I I

H—CI H—O—H H—N—H H—C—H

I
H

The position of the lines is of course immaterial, the only important things to be observed are their number, and the elements to which they are attached. Thus carbonic anhydride may be written A = C = 0 or O^C^O.

The atomicity of an element appears to be variable to a certain extent; thus nitrogen is pentad in ammonic chloride, triad in ammonia and monad in nitrous oxide, but it will be noticed that this variation takes place by the suppression of two bonds at a time, and it is quite easy to imagine that these attractions neutralize or saturate one another; so that the three compounds named might be thus formulated:—

Ammonia Chloride. Ammonia. Nitrous Oxide.

CI H

H^i/H H_I_H n^n

The maximum or absolute atomicity of nitrogen is said to be 5, all the attractive forces being active in ammonia chloride; in ammonia three are active and two latent; and in nitrous oxide one is active in each atom of nitrogen and four latent.

17-] WEIGHTS AND MEASURES. 37

To bring the symbolic mode of representing chemical compounds into relation with the graphic notation, Dr. Prankland has proposed that the symbol of the element having the highest atomicity should be placed at the commencement of the formula, and to show that the other elements of the compound are directly combined with this binding element its symbol is printed in a thicker type. Thus the symbolic formulae for the seven compounds above mentioned would be:—

[table]

The details of this introductory chapter are necessary to the student at the commencement of his chemical course; but the full consideration of some of the subjects here alluded to, such as the determination of the atomic weights, and the discussion of the equivalency of the elements, will be resumed as preliminaries to the study of organic chemistry.

CHAPTER II.

WEIGHTS AND MEASURES DENSITY.

(17) Weights and Measures.—The foundation of all accuracy in experimental science consists in the possibility of determining with exactness the quantity or mass, and the size or volume of those substances which are submitted to examination. In gravity we possess an unvarying standard of comparison.

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 at the equator, would act upon it with a force of about 195 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 attraction with which any given mass is drawn 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

38 FRENCH STANDARDS OF WEIGHT AND MEASURE. [17.

of tension or pressure which is required to prevent the body under examination from falling to the ground.

The standard of mass or weight used in this country is an arbitrary quantity called the avoirdupois pound, which is subdivided into 7000 grains. It was enacted in 1855, 'that the platinum weight deposited in the Exchequer shall be denominated the Imperial standard pound avoirdupois, and that the ,-oVoth of it shall be a grain, while 5760 such grains shall denote one pound troy/

The system of weights is connected with the measures of volume 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 volume 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 620, with the barometer at 30 inches, 252'456 grains; in vacuo (24) it weighs 252722 grains. The standard of length is the yard measure, and is subdivided into 36 inches.* The standard yard is defined by an Act of Parliament passed in 1855, whereby it is enacted, 'that the straight line or distance between the centres of two gold plugs in the bronze bar deposited in the office of the Exchequer shall be the genuine standard yard at 62° Fahr., and if lost it shall be replaced by means of its copies.'

(18) French or Metrical System of Weights and Measures.— The French system of weights and measures is connected together in a manner far more philosophical than the foregoing; it is the one generally adopted by scientific men abroad, and is now being introduced into the writings of men of science in this country. Its advantages indeed are so great that it will be largely used in this work,

* In order further to connect the measures of length with those of weight, Captain Eater determined the length of a seconds pendulum, the oscillations of which are produced by the action of gravity. The length of a pendulum, which beats seconds at the level of the Bea in vacuo, and in the latitude of Greenwich, he found to be 30/13929 inches. See also the reports of Prof. W. H. Miller on the standard pound (Phil. Trans. 1856, 753), and of Mr. (now Sir George) Airy on the new standard of length (Phil. Trans. 1857, 621).

t The repugnance experienced by most persons to adopt a new system of weights and measures arises chiefly from the difficulty of mentally realizing the values of the new denominations in terms of those to which the mind is accus

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