CHAPTER NINTH.

CHEMICAL.

SECTION LIV.--COMPOSITION.

ART. 256.-Composition of a Substance. Suppose that a substance is formed by the combination of three elementary substances, A, B, and C, and that a M of A, b M of B, and c M of C have been used up in forming the substance. A substance is perfectly homogeneous throughout; hence, as in the case of a homogeneous mixture (Art. 25) but with much greater exactness, a M of Д + b M of B+ c M of C=a+b+c M of substance. Partial equivalences and equivalences of the tangent kind are derived as in the case of mixture. Thus the composition is fully given by taking together the equivalences

b/a M of B = M of A; and c/a M of CM of A.

From these two ratios the third

c/b M of CM of B

follows by necessary consequence.

These ratios differ from the corresponding ratios of a mixture in the respect that they are each limited to a few values.

ART. 257.-Combining Weight. Suppose that a M of A and b M of B unite to form a substance X. The composition then is a M of A+ b M of B = a + b M of X,

and the combining weight (by which is meant the combining mass) of B relatively to A for the substance X is

ba M of BM of A.

But M of B may combine with a Μ of A to form another substance Y, so that

a M of A+ M of B=a+b' M of Y;

and the combining weight of B relatively to A is in this case b'a M of BM of A.

It is found that the value ba is either a multiple of ba, or is related to ba by a simple fraction.

For example, the ratio of oxygen to hydrogen in water is

8 M of oxygen = M of hydrogen;

while in the case of peroxide of hydrogen it is

16 M of oxygen = M of hydrogen.

By comparing the several elements directly or indirectly with one element, a set of combining weights can be formed. The standard element is hydrogen, and the set of combining weights which has been chosen is exhibited in the following table. The values have been deduced from a comparison of those published by Meyer and Seubert with those published by Clarke.

COMBINING WEIGHTS OF THE ELEMENTS.

r M of element = M of Hydrogen.

ART. 258. Empirical Formula. The symbols Ag, Al, etc., may be used in a qualitative sense merely, or they may have a quantitative signification attached. In the latter case the quantity attached to the qualitative meaning of the symbol is the value of the combining weight.

If the combining weights of A, B, C, the elements forming a compound are a, ß, y relatively to hydrogen, and the multiples according to which these combining weights occur in the par

ticular substance p, q, r; then the composition of the substance is pa M of A+qẞ M of B + ry M of C' = pa + qß + ry M of compd.; and an empirical formula for the compound is derived by a short notation for this equivalence, namely, A, B, C, reduced to its simplest form.

The notation A, B, C, in its simplest form expresses the combining weight of the compound, and it also serves for a name, excepting in the case of the class of substances which have the same chemical composition, but different physical properties.

ART. 259. Percentage Composition. The composition of a substance A, B, C, is

pa M of A+ qẞ M of B+ry M of C = pa + qß + ry M of subst. This equivalence will remain true, after each term is multiplied by 100 and divided by pa +qß+ry.

100 pa

pa + yẞ + ry

Hence
100 qB
M of 4+
M of B +
pa + qẞ + ry

100 ry pa+qB + ry

M of C

=

100 M of substance.

Let the values of the three percentages be respectively n1, no, no. Then we deduce the equations

ρα η pa ni

There are only two independent equations. Hence from the percentage data given by analysis we can determine only the ratios to one of the elements of each of the remaining elements, that is, the empirical formula. To determine the molecular formula an additional datum is required (Art. 266).

ART. 260.-Composition by Volume. Suppose that the three elements A, B, C are gases, having at a common temperature and pressure the bulks x, y, z V per M. Let the composition by mass of a substance X formed by them be

pa M of A+qẞ M of B+ry M of C = pa + qß + ry M_of_X. The composition by volume is obtained by finding the volumes of

the several components. The volume of A is pax, etc.; therefore the composition by volume is

pax V of A+ qßy V of B + ryz ▼ of C=pa + qẞ +ry Μ of X.

The mass of a compound is necessarily equal to the sum of the masses of its components; but a similar statement is not true for volume.

Here the composition by volume is deduced from the composition by mass, in the same manner as in the case of mixture of a commodity the composition by value is deduced from the composition by mass (Art. 26).

ART. 261.-Strength of a Solution; Solubility. The composition of a solution is expressed in the form

q M of solid + (1 − q) M of solvent = M of liquid.

The strength of the solution is expressed by

q/(1-q) M of solid dissolved = M of solvent.

The ratio q/(1 - q) is not restricted to a few values like a ratio of combination, but may have any fractional value up to a certain limit, which depends on the temperature of the solvent.

By the solubility of a solid in a liquid at a given temperature is meant the maximum value of the strength. The reciprocal idea is the minimum value of

(1-q)/g M of solvent M of solid dissolved;

=

it gives the idea of resistance to solution.

ART. 262.-Absorptive-power. The solution of a gas in a liquid is called absorption. The composition of the solution is expressed by

4 M of gas absorbed + (1 − q) M of solvent M of liquid, and the strength of the solution by

1/(1-9) M of gas absorbed = M of solvent.

If the bulk of the gas before absorption is v V per M, then qv/(1-q) V of gas absorbed = M of solvent ;

and if the density of the absorbing liquid is p M per V, then gup/(1-9) V of gas absorbed = V of solvent.