Page images
PDF
EPUB

ought, if the formula CH2O were correct, to be 15, whereas experiment shows it to be 30.

3. Specific Heat.-Atomic Heat.-This method gives very good results with elements in the solid state, and is of the utmost importance. Equal weights of different substances, heated to an equal extent, require very different times for cooling down to the same point. If one pound of water and one pound of mercury, at a temperature of 100° C. (boiling point) be allowed to cool down to 50°, it will be found that the water is 30 times as long in doing so as the mercury, and gives out 30 times as much heat. Water at any one temperature contains 30 times as much heat as an equal weight of mercury at the same temperature, and analogous differences are observed with all other substances. The quantities of heat which equal weights of different bodies contain at the same temperature (measured by the time required for their cooling, or in some other way) are called their specific heats. The specific heat of water is greater than that of any other known substance (except hydrogen), and it is therefore taken as the standard, and called 1. The specific heat of liquid mercury is said to be 0.033.

Now if we compare together the specific heat of different elements in the solid state, we find, as the following table (page 82) will show, that no relation can be observed between them. But if we compare together the specific heats, not of equal weights, but of atomic weights, which we can easily do by multiplying the specific heats of the first column by the atomic weights, we find that the numbers are very similar to one another, and approximate to 6.3. It is therefore inferred that if we could compare all elements in the solid state and at equal temperatures, in quantities proportionate to their atomic weights, and could avoid all errors of experiment, they would all contain the same quantity of heat. This quantity, found by multiplying the specific heat by the atomic weight, is called the atomic heat of the element.

It is easy to understand the assistance which this theory, discovered by Dulong and Petit (we must remember that it is only a theory, for its truth cannot be demonstrated), affords in fixing the atomic weights of elements. Let us take the case of zinc to illustrate it. The equivalent of zinc, as compared with hydrogen, is 32.5, and that number

G

was formerly taken as the atomic weight of the metal. But the specific heat of zinc is 0.0955, and this number multiplied by 32.5 only gives 3·1, so that the atomic heat of zinc must be regarded as exceptional and only half of the usual quantity. But if we take 65 as the atomic weight of zinc, the anomaly vanishes, and the atomic heat becomes 6.2. The atomic weights of many other elements have of late years been doubled for the same reason.

Some few exceptions to this rule of atomic heat are known, of which carbon and silicon are the most important.

It will be seen that if we wish to find the atomic weight of an element from its specific heat, we must divide the average atomic heat 6.3 by the specific heat. The atomic weight so obtained will not be exact, but it will serve to indicate whether a particular number or a fraction or multiple of it is to be chosen as the true atomic weight.

6.3 ⚫0562

E.g. Atomic weight of tin is = 112, which sufficiently indicates that 118, and not 59, should be taken as the atomic

weight of tin.

[blocks in formation]

4. Isomorphism.-It was discovered by Mitscherlich, that compounds which might be supposed to have a similar chemical composition, very commonly crystallized in the same form, even though quite different in properties. Similar compounds of calcium, strontium, barium, and lead; similar sulphates, selenates, and chromates, and similar phosphates and arsenates, are examples of this law, and many others might be quoted. Compounds which exhibit this peculiarity are said to be isomorphous with one another (from toros, like, and μopon, form). Mitscherlich's law, though not universal, is yet so frequently true that when two analogous substances yield crystals having the same angular measurement, there is a great a priori probability that their constituents are arranged in a similar manner, and this probability is sometimes of great use in fixing the atomic weight of an element. For example, the metal aluminium has only one oxide, alumina, which contains 18.3 of the metal to 16 of oxygen. As 16 is the atomic

weight of oxygen, it would be natural to suppose that that of aluminium was 18.3, in which case the formula of alumina would be A10. But alumina is isomorphous with red oxide of iron (ferric oxide) which is known to have the formula Fe2O3, and it is therefore concluded that the formula of alumina must be Al2O,, in which case the atomic weight of aluminium must be 27.5 Of course either formula agrees equally well with the result deduced from analysis. For if:

=

Al 18.3, then A10 = 18.3 Aluminium: 16 Oxygen. 27.5, then Al,O, = 55

Al =

وو

: 48

[ocr errors]

the proportion being the same in both cases. The number 27.5 is confirmed by the specific heat of aluminium, for—

[blocks in formation]

which is as near as could be expected.

The atomic weights of the elements, determined by the collation of all these various facts and theories, are given in the table at the commencement of this volume.

EQUIVALENT VALUE OF ATOMS.-ATOMICITY, OR

QUANTIVALENCE.

The above remarks will have made it evident that the quantity which is known as the atom of an element is not always the same as its equivalent. The equivalent of oxygen is 8, because 8 parts of oxygen will replace 35 5 of chlorine, or 1 of hydrogen, but the atomic weight of oxygen is held to be 16 the equivalent of nitrogen is 4.6, but its atomic weight is 14; and lastly, in its simplest compound, the equivalent of carbon is 3, whereas its atomic weight is 12. It follows from this, that an atom of oxygen is equivalent to two, an atom of nitrogen to three, and an atom of carbon to four atoms of hydrogen. For if 8 of oxygen be equivalent to 1 of hydrogen, 16 must evidently be equivalent to 2. The atom of each element has therefore its own equivalent value, represented by the number of atoms of hydrogen to which it is equivalent. This equivalent value, as compared with that of the atom of hydrogen, is often spoken of as the atomicity, or quantivalence of the element, and its amount is described by the words monad, diad, triad, tetrad, pentad, &c., according as the atom is equivalent to one, two, three, four, or five atoms of hydrogen.

Classification of Elements by their Atomicity.-All those elements of which one atom is equivalent to one atom of hydrogen, either in replacing it or combining with it, are classed together as monads. The most important monad elements are, hydrogen, chlorine, bromine, iodine, and fluorine among the non-metals, and potassium, sodium, and silver, among the metals.

The chief diad elements are oxygen and sulphur among the non-metals, and barium, strontium, calcium, magnesium, zinc, copper, mercury, and lead among the metals.

As examples of triads may be mentioned nitrogen, phosphorus, arsenic, and boron, and the metals bismuth and gold.

Carbon, silicon, and most of the other metals are tetrads.

Mode of Marking Atomicity.-As the symbol denotes one atom, it is easy to mark the equivalent value, or atomicity, of that atom by dashes or Roman figures placed above and to the right of the symbol, in the following manner :-H' O" N'"'

C""" or Civ. The monad atoms, however, are seldom marked at all.

Let it be understood distinctly that a diad atom will take the place of or will combine with any two monad atoms. One atom, or 16 parts of oxygen, for instance, may very often, by mere substitution, be made to displace and take the place of two atoms (2 parts) of hydrogen, and we already know that 16 parts of oxygen will combine with 2 of hydrogen. The formulæ for a few important compounds will illustrate the different values of the atoms more clearly than words.

HCl, H2O", H ̧N"', H1C1, KCl, K2O", Zn"Cl2, Bi""'Cl3. Mode of Fixing Atomicity.-When an element combines with hydrogen, the hydrogen compound decides the atomicity, as with chlorine, oxygen, nitrogen, and carbon. In other cases the chlorine compound, or any other compound that the element may form with a monad, may be used instead. Thus platinum is reckoned a tetrad because it forms the chloride, PtivCl1, in which the atom of platinum is united with four of the monad atoms of chlorine.

Perissiad and Artiad Atoms.-Those elements whose atomicity is an uneven number are called perissiads (πepiσσós, odd). and those in which it is even, artiads (aprios, even). There is great convenience in the use of these words, which were suggested by Dr. Odling.

Variations of Atomicity. Many elements have different atomicities in different compounds. For example, a second compound of platinum and chlorine is known, which has the formula Pt Cl2, and here the metal is clearly a diad. So carbon, which is tetrad in CH, and CivO"2 (two diad atoms act, of course, like four monad ones), is diad in CO. And nitrogen, which is triad in N""H,, is monad in N'2O", and pentad in N'H',Cl'. But these variations are subject to one rule, which is of almost universal application. Perissiads are never artiads, nor artiads perissiads. A pentad, for example, which of course is a perissiad, may sometimes act as a triad, or a monad, but not as a tetrad, or diad. Two of the oxides of nitrogen, NO and NO2, are almost the only certain exceptions to this important rule, which must be borne carefully in mind in constructing formulæ.

Radicals.-The formula for nitric acid is HNO3.

Now a

« PreviousContinue »