The chemical union of different kinds of atoms and volumes of matter in the definite proportions of whole numbers entirely changes their character and properties, as for example the chemical combination of one atom of hydrogen and one atom of oxygen into water. The condensation is often unexpected and wonderful; two different liquids are often condensed into a solid, and the result of the chemical combination of two different gases or vapours in quantitative proportions may be solid, liquid or aëriform, a fact which could only have been discovered by experiment. The powers of the atoms are changed and often highly exalted by chemical union as in ammonia, a chemical compound of three atoms of hydrogen and one of nitrogen, which absorbs 1,195 times more radiant heat than its constituents whether simple or mixed. During chemical combination light and electricity are often evolved, heat always. The quantity given out is exactly proportional to the energy of the chemical action, and is often so great and so rapidly evolved as to produce an explosion by the sudden expansion of the air around. Whatever the temperature may be, which is given out during the union of the atoms, the very same quantity of heat is requisite to dissolve their union, and the atoms are separated in the same definite proportions in which they were combined.
Voltaic electricity both combines and resolves substances into their component parts, strictly according to the law of definite proportions. It combines eight parts by weight of oxygen and one part by weight of hydrogen into water; and again when it decomposes water, one part by weight of hydrogen is given out at the negative pole of the battery, and eight parts by weight at the positive or zinc pole. For an electric current weakens or neutralizes the force of affinity in one direction and strengthens it in the other, so that the heterogeneous atoms of the substance under its influence have a tendency to go in different directions and appear at opposite poles. Mr. Faraday has established as a general law, that the quantity of electricity requisite to unite the atoms of matter, is precisely equal to the quantity requisite to separate the same atoms again. Electro-chemical action, or the power of electricity to combine and separate the heterogeneous atoms of matter, is in direct proportion to the absolute quantity of electricity that passes in the current. Hence the superior analytical power of voltaic over static electricity, which has enormous intensity, but is very small in quantity. The electric current separates molecular combinations which yield to no other means: it is the most powerful instrument of analysis; light is the most delicate.
Two simple substances are only capable of a certain number of chemical combinations, which form a regular series of new substances; as for example oxygen and nitrogen. Two measures of nitrogen gas will unite with one measure of oxygen to form the protoxide of nitrogen; with two measures of oxygen it unites to form the binoxide of nitrogen; with three measures of oxygen it forms the hyponitrous acid; with four it forms nitrous oxide; and with five measures of oxygen it forms nitric acid. Thus there are five compounds of nitrogen and oxygen, no more. Affinity of kind is merely the attraction of one element or atom of matter for another; affinity of degree consists in the grades and limits of combination; the preceding series is of the fifth degree; the limit is the last term, for no further combination of these two gases can take place, and these are accomplished by art. All the five substances are deleterious, most of them deadly poisons, for the protoxide of nitrogen, which is the laughing gas, could not be long inhaled with impunity. For a long time the middle term of the preceding series was wanting, but Gay-Lussac formed it by attending to the laws of definite proportion and sequence.
The atoms of different kinds of matter possess an affinity, or attractive force, which binds them together chemically in different and very unequal degrees. Two substances may unite and form a third differing from both, as water does from oxygen and hydrogen; but if a new substance be added which has a greater attraction for one of the substances than for the other, it will dissolve their union, combine with that for which it has the strongest attraction, and set the other free. Thus the metal potassium, which has a greater attraction for oxygen than it has for hydrogen, decomposes water, combines with the oxygen, and sets the hydrogen free. Both chlorine and ozone have the property of liberating the iodine in a weak solution of the iodide of potassium; the liquid stains starch blue, a proof of the free iodine. The facility with which acids and alkalies combine affords the means of eliminating either the one or the other from a compound so as to liberate what remains.
The constituents of compound substances may be separated from one another by a variety of means depending upon their greater or less fusibility, volatility, and other properties. Water, acids, alcohols and other liquids hot or cold, different degrees of temperature, sublimation, solution, distillation, evaporation, together with static and voltaic electricity, are the most powerful means of analysis.
But the animal and vegetable creation rear their fabrics by a synthetic process. A plant after having absorbed carbonic acid and water, decomposes the carbonic acid, returns the oxygen to the atmosphere, and combines the carbon and water into wood, leaves, and a variety of organic substances. Now MM. Berthelot, Wöhler, and other distinguished chemists, by following this example of nature, have established a system of synthetic chemistry, by which they have produced from the chemical combination of the three elementary gases and carbon alone more than 1,000 complete organic substances, precisely the same with those formed within the living plants and animals. Yet we are as far as ever from any explanation of the mystery of life, whether animal or vegetable.
Carbon and hydrogen will not combine at any artificial heat however great; but when the electric arc between highly purified charcoal terminals passes through hydrogen gas, acetylene, a new carburet of hydrogen, is formed, consisting of four equivalents of carbon and two of hydrogen. This substance, which no organized being is capable to form, was discovered by M. Berthelot, and being assumed as a base, yielded an extensive series of organic substances. Thus when two atoms of carbon are added to acetylene it becomes olefiant gas; when two equivalents of oxygen are added to olefiant gas, the result is alcohol, which is transformed into acetic acid by the addition of two atoms of oxygen, and from this by a similar process have been obtained the malic, tartaric, succinic, and the other acids; glycerine also, which is the sweet principle of the oils, wax, essential oils, the perfumes of fruit and flowers, the principle of the balms, the essential oil of mustard, and numerous other organic substances, simply from carbon, oxygen and hydrogen; but nitrogen was introduced by combining alcohol with ammonia, an inorganic substance consisting of three equivalents of hydrogen and one of nitrogen, from whence a vast number of nitrogenized substances were derived, both animal and vegetable.
Chemical combination, which has from the beginning of created things, and still is, building up organic and inorganic matter in the earth, in the air, and the ocean, exerts forces of transcendent power, though silent, unperceived, and for the most part unknown. Professor Tyndall has given a striking instance of this in water, the most simple compound of oxygen and hydrogen, a constituent alike of organic and inorganic nature. ‘In the combustion of the two gases to form a gallon of water weighing ten pounds, an energy is expended, the atoms clash together with a force, equal to that of a ton weight let fall from a height of 23,757 feet; and in the change from the state of vapour to water, an energy is exerted equal to that of a ton weight falling from a height of 3,700 feet, or of a hundredweight falling from a height of 74,000 feet. The moving force of the stone avalanches of the Alps is but as that of snowflakes compared with the energy involved in the formation of a cloud. In passing finally from the liquid to the solid state,’ that is from water to ice, ‘the atoms of ten pounds exercise an energy equal to that of a ton weight falling down a precipice of 550 feet of perpendicular height.’
From Mr. Joule’s investigation of the relation existing between chemical affinity and mechanical force, it appears that when affinity is feeble it can be overcome mechanically. He formed amalgams of different metals, that is he combined them with mercury, by electricity. The affinity of iron for mercury is so feeble that the amalgam is speedily decomposed when left undisturbed by the pressure of the atmosphere, and if a greater pressure be added, almost all the mercury is driven out. The efficacy of mechanical force to overcome feeble chemical affinities is strikingly illustrated by the amalgam of tin, out of which nearly the whole of the mercury is driven by long continued pressure. In these cases the force of affinity did not amount to chemical equivalency, otherwise the mercury could not have been driven out by so small a force. Instances from the weakest to the strongest affinity show that it is only when the power reaches a definite point that the law of chemical equivalents comes in. The intense energy which then begins to be exerted has just been shown.
It is vain to hope for a knowledge of the absolute weight of the ultimate atoms of matter, and nothing seems to be more beyond the power of man than to determine even their relative weights; yet the definite proportions in which they combine have enabled him to do so. Thus, an atom of oxygen unites with an atom of hydrogen to form water; but as every drop of water, however small, contains eight parts by weight of oxygen, and one part by weight of hydrogen, it follows that an atom of oxygen is eight times heavier than an atom of hydrogen. Now, since hydrogen gas is the lightest body known, its atom has been assumed as the unit of comparison. Hence, if the unit of hydrogen be represented by 1, that of oxygen may be represented by 8. Again, carbonic acid gas contains six parts by weight of carbon, and eight parts by weight of oxygen, and as an atom of oxygen is eight times heavier than an atom of hydrogen, therefore an atom of carbon is six times heavier than an atom of hydrogen, and consequently may be represented by 6. In this manner the relative weights of many substances have been determined. But the property of isomorphism also affords the means of ascertaining the atomic weights of certain substances with unerring certainty. It is exactly the contrary of dimorphism, for in the latter substances are chemically the same under different forms; whereas isomorphic bodies are chemically different under the same form. Now the peroxide of manganese contains one atom of oxygen for one atom of metal; but in 100 parts of the protoxide there are 21·94 parts of oxygen and 78·06 of manganese. Comparing these numbers with 8 the atomic weight of oxygen, the result is 28 the weight of an atom of manganese. The same number is obtained from two other isomorphic compounds of oxygen and manganese, which proves the accuracy of this result. The atomic weights of many bodies have been determined, of which the following are the most important.