The condition requisite to the production of chemical action between bodies is that they should be dissimilar. Two elementary atoms are placed within the spheres of each other’s influences, and a compound molecule results. Oxygen and hydrogen form water; oxygen and carbon give rise to carbonic acid; nitrogen and hydrogen unite to form ammonia; and chlorine and hydrogen to produce hydrochloric acid. In all these cases an external force is required to bring the atoms within the range of mutual affinity: flame,—the electrical spark,—actinism,—or the interposition of a third body, is necessary in each case. There are other examples in which no such influence is required. Potassium and oxygen instantly unite: chlorine, iodine, and bromine immediately, and with much violence, combine with the metals to form chlorides, iodides, or bromides.
With compound molecules the action is in many cases equally active, and combination is readily effected, as in the cases of the acids and the oxides of some metals, which are all instances of the most common chemical attraction.
An elementary or simple molecule and molecules of a compound and different constitution are brought together, and a new compound results from an interchange of their atoms, whilst an element is liberated. These are essentially illustrations of analytical chemistry. Sulphuretted hydrogen is mixed with chlorine; the chlorine combines with the hydrogen, and sulphur is set free. Potassium is put into water, and it combines with the oxygen of the water, whilst the hydrogen is liberated.
Two compound molecules being brought together may decompose each other, and form two new compounds by an interchange of their elements.
One element may be substituted for another under certain circumstances. Gold may be replaced by mercury; copper will take the place of silver; and iron will occasion the separation of copper from its solutions, the iron itself being dissolved to supply its place; chlorine will substitute hydrogen in the carburetted hydrogen gases; and many other examples might be adduced.
Chemical phenomena very frequently become of a complex character; and one, two, or three of these cases may be occurring at the same time in the decomposition of one compound by another. Such are the general features of chemical science. Many peculiarities and remarkable phenomena connected with chemical investigations will be named, as the examination of the elementary composition of matter is proceeded with; but, although the philosophy of chemical action is of the highest interest, it must not be allowed to detain us with its details, which are, indeed, more in accordance with a treatise on the science than one which professes to do no more than sketch out those prevailing and striking features which, whilst they elucidate the great truths of nature, are capable of being employed as suggestive examples of the tendency of scientific investigation to enlarge the boundaries of thought, and give a greater elevation to the mind, leading us from the merely mechanical process of analysis up to the great synthetical operations, by which all that is found upon the earth for its ornament, or our necessities, is created.
Among the most remarkable phenomena within the range of physical chemistry are those of Catalysis, or, as it has also been called, the “Action of presence.”[205] There are a certain number of bodies known to possess the power of resolving compounds into new forms, without undergoing any change themselves. Kirchoff discovered that the presence of an acid, at a certain temperature, converted starch into sugar and gum, no combination with the acid taking place. Thenard found that manganese, platinum, gold, and silver, and, indeed, almost any solid organic body, had the power of decomposing the binoxide of hydrogen by their presence merely, no action being detected on these bodies. Edmund Davy found that powdered platinum, moistened with alcohol, became red-hot, fired the spirit, and converted it into vinegar, without undergoing, itself, any chemical change. Döbereiner next discovered that spongy platinum fired a current of hydrogen gas directed upon it, which, by combining with the oxygen of the air, formed water. Dulong and Thenard traced the same property, differing only in degree, through iridium, osmium, palladium, gold, silver, and even glass. Further investigation has extended the number of instances; and it has even been found that a polished plate of platinum has the power of condensing hydrogen and oxygen so forcibly upon its surface, that these gases are drawn into combination and form water, with a development of heat sufficient to ignite the metal.
This power, whatever it may be, is common in both organic and inorganic nature, and on its important purposes Berzelius has the following remarks:—
“This power gives rise to numerous applications in organic nature; thus, it is only around the eyes of the potato that diastase exists: it is by means of catalytic power that diastase, and that starch, which are insoluble, are converted into sugar and into gum, which, being soluble, form the sap that rises in the germs of the potato. This evident example of the action of catalytic power in an organic secretion, is not, probably, the only one in the animal and vegetable kingdom, and it may hereafter be discovered that it is by an action analogous to that of catalytic power, that the secretion of such different bodies is produced, all which are supplied by the same matter, the sap in plants, and the blood in animals.”[206]
It is, without doubt, to this peculiar agency that we must attribute the abnormal actions produced in the blood of living animals by the addition of any gaseous miasma or putrid matter, of which we have, in all probability, a fearful example in the progress of Asiatic cholera; therefore the study of its phenomena becomes an important part of public hygiène.