There are certain groups of substances, especially among the metals, whose atomic weights are in regular arithmetical series, as those of titanium, tin, and tantalum, which are 25, 59, and 92, the common difference being 34.
Certain groups of combined atoms called compound radicles are much more important than the preceding. They unite chemically with one another, and with other substances in definite proportions, precisely as if they were ultimate atoms. They are even capable of being substituted one for the other, forming groups of infinitely varied properties, and thus chemical equivalency extends to them.
Cyanogen, amidogen, and the peroxide of hydrogen are compound radicles which combine with other substances and with simple atoms as if they themselves were simple elements; though the first is a chemical compound of two atoms of carbon and one of nitrogen, the second a chemical compound of one atom of nitrogen and two of hydrogen, and the peroxide contains as before mentioned two atoms of oxygen and one of hydrogen. All three are capable of replacing hydrogen, chlorine, and metals by equivalent substitutions. For example, the chlorate of potash consists of one atom of potash, an atom of chlorine, and five atoms of oxygen; if then an atom of cyanogen whose weight is 26, be put for the atom of chlorine, the result would be the cyanate of potash.
Cyanogen, formed by passing nitrogen over red-hot carbon, consists of two equivalents of carbon and one of nitrogen. It is a frequent constituent of organic and inorganic compounds, and travels in the voltaic circuit as if it were a simple substance.
Ammonia consists of three equivalents of hydrogen and one of nitrogen; now, when the radical phenyle, which consists of twelve equivalents of carbon and five of hydrogen, is put in the ammonia for one equivalent of hydrogen, the result is aniline, whence most of the coal tar colours are obtained. In like manner carbazotic acid, a beautiful yellow dye from coal tar, is carbolic acid, three of whose equivalents of hydrogen have been replaced by three equivalents of an oxide of nitrogen.
Compound radicles, consisting of carbon and the three elementary gases, have been discovered which enter into combination in definite proportions as simple atoms, and all compound radicles travel in the galvanic circuit as equivalents to the elementary substances. Hitherto they have been regarded as representatives or equivalents of one atom of hydrogen. Now it is generally admitted that each has the property of replacing two, three, or more atoms of hydrogen by equivalent substitution. This multiple equivalency among compound radicals forms the basis of what is called the polyatomic theory, now so much employed by MM. Hofmann, Berthelot, and other great modern chemists.
Water is the most common radicle both in the inorganic and organic world. Though a compound of oxygen and hydrogen, it enters, according to the law of definite proportion, into the composition of various amorphous bodies in a dry state, that is in the form and proportion of its gases. It is an essential element in the greater number of crystals, and abounds in organic matter. In certain cases the same substance crystallizes at different temperatures, unites with different quantities of water under the form of oxygen and hydrogen, and assumes corresponding forms. For example, the seleniate of zinc unites with three different portions of water and takes three different forms, according as its temperature is hot, lukewarm, or cold. Thus each particle of water, containing one atom of oxygen and one of hydrogen, combines with one atom of zinc in three different proportions as if it were a simple atom.
The water of crystallization may be driven off from many substances by heat, as from the hydrates of lime, iron, copper, &c., but when combined with the oxides of certain metals, potassium for instance, it cannot be driven off by any means whatever. In general a heat of 212° Fahr. is sufficient, but some crystals lose their water of crystallization at the ordinary atmospheric temperature.
Crystals whose atoms are in unstable equilibrium, are readily altered both externally and internally by a very moderate degree of heat. Arragonite and calcareous spar are isomeric, that is, they are chemically the same but differ in form and hardness, which shows that their molecules are grouped differently. When the arragonite is heated, the inertia of its atoms is overcome, the crystal explodes with force, and becomes a mass of crystals of calcareous spar. The expansive force of the heat suddenly overcoming the force of cohesion causes the explosion, and at the same time disturbs the unstable repose of the atoms, which immediately obey their natural attractions and assume the stable form of calcareous spar.
Dialysis is a method of separating and analysing substances by means of their diffusion in alcohol or water. If a wide-mouthed vial nearly full of a solution of common salt be placed in a jar of water, after a few days it will be found that the particles of salt have come out of the vial and have diffused themselves through the superincumbent water, even to its surface. Now Professor Graham, Master of the Mint, with whom this subject originated, made three arrangements precisely like that described; the three vials were exactly similar and equal, the three jars exactly the same in size and form, and contained the same quantity of water; but the first vial contained a solution of gum arabic, the second a solution of Epsom salt, and the third a solution of common salt. After fourteen days the diffusion of the gum had risen through one half of the superincumbent water, while the particles of both the salts had risen to the surface. However the common salt would have risen much higher, for when the strata of water at the two surfaces were drawn off by a siphon and evaporated to dryness, there was fifteen times as much common salt as Epsom salt. The three solutions are heavier than water, yet they rise notwithstanding their gravitation, whence Mr. Graham thinks that there is probably an attraction between the particles of the dissolved substances and those of the water. The force of molecular attraction is more powerful than gravitation, hence the particles must rise by the difference of the two forces.