FLUORINE.

61. Properties of Fluorine. This element occurs combined with the metal calcium, as calcic fluoride (fluoride of calcium) or fluor-spar. It is remarkable as forming no compound with oxygen, and as being extremely difficult to prepare in a pure state. Many unsuccessful attempts have been made to obtain fluorine; but, by the action of dry iodine upon dry argentic fluoride (fluoride of silver), it appears to have been isolated; and it is found to be a colorless gas.

62. Hydric Fluoride, or Hydrofluoric Acid.-When calcic fluoride (fluor spar) is acted upon by sulphuric acid, a very corrosive gas is obtained, called hydric fluoride (hydrofluoric acid). The reaction is shown in the following equation:

H2SO4 + CaF2 = CaSO, + 2HF.

The calcium and the hydrogen change places, forming calcic sulphate (sulphate of lime), CaSO4, and hydric fluoride.

This acid must be prepared in vessels of lead or platinum, since it rapidly attacks glass. It acts violently upon the skin, producing painful sores, and the fumes are very poisonous to the lungs. It is used for etching on glass. The glass is first covered with wax, through which the design is traced with a sharp point. The glass is then exposed to the gas, which corrodes it where the wax has been removed. The solution of the gas in water is also used for the same purpose.

63. The Chlorine Group.-The four elements just described form a well-marked group. They are especially remarkable for their gradation of properties. Thus chlorine is a gas, bromine a liquid, and iodine a solid, at the ordinary temperature. Chlorine may be easily con

densed into a liquid, and the specific gravity of liquid chlorine is 1.33, of bromine 2.97, and of iodine 4.95. Liquid chlorine is transparent, bromine slightly so, and iodine opaque. The atomic weight and density of bromine are nearly the mean of those of chlorine and iodine, 35.5+127 81.25; and in its general chemical deportment bromine stands half-way between the other two elements.

2

These elements all form powerful acids with hydrogen, and have a strong affinity for the metals. They all have a very weak affinity for oxygen.

PHOSPHORUS.

64. Properties of Phosphorus. - This element does not occur free in nature, but is found combined with oxygen and calcium in large quantities in the bodies, especially the bones, of animals, and in the seeds of plants. Animals obtain their phosphorus from plants, plants from the soil, and the soil from the slow disintegration of the oldest rocks. Phosphorus was accidentally discovered by Brande, in 1669; but Scheele, in 1769, pointed out its existence in the bones, and carefully examined its properties. It is prepared from bones, and is used chiefly in the manufacture of lucifer matches.

Phosphorus is a yellowish, semi-transparent solid, resembling wax. Its specific gravity is 1.83. It melts at III°, and boils at 550°. In the air it gives off white fumes, and emits a pale light in the dark, whence its name, which means light-bearer. While emitting this light, it appears to be undergoing a slow burning. At a temperature a little above its melting-point, phosphorus takes fire in the air, and burns with great energy. It

may be set on fire by slight friction, by a blow, and even by the heat of the hand. Hence it should be handled with great care, and always be cut under water. Phosphorus has several well-marked allotropic states, the most important of which is that known as red phosphorus. This is much less inflammable than ordinary phosphorus, and is not poisonous.

65. Compounds of Phosphorus. — Phosphorus combines with oxygen, to form several acids, the most important of which are known as hypophosphorous acid and phosphoric acid. From the former are derived salts (hypophosphites), used in medicine; while the latter furnishes a large class of salts (phosphates and superphosphates), which are valuable to the farmer as fertilizers. Phosphorus unites with hydrogen, to form hydric phosphide (phosphuretted hydrogen), H.P, a poisonous gas, with a very offensive odor. When this gas is prepared by the action of caustic potash on phosphorus, each bubble of the gas takes fire as it comes into contact with the air, forming beautiful rings of phosphoric anhydride (phosphoric acid), which expand as they rise. This spontaneous combustion is owing to the presence of a small quantity of a liquid compound of phosphorus and hydrogen, whose symbol is H2P. Pure phosphuretted hydrogen does not take fire in this way.

ARSENIC.

66. Properties of Arsenic.-Arsenic closely resembles phosphorus and nitrogen in its chemical properties, and in those of its compounds, and is therefore to be counted as a non-metallic element. In physical features, such as specific gravity and lustre, it bears a greater resemblance to the metals, with which it was formerly classed. (See, in Appendix, Chemical Philosophy, § 19.)

Arsenic is sometimes found in a free state, but more often combined, chiefly with iron, nickel, cobalt, and sulphur. One of the compounds of this element with oxygen, As,O,, is the well-known poison, white arsenic.

67. The Nitrogen Group.-Nitrogen, phosphorus, and arsenic form a well-marked group. They all form similar compounds with oxygen and with hydrogen. Thus we have H,N (ammonia), H.P (phosphuretted hydrogen), and HAs (hydric arsenide); N,O, (nitrous anhydride), N2O, (nitric anhydride), P2O, (phosphorous anhydride), PO, (phosphoric anhydride), and As,Og (arsenious anhydride), As,O, (arsenic anhydride).

BORON.

68. Properties of Boron. Boron is found in nature combined with oxygen as boracic acid, and with oxygen and sodium as borax. In certain volcanic districts in Tuscany, jets of steam and gas are continually escaping from the earth. These steam-jets contain small quantities of boracic acid, which collects in artificial basins formed for the purpose at the mouth of the jet. By means of the heat of natural steam-jets, this solution of boracic acid is concentrated and crystallized. About 2,000 tons of crude acid are thus prepared and exported from Tuscany every year.

Boron is remarkable as combining directly with nitrogen at a high temperature.

SILICON.

69. Properties of Silicon.-Next to oxygen, silicon is the most abundant element known. It does not, however, occur in a free state, but always combined with oxygen (for which it has a very strong affinity) as silica.

Silica is found nearly pure in quartz, flint, sand, and in several minerals, and combined with metals in almost all known rocks. Its symbol is SiO,.

Silicon can be obtained in three modifications, corresponding to the three states of carbon.

70. The Carbon Group.-Carbon and silicon, then, form a well-marked group, resembling each other closely in their allotropic states.

SUMMARY.

Chemistry divides substances into two classes:

Ist, Elements, or substances which cannot be decomposed by any known process. Of these only 65 are known.

2d, Compounds, or substances made up of elements. The force which causes elements to combine, and holds them in combination, is called Affinity.

Affinity changes the properties of the substances which it combines.

Affinity always causes substances to combine in fixed and definite quantities. This law of combination is called the Law of Definite Proportions.

A given element combines with some elements in preference to others.

Matter is made up of molecules; and molecules are made up of atoms.

The compounds of nitrogen and oxygen show that the same elements may combine in more than one proportion, and that in such cases the proportions of the elements in the compounds are always multiples of the atomic weights of the elements. This law of combination is known as the Law of Multiple Proportions.

The symbol of an element is usually the first letter of its name, a second letter being added to distinguish