the elements. Because of this relationship chemists are tending to the belief that all the elements are modifications of some yet simpler forms or form of matter. As to the character of this fundamental substance, nothing is known at present. A recent irregularity in the periodic arrangement results from the fact that the atomic weight of tellurium has been placed at 127.5, or slightly higher than iodine, 126.9, and that argon (39.88) comes before potassium (39.10). This is not startling, however, for the periodic recurrence of properties is only approximate, at the best.

386. Exercises.

1. The specific gravities of some elements are as follows: P (red), 2.14; Cl (liquid), 1.33; Cd, 8.6; In, 7.4; Te, 6.2. Using these figures and those in Appendix v, see if there is a relation between specific gravity and atomic weight in Series 3 (begin with Na), and in Series 7.

2. Three elements that appear upon the list of International Atomic Weights, but are not in the periodic table, are dysprosium (Dy=162.5), europium (Eu=152), and lutecium (Lu=174). Are there places for them? Is the existence of a blank in the proper place the only condition necessary for placing elements in particular groups?

3. If dysprosium belongs where its atomic weight places it, what is the formula of its oxide, chloride, and nitrate? If europium belongs in a blank, according to its atomic weight, what will be the formulas of its hydroxide, carbonate, sulphide, sulphate, and nitrate? Answer the same questions for lutecium.

4. What two kinds of elements might fill the blank in Series 6, Group VII? What atomic weight belongs in this space?

CHAPTER XXVIII.

SILICON AND BORON.

A. Silicon.

387. Occurrence of Silicon. As was stated in § 9, silicon is, next to oxygen, the most abundant constituent of the earth's crust. It is not found free, but in the form of its oxide, SiO2, and in silicates, i. e., the salts of silicic acid. Silicon dioxide (silica) and the silicates make up sand, clay, and almost all the crystalline rocks of the earth's crust.

Silica is taken up by plants. The straw and husks of the grains contain it. The equisetum is called "scouring-rush," from the large amount of silica present in it. Silica is found, also, in the skin, nails, hair, etc., of animals.

Certain microscopic plants, the diatoms, have skeletons of silica; and these have accumulated in large deposits in some places.

388. Preparation. - Silicon, like carbon, exists in several allotropic forms. The amorphous variety may be obtained by heating a mixture of powdered quartz and powdered magnesium.

2 Mg+SiO2 MgO+Si.

Amorphous silicon is a brown powder which burns, when heated in the air, to silicon dioxide. It is attacked by hydrofluoric acid, but not by other acids.

SILICON COMPOUNDS.

Si+2 H2F2

SiF4+2 H2.

357

Silicon is obtained crystalline by heating sodium fluosilicate, Na2SiF6, with sodium and zinc.

Na SiFe+4 Na-6 NaF+Si.

The silicon dissolves in the melted zinc, and separates out in crystals as thę zinc cools. When the zinc solidifies it encloses the silicon. The zinc is then removed by treating the mass with hydrochloric acid; and silicon remains.

The crystalline form of silicon does not burn in air or oxygen, nor does it dissolve in acids. It dissolves in hot sodium hydroxide solution, giving sodium silicate and hydrogen. The simplest equation is,

Silicon is made on a large scale, at Niagara, by heating quartz sand with coke in the electric furnace. It is used in making steel (cf. § 488).

389. Silicon Compounds.

The general structure of silicon compounds is like that of the corresponding carbon compounds. Hydrogen silicide, SiH4, corresponds to marsh gas, CH4; silicon tetrachloride, SiCl4, to carbon tetrachloride, CC; silicon dioxide, SiO2, to carbon dioxide, CO2; silicic chloroform, SiHCl3, to chloroform, CHCl3.

Hydrogen silicide is a colorless gas. It may be obtained, mixed with hydrogen, by treating magnesium silicide with dilute hydrochloric acid.

Mg2Si+4 HCI- SiH4+MgCl2.

air.

As ordinarily made, it ignites spontaneously in the

SiH4+2 02 →→→→SiO2+2 H2O.

Magnesium silicide is prepared by heating powdered quartz with an excess of magnesium powder.

4 Mg+SiO2 Mg Si+2 MgO.

Mg2Si+2

If too little magnesium is used, amorphous silicon results (cf. § 388).

Silicon tetrachloride is a colorless liquid, boiling at 59° C. It is formed from silicon and chlorine. Water decomposes it, giving silicic acid and hydrochloric acid.

SiCl4+4 H2O →→→Si(OH)4+4 HCl.

(Cf. the action of phosphorus trichloride with water, § 354.)

Silicon tetrafluoride is a colorless gas, formed when silicon dioxide is treated with hydrofluoric acid.

is.

SiO2+2 H2F2 →SiF4+2 H2O.

Silicon tetrafluoride is decomposed by water as the tetrachloride

SiF4+4 H2O-Si(OH),+2 H2F2.

Instead of being set free, however, the hydrofluoric acid unites with some of the silicon tetrafluoride, forming fluosilicic acid, H2SiF6. The name fluosilicic acid means silicic acid, H2SiO3, with its oxygen replaced by fluorine, - three bivalent oxygen atoms by six univalent fluorine atoms (cf. § 365, sulpharsenites, and § 268, thiosulphates). Many fluosilicates are known; potassium fluosilicate, KSiF, is one of the few difficultly soluble potassium salts.