168. Fluorine.--F, Cl, Br, I, are called halogens or haloids, and exist in compounds--salts--in sea water. F is the most active of all elements, combining with every element except O. Until recently it has never been isolated, for as soon as set free from one compound it attacks the nearest substance, and seems to be as much averse to combining with itself, or to existing in the elementary state, as to uniting with O. It is supposed to be a gas, and, as is claimed, has lately been isolated by electrolysis from HF in a Pt U-tube. Fluorite (CaF2) and cryolite (Al2F6 + 6 NaF) are its two principal mineral sources. The enamel of the teeth contains F in composition. CHAPTER XXXIII. THE HALOGENS. 169. Halogens Compared.--The elements F, Cl, Br, I, form a natural group. Their properties, as well as those of their compounds, vary in a step-by-step way, as seen below. F is sometimes an exception. They are best remembered by comparing them with one another. Notice: 1. Similarity of name-ending. Each name ends in ine. 2. Similarity of origin. Salt water is the ultimate source of all, except F. 3. Similarity of valence. Each is usually a monad. 4. Similarity of preparation. Cl, Br, I, are obtained from their salts by means of MnO2 end H2SO4. 5. Variation in occurrence. Cl occurs in sea-salt, Br in sea- water, I in sea-weed. 6. Variation in color; F being colorless, Cl green, Br red, I violet. 7. Gradation in sp. gr.; F 19, Cl 35.5, Br 80, I 127. 8. Gradation in state, corresponding to sp. gr.; F being a light gas, Cl a heavy gas, Br a liquid, I a solid. 9. Corresponding gradation in their usual chemical activity; F being most active, then Cl, Br, and I. 10. Corresponding gradation in the strength of the H acids; the strongest being HF, the next, HCl, etc. 11. Corresponding gradation in the explosibility of their N compounds; the strongest NCl3, the next, NBr3, etc. 12. Corresponding gradation in the number of H and O acids; Cl 4, Br 3, I 2. 170. Compounds.--The following are some of the oxides, acids, and salts of the halogens. Name them. CI2O (+H2O=) 2 HClO. The salts are hypochlorites, as Ca(ClO)2. Cl2O3 (+H20=) 2 HClO2. The salts are chlorites, as KClO2. Cl2O4 -- HClO3 The salts are chlorates, as KClO3. -- HClO4 The salts are perchlorates, as KClO4, -- HBrO The salts are ? KBrO, -- -- The salts are wanting. -- HBrO3. The salts are ? KBrO3, -- HBrO4. The salts are ? KBrO4, -- -- The salts are wanting. -- -- The salts are wanting. I2O5 (+H2O=) 2 HIO3. The salts are ? KIO3. -- HIO4. The salts are ? KIO4.