_Nitrogen_ is a very important element, entering into many chemical combinations. It forms the basis of explosives, used in war. Until relatively recently, this element had to be obtained from substances dug out of the ground, but during the late war, methods were devised for obtaining it from the air. “Nitrogen fixation” became possible. If it had not been for this discovery, Germany would have had to give up the war in 1916, at the latest. Nitrogen combines with hydrogen, to form ammonia; with oxygen and water, to form nitric acid; with nitric acid and potash to form gunpowder, etc. Certain oxides combine with water, to form what are known as _bases_. Bases can combine with acids, giving rise to _salts_. _Carbon_ is an essential element for all living matter; it combines with oxygen, to yield carbonic acid; with hydrogen, giving rise to a great number of compounds, such as benzene, turpentine, etc. Marsh gas, illuminating gas, acetylene, etc., are compounds of carbon. _Chlorine_ is a very important element, combining with sodium to form common salt. As we have seen, the saltness of the sea is due to this substance. Owing to its great affinity for hydrogen, chlorine decomposes water, setting free oxygen. The result of this is that a mixture of chlorine and water has strong bleaching qualities. Chlorine also combines with hydrogen to form hydrochloric acid. On the other hand, it shows little sympathy for oxygen, forming but few stable compounds. Chloroform, so long useful in surgical anaesthesia, is a compound of chlorine, carbon and hydrogen. _Sulphur_ can assume a variety of appearances (allotrophic variation) as we have seen. Sulphuric acid, etc., are its compounds. The latter substance is used for bleaching violets, but the flowers become violet again when put into an ammonia solution. _Sodium_ is a metal, which burns when thrown into water. It is the other constituent of common salt, and enters into a great variety of combinations. Carbonate of soda is one of these. Many of the elements--iron, nickel, gold, platinum, silver, etc.,--are too well known to necessitate more than a brief note. It is interesting to notice, however, that there are certain “family relations” among a number of the elements. Thus, sodium and potassium are “related”; and so are barium, strontium and calcium. Again, oxygen and sulphur have a number of points in common,--although one is a solid and the other a gas! Gold stands rather apart from the rest. Two very interesting groups should be mentioned in this place. The first is the group of _rare gases_--argon, neon, etc.,--most of which have been discovered only recently. They are inert, and partly on account of this, and partly on account of their rarity, their discovery was so long delayed. RADIO-ACTIVITY