Providentially, chlorine is never found free; but in a combined state it exists in enormous quantities in the salt of the ocean, in salt lakes, brine springs, and in extensive deposits of rock-salt, as well as in organic liquids. It has a strong affinity for hydrogen, and forms muriatic acid. A mixture of these two gases remains inactive in the dark, but explodes in sunshine. By chemical means chlorine is made to combine with oxygen so as to produce four substances, two of which are gases of such unstable equilibrium and weak affinity that the slightest cause makes them detonate violently; the other two are more stable, though they contain a greater quantity of oxygen. The only combination of chlorine with nitrogen is the most powerful and dangerous explosive compound known. Chlorine combines naturally with sulphur, and with the metals so as to form ores. Common salt affords a remarkable instance of change of volume by chemical combination. Twenty-four parts in bulk of salt contain 20·7 parts of sodium and 23·3 parts of liquid chlorine; hence by chemical combination a bulk of 44 is compressed into a bulk of 24, yet that great compression is consistent with perfect transparency, crystallized salt being perfectly transparent to light, and more so as regards radiant heat than any other substance. Thus chemical affinity does what no mechanical power could accomplish. At an ordinary temperature and barometric pressure, bromine is an orange red, extremely volatile fluid, which congeals and becomes brittle at a temperature a little below the zero of Fahrenheit’s thermometer, and if combined with water at that degree of cold it crystallizes in octohedral crystals which are permanent even at 50° Fahr. Bromine is very poisonous, corrodes the skin, has a disagreeable taste, and a smell similar to that of chlorine, but more pungent and hurtful. It possesses a powerful bleaching property, does not conduct electricity, and like chlorine a taper will not burn in its gas, though it spontaneously sets fire to phosphorus, and some of the metals. Reasoning from analogy Professor Schönbein believes that chlorine and bromine are not simple substances; he considers them to be ozonides analogous to the peroxides of manganese, lead, &c. He believes chlorine to be the peroxide of murium, and bromine to be the peroxide of bromium. Professor Tyndall’s experiments on the absorption and radiation of gases show that the action of these two substances is very different from that of the simple gases.