Two simple substances are only capable of a certain number of chemical combinations, which form a regular series of new substances; as for example oxygen and nitrogen. Two measures of nitrogen gas will unite with one measure of oxygen to form the protoxide of nitrogen; with two measures of oxygen it unites to form the binoxide of nitrogen; with three measures of oxygen it forms the hyponitrous acid; with four it forms nitrous oxide; and with five measures of oxygen it forms nitric acid. Thus there are five compounds of nitrogen and oxygen, no more. Affinity of _kind_ is merely the attraction of one element or atom of matter for another; affinity of _degree_ consists in the grades and limits of combination; the preceding series is of the fifth degree; the limit is the last term, for no further combination of these two gases can take place, and these are accomplished by art. All the five substances are deleterious, most of them deadly poisons, for the protoxide of nitrogen, which is the laughing gas, could not be long inhaled with impunity. For a long time the middle term of the preceding series was wanting, but Gay-Lussac formed it by attending to the laws of definite proportion and sequence. The atoms of different kinds of matter possess an affinity, or attractive force, which binds them together chemically in different and very unequal degrees. Two substances may unite and form a third differing from both, as water does from oxygen and hydrogen; but if a new substance be added which has a greater attraction for one of the substances than for the other, it will dissolve their union, combine with that for which it has the strongest attraction, and set the other free. Thus the metal potassium, which has a greater attraction for oxygen than it has for hydrogen, decomposes water, combines with the oxygen, and sets the hydrogen free. Both chlorine and ozone have the property of liberating the iodine in a weak solution of the iodide of potassium; the liquid stains starch blue, a proof of the free iodine. The facility with which acids and alkalies combine affords the means of eliminating either the one or the other from a compound so as to liberate what remains. The constituents of compound substances may be separated from one another by a variety of means depending upon their greater or less fusibility, volatility, and other properties. Water, acids, alcohols and other liquids hot or cold, different degrees of temperature, sublimation, solution, distillation, evaporation, together with static and voltaic electricity, are the most powerful means of analysis.