Second, _Erroneous ideas regarding the phenomena of combustion_. While it was recognised that a burning candle was extinguished if placed in a confined space, its extinction was attributed not to the absence of air, but to the impossibility of the escape of flame. Indeed, flame was regarded as possessing the same semi-spiritual, semi-material nature as air. Together with earth and water, air and flame or fire formed the four elementary principles of the Ancients; and all substances--stones, metals, animals, and vegetables--were regarded as partaking of the properties of these elements, and often as being constituted of the latter in varying proportions, according as they were cold and dry (earth), cold and moist (water), hot and moist (air), or hot and dry (fire). It is not within the scope of this book to enter into details regarding such ancient views. Those who are interested in the matter will find them expounded in Kopps’ _History of Chemistry_, Rodwell’s _Dawn of Chemistry_, E. von Mayer’s _History of Chemistry_, and in other similar works. But we shall be obliged to consider the later developments of such ideas in the _phlogistic theory_, by means of which all chemical changes connected with combustion were interpreted from the latter part of the seventeenth to the end of the eighteenth century. With erroneous views regarding the nature of combustion, and ignorance as to the part played by the atmosphere in the phenomena of burning, the true nature of air was undiscoverable.