combustion unconscious of the fact and meaning of the relationship in weight subsisting between the consuming and the consumed body, but they were altogether ignorant of the part played by the air in the phenomena which they so boldly and successfully attempted to explain. Torricelli's invention of the barometer, and Guericke's invention of the air-pump, were both, indeed, made during Beccher's early boyhood; but years had to elapse before the consequent idea of-the materiality of air could be domiciled, as it were, in human understandings. And not until more than a century after Torricelli's discovery of the weight of air—not, indeed, until the time of the great pneumatic chemists, Black, Cavendish, Priestley, and Scheele—was it ever imagined that the aërial state, like the solid or liquid state, was a state common to many distinct kinds of matter; and that the weight or substance of a rigid solid might be largely contributed to by the weight or substance of some constituent having its independent existence in the aerial or gaseous form. The notion that 100 pounds of smithy-scales might consist of 73 pounds of iron and 27 pounds of a particular kind of air, and that 100 pounds of marble might consist of 56 pounds of lime and 44 pounds of another kind of air, was a notion utterly foreign to the elder philosophy. Air, it was allowed, might be rendered mephitic by one kind of contamination, and sulphurous by another, and inflammable by a third; it might even be absorbed in, and so add to the weight of, a porous solid, as water is absorbable by sand; but still air was ever indisputably air, essentially alike and unalterable in its mechanical and chemical oneness. This familiar conception had to be overcome, and the utterly strange notion of the largely aërial constitution of solid matter to be established in its stead, by the early pneumatic chemists, Black, Cavendish, and Bergmann, before the deficiencies rather than positive errors of the phlogistic theory could be perceived.
But long ere the foundation of modern chemistry had thus been laid, in 1756, by Black's discovery of fixed air or carbonic acid as a constituent of mild alkalies and limestone, those old German doctors, Beccher and Stahl, though ignorant of the nature of air, and neglectful of the import of gravity, had yet found something to say about the chemistry of combustion worthy of being defended a century afterward by men like Priestley and Cavendish—worthy, it is believed, of being recognized nearly two centuries afterward as the expression of a fundamental doctrine in chemical and cosmical philosophy. They pointed out, for example, that the different and seemingly unlike processes of burning, smouldering, calcining, rusting, and decaying, by which combustible is changed into incombustible matter, have a community of character; that combustible bodies possess in common a power or energy capable of being elicited and used, whereas incombustible bodies are devoid of any such energy or power; and, lastly, that the energy pertaining to combustible bodies is the same in all of
