The fundamental notion of this theory was, as we have mentioned, a development from the combustible and heat-giving sulphur of Paracelsus to the notion of a heat substance, phlogiston, which constituted a part of all combustible or, as we should say, oxidizable substances. The phenomena of combustion or oxidation were in terms of this theory due to a loss of phlogiston—the phenomena of reduction to a gain of phlogiston. It is just to say of this theory that it proved a fertile and valuable hypothesis to the science of chemistry in developing a vast amount of excellent experimental work and of comprehensive generalizations. We have only to recall the names of Scheele, Priestley, Marggraf, Black and Cavendish to realize the class of chemists whose labors were influenced and stimulated by the adoption of this theory.
Two serious obstacles to continuous progress were, however, inherent in this theory. The supposed phlogiston could not be separated or isolated and weighed. It could not be known whether it had a positive weight in combination, or whether it could affect in any definite or determinable way the weight of other substances. It might even have the effect of buoyancy or of diminishing the weight of substances with which it was combined, and so long as such ideas were held the weights as given by the balance could not be depended upon to give the real quantitative relations of chemical reactions.
The second obstacle this theory offered to chemical development lay in the fact that so long as this theory was maintained no identification of substances as elements was possible. Boyle had given us a proper definition of an element, but so long as such oxidizable substances as phosphorus, sulphur, iron, zinc or carbon were considered as combinations of phlogiston with other substances (viz., their oxides) and so long as the products of combustion, such as we now know, as the oxides of phosphorus, sulphur, iron, etc., were considered as products of the loss of phlogiston, and therefore to that extent simpler or more nearly elementary than the combustibles from which they were produced, it is manifest that the elementary character of most of the known elements could not have been recognized. It required the insight of Lavoisier to discern the real nature of combustion and reduction, and to banish at last the element phlogiston from the weighable factors of chemical reactions.
But with this period of chemistry, the dawn of modern chemistry was past and the sun was shining brightly above the horizon.
