THE DAWN OF MODERN CHEMISTRY 21 



The fundamental notion of this theory was, as we have mentioned, a 

 development from the combustible and heat-giving sulphur of Para- 

 celsus 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 generali- 

 zations. We have only to recall the names of Scheele, Priestley, Marg- 

 graf, 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, inher- 

 ent in this theory. The supposed phlogiston could not be separated 

 or isolated and weighed. It could not be known whether it had a posi- 

 tive 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 

 witli 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 combi- 

 nations 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. 



