18 Mr. G. F. Rodwell on the Theory of Phlogiston. 



Hooke's theory of combustion was published nearly five years 

 before the publication of Becher's Physica Subterranea (which 

 laid the foundation of the theory of phlogiston). At this time 

 Stahl was four years old ; yet M. Chevreul asserts, " On doit a 

 St ahl la premiere explication de la combustion"*; and still more 

 recently Sir B. Brodie tells us that the theory of phlogiston was 

 the first chemical theory j\ While this idolum specus remains 

 in the mind of the scientific, I despair of a just recognition of the 

 services of Hooke — of a just appreciation of one of the most 

 sound and comprehensive theories which has appeared in the 

 whole history of scientific thought. 



IV. Of the ideas regarding the calcination of metals which prevailed 

 prior to the rise of the theory of Phlogiston. 



We have finally, before considering the rise of the theory of 

 phlogiston, to glance at the views which prevailed before and 

 during the time of Becher in regard to the calcination of metals. 



The process of calcination has always been one of the most 

 important operations in chemistry, and is prominently described 

 in all old chemical treatises. The term calx, originally applied 

 to lime, soon came to have a wider meaning, and it was found 

 a convenient term to express anything that was burnt to powder. 

 Geber defines calcination as "the pulverization of a thing by 

 fire by the deprivation of the humidity consolidating the parts" J; 

 and this broad definition was repeated almost verbatim in che- 

 mical works for at least eight centuries after the time of Geber. 

 The ash of wood was its calx ; the oxide of a metal was its calx ; 

 the ignited residue of a substance dissolved in acid was its calx, 

 as also was the product produced by fusing it with sulphur. 

 Expressed according to the salt, sulphur, and mercury theory, 

 calcination is the expulsion of the combustible and volatile prin- 

 ciple of the body calcined, and the resulting ash is its salt or 

 principle of solidity. This last product varied considerably ; for 

 while organic matter contains much mercury and sulphur and 

 but little salt, the metals, as a rule, contain much fixed prin- 

 ciple, and but little principle of combustibility or of volatility. 

 Such metals as refused calcination by the ordinary means of 

 heating in air were calcined by the addition of nitre or sulphur ; 



* " Note historique sur les manieres diverses dont Fair a ete envisage 

 dans ses relations avec la composition des corps/' Comptes Rendus for 

 December 12, 1864. 



t Vide his discourse delivered before the Chemical Society on June 6, 

 1867, " On the mode of Representation afforded by the Chemical Calculus, 

 as contrasted with the Atomic Theory. " 



J " Est ergo calcinatio, rei per ignem pulverifatio, ex privatione humi- 

 ditatis partes consolidantis." — De Alchimia libritres. 



