394 THE POPULAR SCIENCE MONTHLY. 



perhaps the commonest of all chemical changes, the question became 

 narrowed, and chemists eagerly sought for an answer to the query, 

 " What happens when a chemical substance burns ? " 



In those days natural phenomena were referred to the presence of 

 " principles " or " essences " in the matter exhibiting the phenomena. 

 A new principle was added to the list ; and the question was sup- 

 posed to be solved by saying that combustible substances are char- 

 acterized by richness in phlogiston (Gr. phlogizo = burn, or inflame), 

 and that when they burn they lose this principle, so that the burned 

 substance, or calx, consists of the original substance minus phlogis- 

 ton. 



The phlogisteans seem to have regarded their hypothetical prin- 

 ciple as a modified form of fire, as fire confined in a material substance ; 

 but, as they gave no definition of fire, beyond saying it was one of 

 the four elements, it was scarcely to be expected that they should 

 define phlogiston. By restoring phlogiston to the burned substance, 

 said the theory, the original matter is regenerated. Some substances, 

 e. g., charcoal, are especially rich in phlogiston, and metallic calces 

 may be converted into metals, i. e. , may be unburned, by heating them 

 with charcoal. Thus the phlogisteans regarded the phenomena which 

 they studied in a purely qualitative manner : they asked only, What 

 does this or that substance do under given conditions? not being 

 aware that a full answer to this question can only be given when the 

 other question How much of some given effect is produced by a 

 given quantity of this body under stated conditions? had been an- 

 swered. 



The introduction and use of the balance carried the day in favor 

 of those who opposed phlogistic views. If a substance loses some- 

 thing when it burns, it must weigh less than before burning as a fact 

 it weighs more therefore it has not lost but gained something. 



" Nay," replied the phlogistean, " it has lost something, but the 

 weight of this something can only be expressed by a negative quantity." 



" But a something with such properties is an absurdity," replied 

 the opponent ; " therefore it has no existence, and therefore your theory 

 is utterly false." 



The anti-phlogistean triumphed, and the principle of levity was 

 banished from chemical science. But the principle returned in a 

 modified form. Lavoisier, who opposed the Beccheriah theory of 

 phlogiston with signal success, himself propounded a theory of the 

 constitution of solids, liquids, and gases, in which the " subtle prin- 

 ciple " " caloric " played an important part. Lavoisier regarded oxy- 

 gen as what he termed " concrete oxygen " plus a something caloric ; 

 indeed, he appears to have looked on all substances in the concrete 

 state as solids, and to have supposed that the addition of a certain 

 quantity of caloric to these caused them to become liquids, while the 

 addition of a further quantity of caloric produced gases. 



