16 PRINCIPLES OF CHEMISTRY 



success, for they groped in the dark, making all kinds of mixtures and 

 experiments, without setting themselves clear and simple questions 

 whose answers would aid them to make further pm^ros. Thus they 

 did not form one exact law, but, nevertheless, they left numerous and 

 useful experimental data as an inheritance to chemistry ; they studied, 

 in particular, the transformations proper to metals, and for this reason 

 chemistry was for long afterwards entirely confined to the study of 

 metallic substances. 



In their researches, the alchemists frequently made use of two 

 chemical processes which are now termed 'reduction ' and 'oxidation/ 

 The rusting of metals, and in general their conversion from a metallic 

 into an earthy form, is called ' oxidation,' whilst the extraction of a 

 metal from an earthy substance is called * reduction.' A large number 

 of metals for instance, iron, lead, and tin are oxidised by heating in 

 air alone, and may be again reduced by heating with carbon. Such oxi- 

 dised metals are found in the earth, and form the majority of metallic 

 ores. The metals, such as tin, iron, and copper, may be extracted from 

 these ores by heating them together with carbon. All these processes 

 were well studied by the alchemists. It was afterwards shown that 

 all earths and minerals are formed of similar metallic rusts or oxides, 

 or of their combinations. Thus the alchemists knew of two forms of 

 chemical changes : the oxidation of metals and the reduction of the 

 oxides so formed into metals. The explanation of the nature of these 

 two classes of chemical phenomena was the means for the discovery of 

 the most important chemical laws. The first hypothesis on. their 

 nature is due to Becker, and more particularly to Stahl, a surgeon to 

 the King of Prussia. Stahl writes in his * Fundamenta Chymise,' 

 1723, that all substances consist of an imponderable fiery substance 

 called ' phlogiston ' (materia aut principium ignis 11011 ipse ignis) and of 

 another element having particular properties for each substance. The 

 greater the capacity of a body for oxidation, or the more combustible it 

 is, the richer it is in phlogiston. Carbon contains it in great abundance. 

 In oxidation or combustion phlogiston is emitted, and in reduction it 

 is consumed or enters into combination. Carbon reduces earthy sub- 

 stances because it is rich in phlogiston, and gives up a portion of its 

 phlogiston to the substance reduced. Thus Stahl supposed metals to 

 be compound substances consisting of phlogiston and an earthy sub- 

 stance or oxide. This hypothesis is distinguished for its very great 

 simplicity, and for this and other reasons it acquired many supporters. 24 



a4 It is true that Stahl was acquainted with a fact which directly disproved Ins 

 hypothesis. It was already known (from the experiments of Geber, and more especially 

 of Ray, in 1630) that metals increase in weight by oxidation, whilst, according to Stahl's 



