57 
Much progress was made during the period of medical 
chemistry ; clearer ideas were obtained of chemical com- 
pounds and chemical affinity. 
For some time chemistry remained under the influence of 
the medical school. From the time of Robert Boyle, however, 
the aim of chemistry gradually changed, and the discovery 
of new facts purely for the sake of arriving at the truth became 
the chief aim of the science. Boyle (1626-1696) rendered 
undying service to chemistry. He taught that the aim of 
chemists was to set themselves diligently to make experiments 
and to collect observations and thus work out the funda- 
mental principles of chemistry ; that it begins with an ordered 
knowledge of fact, makes deductions from this and from the 
deductions again designs new experiments. With methods such 
as these chemistry can now claim to be admitted amongst 
the exact sciences. No one before Boyle had treated so 
successfully the main problem of chemistry, namely the 
investigation of the composition of substances, and his work 
in this direction gave a great impetus to analytical chemistry. 
For about a hundred years after Boyle’s time all the 
eminent chemists gave their attention to the question of 
combustion and the calcination of the metals. John Mayo, 
a practising physician in the middle of the seventeenth century, 
stated that atmospheric air contained a substance which 
combined with metals, sustained respiration and converted 
veinous blood into arterial. He died early, otherwise 
the theory of combustion would have been satisfactorily 
explained long before it actually was. The increased weight 
resulting from combustion was recognised, but the explana- 
tions given to it by the chemists were ridiculously short of 
the truth. The chief theory was that all combustible sub- 
stances contained a “ fire material,” or a “ phlogiston.” 
Stahl (1660-1734) formulated the theory that this phlogiston 
was a constituent of all combustible bodies and that in the 
process of combustion it escaped. No attempt, however, 
to isolate this phlogiston was made by the chemists of the 
time. Phlogiston was said to have a negative weight, i.e., 
it was lighter than nothing ; hence the increase in weight on 
combustion. 
Joseph Black (1728-1799) professor of chemistry in the 
Universities of Glasgow and Edinburgh, and Henry Cavendish, 
both great chemists, were believers in this phlogiston theory. 
Cavendish discovered hydrogen as a gas and regarded it as 
the phlogiston. Even Joseph Priestley, who in 1774 discovered 
oxygen, remained a firm disciple of the phlogiston theory. 
