MODERN SCIENCE. 173 



^ r 1660 1734. Stahl pointed out numerous chemical phe- 

 nomena, and showed the way in which many different 

 substances combine together to form new compounds. 

 Ignoring or disregarding Mayow's experiments, he formu- 

 lated the PHLOGISTIC THEORY of combustion, by which he 

 assumed that all combustible bodies contained a substance 

 (Phlogiston) which dispersed into air during the process of 

 burning. This theory explained some chemical facts so 

 plausibly that chemists accepted it at once whereas serious 

 verification would have upset it. The very fact that bodies 

 grow heavier by burning, as Geber had shown nine hundred 

 years before, should have confuted it, for if bodies lost 

 something (their phlogiston, so called) by burning they 

 would become lighter instead of heavier. As it was left 

 unverified, the phlogistic theory held the field for nearly 

 a century, and retarded the progress of chemistry all the 

 time. Even when Lavoisier exploded it, men like Priestley 

 adhered to it still. This baneful effect of a preconceived idea 

 was the last serious delusion which misled science. But the 

 lesson bore fruit : after that very few theories were accepted 

 without repeated verification. 



1728 99. Black (Joseph) found the causticity (burning 

 property) of quicklime (1756), and detected its loss of weight 

 after it was burnt. From this fact he was led to the discovery 

 of FIXED AIR (carbonic acid gas*), failing, however, to 

 discover that it was an acid. He was further led by a series 

 of clever experiments to see that a flame would not burn nor 

 animals live in it ; also that our breath contains fixed air ; at 

 the same time he showed air to be necessary for combustion, 

 thus corroborating Mayow's demonstration from end to end. 

 The discovery of fixed air was an important stride in 

 chemistry. Another capital service rendered to science by 

 Black was his discovery of LATENT HEAT the heat that 

 is hidden in water and steam (1760) the latent heat of water 

 at o (Cent.) being 79-25 C, and the heat of steam at 100 C. 

 being 537*2 C. ; so that we have to remember that water 



* In one million gallons of London air carbonic acid gas exists to the 

 extent of 380 gallons ; over one square mile in the country the carbonic 

 acid gas present weighs 371,875 tons. 



