174 PROGRESS OF SCIENCE. 



is a solid (ice) plus heat, because the heat used to melt ice 

 is absorbed in the water ; whilst a gas is a liquid (say water) 

 plus heat, because the heat used to turn water at boiling point 

 into gas (steam) is absorbed in the steam. By this discovery 

 Black found a law which together with the known law of 

 elasticity of gases (Boyle's law) enabled James Watt, under 

 the guidance of Black himself, to invent practical steam 

 machinery the new power of the modern world. Black 

 demonstrated also that certain chemical substances are 

 possessed of definite and invariable composition and pro- 

 perties thereby PARTLY establishing THE LAW (which 

 Lavoisier defined) of the FIXITY OF COMPOSITION of chemical 

 compounds. Black's " Lectures on the Elements of 

 Chemistry " contain a large amount of practical knowledge 

 his researches on alkalis and fixed air (carbonic acid gas) are 

 excellent examples of the scientific process and manipulation. 

 It may be mentioned in honour to him that he was one of the 

 first to abandon the phlogistic theory after Lavoisier had 

 shown its imaginary character. 



1731 1810. Cavendish found the accurate composition 

 of nitric acid, discovered HYDROGEN (water producer, 1766), 

 which he called INFLAMMABLE AIR an effect of which was 

 to make aerostation safer ; formed WATER by sending an 

 electric spark through a mixture of oxygen and hydrogen 

 (1784), which showed water to be a compound of these two 

 g ases another important discovery, also made by Lavoisier. 

 Cavendish invented the EUDIOMETER, for testing the purity 

 of air by ascertaining the quantity of oxygen it contains 

 now used for examining the composition of gases ; he 

 calculated the weight of the earth by means of the Torsion 

 Balance a wonderful determination.* Cavendish has been 

 called, prematurely we think, the Newton of Chemistry a 

 title better deserved by Dalton, who came later and made 

 a discovery which is absolutely fundamental. In the absence 

 of Dalton, Lavoisier would no doubt deserve the title of 

 honour. 



1733 1804. Priestley discovered f oxygen (1774) from 



* Modern computation gives the round number of six thousand 

 trillion tons. t Discovered also laughing gas. 



