76 ESSAYS BIOGRAPHICAL AND CHEMICAL 



The portrait of Dr. Cullen, Black's predecessor both 

 in Glasgow and Edinburgh, and his life-long friend, 

 is also given by Kay. Cullen died in 1790, at the age 

 of eighty-one. 



In the olden days it was considered quite as marvellous 

 that a gas could be made to occupy a small volume, or 

 that ' air ' could be produced in quantity from a stone, as 

 that an Arabian 'djinn' of enormous size and ferocious 

 mien could issue from a bottle, as related in the ' Tale of a 

 Fisherman,' one of the charming stories of the Arabian 

 Nights Entertainments. It is true that in the middle of 

 the seventeenth century Robert Boyle had enunciated his 

 famous discovery, ' Touching the Spring of the Air ' ; in 

 which he proved that the greater the pressure to which a 

 gas is exposed the smaller the volume it will occupy. 

 But however great the pressure, Boyle's air remained air. 

 It might have been thought that the boiling of water into 

 steam should have convinced men that a liquid, at least, 

 could be changed into a gas; but the fact that steam 

 changed back to water probably prevented attention being 

 paid to its comparative large volume while hot. It was 

 Black's discovery of the production of carbonic acid gas 

 from marble, or as he named it, 'fixed air,' which first 

 directed notice to this possibility of the production of a 

 gas from a solid; and further, the peculiar property of 

 this gas its power of being fixed was one which com- 

 pletely differentiated it from ordinary air. Stephen Hales 

 the botanist, it is true, had distilled many substances of 

 vegetable, animal, and mineral origin; among them he 

 treated many which must have produced impure hydrogen, 

 marsh-gas, carbonic acid gas, and oxygen; but Hales 

 contented himself with measuring the volume of gases 

 obtained from a known weight of material, without con- 

 cerning himself about their properties. And as the result 

 of many experiments, he concluded that ' our atmosphere 



