478 Professor William Stirling [March 4, 



combination in the red part of the feathers of the Plantain-eater or 

 Turaco. 



The real aristocracy with genuine blue blood are the crabs, 

 lobsters, squids, and cuttle-fishes. 



Perhaps one of the most striking ways of dissociating this 

 accessory mechanism from the activity of the cell itself is by the use 

 of a poison. When a person is poisoned by coal gas, what happens ? 

 The coal gas contains carbon monoxide. This gas does not poison in- 

 vertebrate animals or plants. Still it kills vertebrate animals. Why ? 

 It does not kill by acting on the living cells, only by depriving them 

 of oxygen and asphyxiating them. It combines with the respiratory 

 pigment haemoglobin. Chloroform, ether, and similar drugs destroy 

 the actual life of the cell elements by destroying their irritability. 



As this year of grace marks the centenary of the death of Joseph 

 Priestley, I may be permitted to refer to his early discovery of the 

 action of green plants. 



In 1771, Priestley found that air vitiated by combustion of a 

 candle, or by the breathing of animals — such as mice — could be made 

 pure or respirable again by the action of green plants. 



Under certain conditions, however, Priestley found that plants gave 

 oif carbonic acid, and the air did not support combustion or animal 

 life. He regarded these as " bad experiments," and he selected what 

 he was pleased to regard as " good experiments," i.e. those in which 

 the air, rendered impure by the respiration of animals, was rendered 

 respirable by the action of green plants. 



In 1779 John Ingen-Housz published his " Experiments on Vege- 

 tables, discovering their great power of purifying the common air in 

 sunshine, and of injuring it in the shade and at night." 



He confirmed Priestley's observations that green plants thrive in 

 putrid air ; and that vegetables could convert air fouled by burning of 

 a candle, and restore it again to its former purity and fi.tness for sup- 

 porting flame, and for the respiration of animals — or, as he j^^ts it 

 " plants correct bad air." 



In 1787 Ingen-Housz, an English physician at the Austrian court, 

 found that only in daylight did green plants give oft' oxygen. In 

 darkness, or where there was little light, they behaved like animals so 

 far as exchange of gases is concerned, i.e. they used up oxygen and ex- 

 haled carbonic acid. He found also that all roots, when left out 

 of the ground, yielded by day and by night foul air, i.e. carbonic 

 acid. 



In the same year, 1804 — the year of Priestley's death — Nicolas 

 Theodore de Saussure, a Swiss naturalist and chemist, published liis 

 " Recherches Chimiques sur la Vegetation" (Paris, 1804), a veritable 

 encyclopaedia of experiments of the effects of air on flowers, fruits, 

 l)lants and vegetation generally, and on the efi"ects of these on atmo- 

 sj^heric acid. 



It is an old adage — the exception proves the rule. The exception 

 *'probes" the rule as the surgeon's probe probes a wound. The tactus 



