78 PERIOD III. 



burned out, and after ten days found that a candle would 

 now burn perfectly well in the same air. Air kept with- 

 out a plant, in a glass vessel immersed in water, did not 

 regain its power of supporting combustion. Balm, 

 groundsel, and spinach were found to answer just as 

 well as mint. Air vitiated by the respiration of mice 

 was restored by green plants as readily as air which 

 had been vitiated by combustion. 



Priestley did not remark that the glass vessels 

 employed in his experiments had been set in a window, 

 and inattention to this point caused some of his 

 attempts to repeat the experiment to fail. He was 

 further perplexed by using vessels which had become 

 coated with a film of "green matter," probably 

 Euglaena. Such vessels restored vitiated air, though 

 no leaves were present, and when placed in the sun, 

 gave off considerable quantities of a gas, Priestley's 

 " dephlogisticated air" (oxygen). Hardly any oxygen 

 was given off when the green matter was screened by 

 brown paper. Water impregnated with carbonic acid 

 was found to favour the production of the green matter. 

 To us, who have been taught at school something about 

 the properties of green plant-tissues, it seems obvious 

 that Priestley ought to have ascertained by microscopic 

 examination whether his " green matter " was not a 

 living plant. But he had always avoided the use of 

 the microscope, his eyes being weak, and after some 

 imperfect attempts in this way he made up his mind 

 that the green matter was neither animal nor vegetable, 

 but a thing sui generis. Neglecting his most instructive 

 experiments, and not waiting till he could devise new 

 ones, or even disentangle his thoughts, he sent to the 

 press a confused explanation, which seemed to teach 

 that vitiated air may be restored by sunlight alone. 



