PRIESTLEY'S DISCOVERY OF OXYGEN GAS. 391 



Priestley's life ? It dissipates the remembrance of all his disputations 

 and all his errors, and shows us that beneath these there was a deeply- 

 pervading and redeeming faith. When his little grandchildren were 

 brought to his bedside to bid him good-night, he uttered his last 

 words : " I go to sleep like you, but we shall wake together, and I hope 

 to eternal happiness." 



To return from his life to his discoveries. Priestley soon found 

 that oxygen — I give it the name under which it has subsequently 

 passed — was absolutely essential, in all cases then known, to the sup- 

 port of flame and fire, and that animal life depended on it; that 

 a man, by breathing in a limited space, would soon exhaust it of 

 so much of this gas that suffocation would ensue ; that the atmos- 

 phere, in reality, is a reservoir of it, from which every thing possess- 

 ing the attributes of an animal abstracts it. It has been shown by 

 succeeding chemists, to such an extent does this abstraction go, that 

 a single man will each year consume about 800 pounds' weight. Con- 

 sidering, therefore, the enormous amount of animal life, the same re- 

 spiratory process being common to the minutest insect and the largest 

 quadruped, there must be a constant tendency to alter the constitu- 

 tion of the air, for, in proportion as we take from it oxygen at each 

 inspiration, we restore at each expiration an almost equivalent bulk 

 of carbonic acid — a double change, the removal of a vital element, and 

 the addition of a poisonous gas. 



But Priestley also showed that, in artificial atmospheres, such as 

 he made, animal life could not possibly be maintained if there were 

 any great reduction of oxygen, or any great increase of carbonic acid. 

 More recent experiments prove that the most striking physical and 

 moral eifects arise when men and animals are made to respire atmos- 

 pheres of a different constitution — efiects such as we witness in the 

 case of chloroform and sulphuric ether — a remarkable discovery, not, 

 as is commonly supposed, cf only a year or two back, but made by 

 Berzelius, who, twenty-four years ago, gave the most extraordinary, 

 and in a scientific point of view the most important, instance of the 

 kind yet produced — the instantaneous and deep sleep brought on by 

 the respiration of hydrogen ; a fact which, in the recent discussions 

 about the priority of that discovery, has been strangely forgotten. 

 From the efiect thus arising when the constitution of the medium we 

 breathe is in any degree disturbed, it necessarily follows that, ever 

 since animal life appeared on this earth, the composition of the air 

 must have been nearly unchanged. But here arises a great and obvi- 

 ous difiiculty. If the life of men and animals can only be conducted in 

 such a medium as our .atmosphere, and if such extensive changes as I 

 have described are constantly impressed on the air by those beings, 

 how does it come to pass that, after the lapse of a few years, it does 

 not gather a poisonous quality? There must be some agency at work, 

 continually tending to prevent that result. The consideration of what 



