Priestley's life? It dissipates the remembrance of all his disputations and all his errors, and shows its 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 support 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 atmosphere, in reality, is a reservoir of it, from which every thing possessing 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. Considering, therefore, the enormous amount of animal life, the same respiratory process being common to the minutest insect and the largest quadruped, there must be a constant tendency to alter the constitution 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 effects arise when men and animals are made to respire atmospheres of a different constitution—effects such as we witness in the case of chloroform and sulphuric ether—a remarkable discovery, not, as is commonly supposed, of 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 effect 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 obvious difficulty. 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
