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 In 1774 Priestley discovered ammonia gas too, which he gave the name of alkaline air; and proved that mixed gases do not separate on standing, but remain uniformly distributed throughout the mass (diffusion of gases).

By passing electric sparks through ammonia gas, he observed that an inflammable gas was formed; and that phlogisticated air (the mephitic air of Rutherford and the nitrogen of Chaptal and later chemists) was evolved on heating ammonia with the calx of lead (lead oxide, PbO). Although a brilliant experimentalist, "with rare quickness and perceptive powers," Priestley had not the genius of the philosopher; "he passed too rapidly from subject to subject even to notice the great truths which lay under the surface"; he left for others to interpret the results of his discoveries. The phlogistic doctrine was always in the way of this remarkable man; he never could free himself of its encumbrances, its wordy phraseology, and its false theory. He was blinded by Stahl's doctrine even to the end of his days. He only "looked at the results of his experiments through the fogs of his prejudices"; but his discoveries proved in the hands of Lavoisier the deathblow to phlogistic ideas. The conflict with error was ended and truth prevailed. His work represents the death of an old science and the birth of a new one. Such was the life of Joseph Priestley, and "le monde comme il va!"