"[N]itrous gas... can combine with oxygen in various proportions, according to the mode in which it is mixed with air... Priestley... and the great majority of his contemporaries were either ignorant or heedless of this fact... travelling from place to place, analysing what they called the good air and the bad air of different localities, and coming to the most extravagant conclusions as to the relative purity of specimens... in which... modern analysis would fail to detect any difference."