Carbon dioxide was known to the ancients. Its presence was perceived by Plenius near volcanoes and mineral springs, and it was called by him "spiritus letales. " It was observed later by others, who confused it with "air. " John Baptista Van Helmont (1577-1644), introducing the word "gas," called it "gas sylvestre." He knew that this gas could extinguish a flame and

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The position of the thermophilic bacteria2 in any adopted system of classification has long been a subject of considerable controversy. Some writers have considered the thermophiles as a distinct group while, according to others, they may be regarded as varieties of the common spore-forming soil and water organisms. The reports of previous investigators and our own study suggest a close relationship between them and the so-called subtilis group. The known thermophilic bacteria are rods which are usually Gram-positive and vary considerably in size. They may occur singly, in pairs or in chains. Long filamentous forms are seen frequently. Their oxygen requirements extend over a wide range, some being aerobic, others facultative, and others anaerobic. They are, with very few exceptions, non-pathogenic and vary to a considerable degree with respect to carbohydrate fermentation, gelatin liquefaction, nitrate reduction, action in milk, indol and hydrogen sulphide production, pigmentation, and growth in the various culture media. Gentian violet inhibits their developnent (Cameron, 1930). Shaw (1928) reported inhibition of supposedly Gram-negative thermophiles by crystal violet. I This paper covers in part the thesis submitted to the Graduate School of Yale University by the senior author as part requirement for the degree of Doctor of Philosophy. 2 The terms "thermophilic bacteria" and "thermophiles" are applied in this paper to bacteria having an optimum growth temperature of 550 or above. Ordinary thermoduric organisms as, for example, certain coccus forms occurring in milk, are not included here.