Those bacteria whose optimum temperature is above 40° are sometimes spoken of as the “_thermophil_” bacteria. The fixing of the “thermal death-point” that is, the minimum temperature at which the bacteria are killed is a matter of great practical importance in many ways and numerous determinations of this have been made with a great many organisms and by different observers. The factors which enter into such determinations are so many and so varied that unless all the conditions of the experiment are given together with the time of application, the mere statements are worthless. It may be stated that all _young, actively growing_ (non-spore-containing) _disease-producing bacteria, when exposed in watery liquids and in small quantities are killed at a temperature of 60° within half an hour_. It is evident, that this fact has very little practical application, since the conditions stated are rarely, if ever, fulfilled except in laboratory experiments. (See Sterilization and Pasteurization, Chapter XIII.) LIGHT. Speaking generally, it can be said that light is destructive to bacteria. Many growing forms are killed in a few hours when properly exposed to direct sunlight and die out in several days in the diffuse daylight of a well-lighted room. Even spores are destroyed in a similar manner, though the exposure must be considerably longer. Certain bacteria which produce colors may grow in the light, since the pigments protect them. Some few kinds, like the sulphur bacteria, which contain a purplish-red pigment that serves them to break up H₂S, need light for their growth. Since disease-producing bacteria are all injuriously affected by light, the advantage of well-lighted habitations both for men and animals is obvious. OXYGEN SUPPLY.