on the surface of the body (pus cocci) need a less amount of water, while those invading the tissues (tuberculosis, black-leg, etc.) seem to be intermediate in this respect. In artificial culture media a water content of less than 30 per cent. inhibits the growth of most bacteria. As a general rule those bacteria which require the largest percentage of water are most susceptible to its loss and are most readily killed by drying. The typhoid and cholera organisms die in a few hours when dried, while pus cocci and tubercle bacilli live much longer. TEMPERATURE. The temperature conditions for bacterial existence and growth have been determined more accurately than any of the other general conditions. The maximum for existence must be placed at or near 100° since it is known that all bacteria including spores may be killed by boiling in time. Nevertheless, certain forms have been reported as thriving in hot springs where the water temperature was 93°. This is the highest known temperature for development. The minimum for existence lies at or near the absolute zero (-273°) since certain organisms have been subjected to the temperature produced by the sudden evaporation of liquid hydrogen (-256° to -265°) and have remained alive. Whether they could withstand such temperatures indefinitely is not known. The minimum for development is near the freezing-point of water, since reproduction by division has been observed in the water from melting sea-ice at a temperature of -1.5°. Thus bacteria as a class have a range for existence of about 373° (-273° to +100°) and for development of 94.5° (-1.5° to +93°) certainly much wider ranges than any other group of organisms.[5] The optimum temperature for development varies within rather wide limits for different organisms. In general it may be stated that the optimum temperature is approximately that of the natural habitat of the organism, though there are exceptions. The optimum of the “hot spring” bacteria just mentioned is apparently that of the springs (93° in this case). Many soil organisms are known whose optimum is near 70° (a temperature rarely, if ever, attained in the soil), _but only when grown in air or oxygen_; but is very much lower when grown in the _absence of oxygen_. Many other soil organisms exhibit very little difference in rate or amount of growth when grown at temperatures which may vary as much as 10° or 15°, apparently an adaptation to their normal environment. The disease-producing organisms show much narrower limits for growth, especially those which are difficult to cultivate outside the body. For example, the bacterium of tuberculosis in man scarcely develops beyond the limits of 2° or 3° from the normal body temperature of man (37°), while the bacterium of tuberculosis in birds grows best at 41° to 45°, the normal for birds, and the bacterium of so-called tuberculosis of cold-blooded animals at 14° to 18°.