THERMAL RELATIONS. 85 



pattern designed by Chamberland and made by the Maison Wiesnegg (P. Lequeux), 

 Paris, France, the steam being generated by gas (plate 9). The steam gage is at 

 the left ; in the middle is the valve through which the hot air is allowed to escape 

 when the instrument is wanned up ; at the right is the steam safety-valve. The 

 temperature is manipulated by regulating this valve. By leaving the vent open the 

 apparatus may be used as an ordinary steam sterilizer. It may also be used as a 

 distilled-water apparatus by attaching a condenser to the exit pipe of the middle 

 vent, but such water must not be used for culture media. A very good autoclave 

 is also made by the Kny-Scheerer Co., New York. Harding recommends for auto- 

 claves the use of steam from the engine-room boiler. This is convenient, provided 

 one can always have steam ready during the summer months. An autoclave, like a 

 steam boiler, which it is, must be watched carefully if it is not some time to explode 

 from excess of heat or lack of water. Each time before use one should see that the 

 apparatus contains sufficient water. 



Soils are rather difficult to sterilize. They may be spread in thin layers and 

 dry-heated for several hours at 150 C., or may be heated in the autoclave for an 

 hour under a pressure of two atmospheres, taking care to drive all the air out of the 

 soil before closing the apparatus. It is not likely, however, that soils can be treated 

 in this way without undergoing certain physical and chemical changes. Small 

 pots of soil may be heated in the steamer at 100 C. for two hours on each of five 

 successive days. 



The reason for preparing all media in the autoclave, or by heating in the 

 steamer at 100 C. on three successive days (the ordinary way), is because we are 

 never certain in what particular case resistant spores may be present. One short 

 steaming is often sufficient to sterilize media prepared in a cleanly way, as every 

 bacteriologist knows who has had much experience, but 'now and then, in spite of 

 all care, resistant spores will find their way into culture media, and for this reason 

 it is best in all cases (especially in teaching students) to adhere to a routine of three 

 steamings. Large masses of fluid (beakers, flasks) require longer steamings than 

 test-tube cultures. The writer gives double time, or triple time. Discontinuous 

 boiling as a means of sterilization was introduced in 1877 by Tyndall, who well 

 says respecting the sterilization of liquids : "Five minutes of discontinuous heating 

 can accomplish more than five hours continuous heating."* 



Most plant-pathogenic bacteria of temperate and cold regions have a lower 

 optimum and maximum temperature for growth and a lower thermal death-point 

 than species pathogenic to warm-blooded animals. The maximum temperature for 

 growth is usually at or below 36 C. We should not, however, expect this to be 

 true of bacterial plant parasites in tropical and sub-tropical regions, about which, 

 however, little is known beyond the mere fact that such parasites occur. Savastano 

 states that the optimum temperature for the olive-knot organism, which is said to 

 be more prevalent at the southern than at the northern limit of olive-growing, 



*This method appears to have been known to housewives for a much longer time. In Dr. Sam- 

 uel Johnson's Dictionary (first Am. from eleventh London ed.) I find the following definition: 

 " Biscuit, A kind of hard, dry bread made to be carried to sea. It is baked for long voyages four 

 times." 



