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 warmed 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 asa 
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.” 
