. Carnegie Institution of Washington publication. 8o BACTERIA IN RELATION TO PLANT DISEASES. lions material used should be the thinnest obtainable film of fluid across a carefully- measured i mm. loop, so as to avoid crowding the plates. The same loop should be used in all cases, and it should be dipped into and out of the fluid always in the same way. After pouring, set the plates on a perfectly level spot (fig. 66), until the agar has hardened. If the work has been well done, there should develop an approximately uniform number of colonies in each plate. The tubes of inocu- lated water or bo
. Carnegie Institution of Washington publication. 8o BACTERIA IN RELATION TO PLANT DISEASES. lions material used should be the thinnest obtainable film of fluid across a carefully- measured i mm. loop, so as to avoid crowding the plates. The same loop should be used in all cases, and it should be dipped into and out of the fluid always in the same way. After pouring, set the plates on a perfectly level spot (fig. 66), until the agar has hardened. If the work has been well done, there should develop an approximately uniform number of colonies in each plate. The tubes of inocu- lated water or bouillon are then immediately lowered into the liquid air and exposed to it for the predetermined time, after which six additional Petri-dish poured plates, of the same size and inoculated in the same way, are made from each tube for comparison with those prepared before the exposure. The tubes may be thawed out by exposure to the air for three minutes and then to tap-water for five to seven minutes. The exposures are best made in Dewar glasses (fig. 67). When the exposures are long, a loose tuft of absorbent cotton should be placed in the moiith of the glass, or it should be covered with a hair-cloth cap, to prevent excessive. Fig. 66.* evaporation. Under these conditions the air remains liquid for a number ot days. At first the temperature is about minus 190° C., rising gradually to minus 180° C., since the nitrogen evaporates somewhat faster than the oxygen. The glasses are fragile and should be handled carefully, especially when filled with the air. As long as they contain liquid air it is safer to keep them in their containing-case, packed about with cotton or felt. One should be careful to avoid cracking the inner wall of the glass, as might happen by dropping some hard substance into the receptacle, otherwise an explosion will occur, the space between the two walls of the Dewar glass being a very perfect vacuum. When the exposures are made in block-tin tubes, the culture
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