56 



THE AMERICAN MONTHLY 



[March, 



articles placed inside are exposed to 

 the direct action of a large volume of 

 steam. 



' The solution is then filtered hot 

 through filter-paper, and it comes 

 through clear and yellow. This is 

 best done by means of a hot-water 

 filter, which keeps the gelatin thor- 

 oughly melted all the time. If this, 

 however, is not at hand, the ordinary 

 glass filter tunnel and its contents may 

 be warmed by carefully and quickly 

 throwing against it the flame of a 

 Bunsen-burner or a spirit lamp. It 

 should be emphasized that the filter 

 and filter-paper, and the test-tubes, 

 together with all the other apparatus 

 used in the process, should be thor- 

 oughly sterilized in a sterilizing oven 

 to a temperature of 300° F. (150° C.) 

 for at least ten minutes. 



' The next process is to fill the test- 

 tubes, which have been fitted with a 

 cotton-wool stopper and sterilized 

 as above described. They are filled 

 for about a quarter or a third of their 

 length and are cooked for fifteen min- 

 utes in the steam sterilizer, when they 

 are probably in a condition for use ; 

 but to make sure, it is better to cook 

 them three days for fifteen minutes 



each day.' 



****** 



' There are two ways of using the 

 gelatin which has been prepared ac- 

 cording to the method I described 

 last week, viz : plate cultures and 

 tube cultures. 



'(a) For plate cultures, which are 

 especially useful to separate different 

 forms of germs in a mixture, there 

 is necessary an apparatus consisting 

 of two bell-jars, a glass-plate, and 

 glass-bridges. Of the bell-jars, which 

 are nbout seven inches in diameter and 

 two inches deep, one should be a trifle 

 larger and a trifle less deep, to set over 

 the other as a cover. The glass plate 

 should be about six inches long and 

 four inches wide, but this should be 

 regulated by the size of the table to 

 one's micrescope stand, for we must 

 be able to examine every portion of 

 it by the microscope. The glass 



bridges should be such as to elevate 

 the plate about one-quarter of an inch 

 above the bottom of the bell-jar. All 

 of this, like everything, as I said be- 

 fore, should be sterilized in a steriliz- 

 ing oven by exposure for ten or fifteen 

 minutes to a temperature of about 

 300° F. (150° C), and then it is put 

 together to cool, the plate resting on 

 the bridges in the lower bell-jar, which 

 is covered by the larger one. 



' When cold it is so placed that the 

 plate is exactly horizontal, and for 

 this purpose especial levelers are 

 sold, which, though convenient, are 

 not absolutely necessary. The cover 

 being removed, gelatin melted in a 

 test tube is poured over the plate, so 

 that when cold there is a layer about 

 an eighth of an inch thick. The cover 

 is replaced and all is set aside for the 

 gelatin to harden, when it is ready for 

 vaccination, which may be done in 

 two ways. (a) Vaccination of the 

 entire surface. This is accomplished 

 by making a very dilute mixture con- 

 taining the several germs it is desired 

 to separate, and pouring this over the 

 surface of the gelatin, (b) Vaccina- 

 tion in stripes. This is accomplished 

 by dipping a platinum wire sterilized 

 by heat into the very dilute mixture 

 of germs and gently scratching the 

 surface of the gelatin. This is re- 

 peated, making the rows from a 

 quarter to a half inch apart. N. B. 

 It is of great advantage to apply 

 moistened filter-paper to the inner 

 surface of the cover to make a moist 

 chamber. Also, N. B. It is almost 

 impossible to make too dilute a mix- 

 ture, and beginners make a great mis- 

 take in this respect. Theoretically it 

 should be so weak that any one germ 

 of a kind drops in a place, and an- 

 other at a little distance, and so on. 

 This apparatus being kept at a tem- 

 perature varying according to the cul- 

 tivation, the plate shows, after twelve 

 to forty-eight hours, little points which 

 gi'ow, and some of which maybe dis- 

 tinguished from the rest. By using 

 these as seed in successive fractional 

 cultures the last becomes quite clean, 



