ZOOLOGY AND BOTANY. MICROSCOPY. ETC. 1043 



plate of glass fitted in a frame of wood and covered with felt. The incubator 

 is encased in wood, and stands 55 cm. high. Three thermometers, each 

 72 cm, long, are employed to indicate the temperature of the water and of 

 the two compartments. The scale is marked from 30°-50°, and subdivided 

 into tenths of a degree in such a way that each division is 1 mm. distant 

 from the next. 



A constant temperature is obtained by means of two simple Bunsen 

 burners fed direct from the meter usually kept at half power. By raising 

 the burner 1 cm. the temperature rises a tenth of a degree, so that to obtain 

 the desired temperature (37°), twenty slips of wood, each 1 cm. thick, will be 

 required. If higher temperatures are desired, the burner must be altered. 

 The heating action of the apparatus must of course be ascertained first of 

 all empirically, but this is only required to be done once. It may be 

 mentioned that the course of the circulation is from the floor upwards 

 through the central partition, then right and left along the top, and then 

 downwards to the floor again by the short sides. The thermometers are 

 all encased in a copper sheath, and the floor of the incubator is also made 

 of copper. 



(2) A perfectly clear Agar Medium, which will withstand a temperature 

 of 40° without melting, is produced in the following manner : — Obtain the 

 raw material, the dried Fucus spinosus (the ordinary agar powder is of no 

 use), and pick out therefrom the clear yellowish transparent pieces. Then 

 weigh the pure agar thus obtained, and wash with a 2 per cent, hydro- 

 chloric acid for five minutes, then with ordinary water frequently changed 

 and perfectly free from dirt. By frequent weighing the quantity of water 

 is ascertained, and by addition of concentrated bouillon the desired con- 

 sistence is attained. It must be noted that for this quality of agar 5-10 per 

 cent, is required to produce a firm medium. The agar bouillon is then left 

 to macerate all night at the ordinary temperature. The next day it is 

 boiled in a water-bath and strained through a linen filter. The usual 

 quantity of pepton and common salt is then added, and after being 

 neutralized with carbonate of potash or soda, is heated once again in the 

 water-bath for about half an hour. The agar solution is then filtered 

 through filter-paper. It flows through clear but slowly. On account of its 

 rapid coagulation it is well to filter direct into sterilized test-tubes or 

 Koch's flasks. Produced in this way the agar medium is perfectly crystal 

 clear, remains quite firm at 40°, but is, however, somewhat softer than the 

 ordinary solution. 



(3) Glass vessels for observing potato cultivations, &c., in various gases may 

 be made by expanding as much as possible the lower part of the neck of 

 Koch's flask of about 200 grm. capacity, and then cutting them off at the 

 middle of the neck. To the upper somewhat conical end an air-tight glass 

 cap is fitted on, and to the side of the bulb a thin glass tube about 10 cm. 

 long is melted in. The latter tube is intended to communicate with the 

 air-pump. The raw potato discs are pushed through the neck opening by 

 removing the glass cap, and after the side tube is plugged with cotton-wool 

 the flask is sterilized. The medium having been inoculated while the flask 

 is held in the oblique position, the air-pump is connected with the side 

 tube ; the air is withdrawn and rejilaced with the desired gas. When full 

 the side tube is melted up with a Bunsen burner. In case the access of 

 impurities should be feared during inoculation, a narrow glass tube termi- 

 nated by a small cap can be fitted to the larger cap, and then inoculation 

 may be performed in a current of the gas selected by quickly removing the 

 smaller cover. Absolute safety is attained by closing the rims with vaselin. 

 The tap connecting with the air-valve must be triply perforated, so that the 



