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to keep the latter sterile. On this account Hansen and his assistants gradually evolved the Carlsberg vessel described below. There are still some who prefer the Pasteur vessel for its window and tap; it is, therefore, not out of place to indicate its failings.
There are two modifications of the Carlsberg vessel, an older, cheaper form, and a newer, more expensive one; they are shown in Figs. 26 and 27. There are two straight side tubes on this vessel, one, a, on the top, and the other, b, a little above
the bottom ; the bent tube is fitted either to screw off (see Figs. 27 and 28), so that both vessel and tube can be easily cleaned, or a part of it is fixed to the vessel and the other part connected by means of a piece of rubber tubing, c (see Fig. 26). The first-named form is the more expensive, as has been remarked, but it is to be preferred. The bent tube, at the end of which a filter, d, can be fitted, is expanded in the middle, e. This filter was in the older model a glass tube filled with cotton wool ; in the new model it
consists of a metal cylinder filled with asbestos, provided with a loose-fitting top, and screwed on to the tube. Rubber tubes are fitted to both of the side tubes, and are closed by glass stoppers ; the rubber on the lower side tube is also provided with a pinchcock. The yeast is introduced into the liquid in the vessel through the upper side tube, the liquid and yeast being removed through the lower one. The chief purpose of this vessel is the growing of yeast in large quantities, as, for example, when such is required for the
Fig. 28.--Mode of connecting the Bent Tube with the Carlsberg Vessel.
FIG. 29. ---Prior's Vessel.
pure culture apparatus mentioned later ; it is also used for fermentation experiments with larger quantities of liquid.
The manner of connecting the bent tube with the vessel in the new model may be seen from Fig. 28. The tube fits into the vessel by means of a conical joint; the connection is effected by means of a female screw. This joint must be made very accurately, otherwise the flask is useless.
The most convenient capacity for this vessel is about 10 litres. The largest amount of wort with which such a vessel can be charged is from 7 to 8 litres if it is to be used for fermentation experiments, for it must not be completely filled. In general 125 to 150 grams of thick yeasty sediment can be grown in the above quantity of wort.
Sometimes it happens that the bent tube becomes blocked during the sterilisation of the wort, so that it is impossible to draw off the contents of the vessel. This difficulty is easily surmounted by passing into the tube a thin, doubled copper wire. As a rule little is gained by heating the tube, since it is not known at what point the tube is stopped. It is advisable, now and then, to boil out the Carlsberg vessels with solution of soda, to remove the hop resin, etc., clinging to the sides.
Prior's Vessel.-E. Prior has constructed a modification of the Carlsberg vessel, which is represented in Fig. 29. A side tube, e, is closed with rubber and glass stopper; the two tubes, r, are connected by means of a piece of rubber tube carrying a pinchcock, and the tube, a, is also provided with a short rubber tube and pinchcock. The filter is fitted on at f. This modification of the original model has been designed chiefly to allow aëration of the wort. For further information see page 77.
The Chamberland Flask and its Modifications. — As is to be seen from Fig. 30, this flask consists of a flatbottomed bulb with a short neck provided with a ground cap which is drawn out into a somewhat long tube filled with cotton wool.
A more frequently used modification of this flask is that known as the Freudenreich flask. It (see Fig. 31) differs from the Chamberland flask in being cylindrical. As regards the size of the flask a total height of 10 cm. and a length of tube 2 cm, may be recommended. The external diameter of the base is about 2:5 cm., the internal diameter of the neck about 1 cm. The inner diameter of the tube ought not to be more than 0-2 cm., and the tube itself should not be shorter than the length mentioned above, otherwise too great an evaporation of the nutrient medium takes place. A flask like this holds about 20 c.c.; but, as a rule, it should not contain more than 10 to 15 c.c. of liquid.
1 Ordinary cotton wool (not fat free) should always be used for plugging flasks. Fat-free cotton wool attracts moisture and can thus set up infection.
Fig. 30.- The Chamberland Flask. Fig. 31.--The Freudenreich Flask.
This flask is very frequently used, as it does not take up much room, and on account of its small size also, working with it is not expensive. But the flask has this failing that it is somewhat easily upset. It is therefore advisable to place these flasks in small tin boxes made of various sizes, e.g., for 6, 10, 15, 25, 50 and 100 flasks. The height of the box may be made 3.5 cm. ; the other dimensions are, corresponding to the above numbers: 6 x 9 cm., 6 x 14 cm., 9 x 14 cm., 14 x 14 cm., 14 x 28 cm. and 19 x 38 cm.
When using the Freudenreich flask, care must be taken
to make the cap fast ; the flask ought never to be lifted by the cap, for it may easily happen that the cap comes off in the hand, and the growth in the flask is thus infected.
The Hansen flask forms another modification of the Chamberland model (see Fig. 32). It is distinguished by having a side tube, the lower part of the flask being either globular or cylindrical. The flask has this advantage, that it can be connected with the side tube of the Pasteur flask.
The side tube is either closed with an asbestos plug and sealing wax, or with a rubber tube and glass stopper. The first method is used specially when cultures are to be kept a long time in such flasks.
Freudenreich and Hansen flasks are used not only for cultures in or upon culture liquids, but also for cultures on nutrient gelatine, which is allowed to solidify obliquely in order to obtain a larger surface. Although the evaporation is but small in Freudenreich and Hansen flasks, it is advis