LIQUEFIABLE SOLID MEDIA 99 



water and main-water of the breweries at Alt-Carlsberg, near Copenhagen, by 

 this method, and found that the spores of mould-fungi are comparatively the 

 most frequent, cells of bacteria capable of thriving in wort and beer being less 

 general, and yeast cells very rare. If the water be used merely for malting and 

 mashing purposes, its germs are unimportant, being, as we have already seen, 

 unable to withstand boiling in the hop-copper. There is, however, one un- 

 avoidable opportunity afforded for the contact of the beer with the water in its 

 unchanged condition, and that is in the washing out of the storage casks and of 

 the trade casks in which the beer is sent out to customers. In Germany these 

 casks, being lined with pitch, will not stand cleaning with hot water or steam, 

 and are cleaned with cold water, a small quantity of which is always left behind 

 in the casks ; so that, if this water be rich in organisms injurious to beer, serious 

 inconveniences may arise. Dr. Will has reported to the author an instance 

 coming under his knowledge where the beer from a brewery was constantly so 

 turbid that no customers would take it. After prolonged investigation the cause 

 was eventually discovered in the well-water, used for swilling out the casks, 

 which was found to be rich in the organisms producing turbidity in beer Subse- 

 quent examination showed that the well was connected with the drains by means 

 of fissures in the soil. 



84.-Liquettable Solid Media. 



If a species is represented in a bacterial mixture by a few individuals only, 

 its isolation by the dilution method requires an inconveniently large number of 

 culture vessels. In order to overcome this difficulty (with others that need not 

 now be touched upon), Robert Koch, utilising a method practised by Schroeter, 

 devised a new method of separation, generally termed plate-culture. The essen- 

 tial part of the method consists in the addition of a gelatinising substance to 

 the nutrient solution, whereby the latter acquires the property of becoming 

 liquid at a moderate warmth but is solid at room-temperature. The medium 

 thus liquefied is inoculated with a little of the bacterial mixture to be separated, 

 and, after being well shaken up, is poured, whilst still fluid, on to sterilised glass 

 plates, on which it sets as a thin film. In this film (under favourable conditions) 

 each one of the cells inoculated therein is held fast and isolated from the others, 

 and can subsequently multiply, undisturbed, into an aggregation of similar cells 

 known as a colony. 



Gelatin is the substance most frequently used for this purpose, and nutrient 

 media containing it are called by the generic name of nutrient gelatin, a distinc- 

 tion being drawn between wort gelatin, meat-juice gelatin, must gelatin, &c., 

 according to the kind of nutrient solution used. The amount of gelatin added 

 is about 9 or 10 per cent., and this produces a medium that is liquid above 

 30 C. and solid below 24 C., so that inoculation can be conveniently performed 

 at 35 C., a temperature exerting no injurious influence on organisms. Bouillon 

 gelatin, often called peptonised bouillon gelatin, is prepared by making up the 

 meat extract prepared as already described to its former volume, i.e. i litre, 

 with distilled water, after boiling, filtering, and mixing it in a glass flask with 

 i per cent, of peptone, 0.5 per cent, of NaCl, and 10 per cent, of gelatin. The 

 flask is carefully warmed in the water-bath or steamer until the gelatin liquefies, 

 and the liquid is then neutralised in the manner prescribed for bouillon. It is 

 next boiled in the steamer for half an hour and filtered hot through a moist 

 folded filter, to remove the precipitated albuminoid matters and those thrown 

 down in neutralising. Samples that clarify badly are improved by egg-albumen, 

 since the filtrate has to be perfectly clear and transparent. The liquid is filled 

 whilst warm and fluid into vessels for use (e.g. test-tubes holding 5-8 c.c.), and 



