NUTRIENT GELATINE 39 



the pores would be clogged by solidification of the gelatine. 

 To avoid using the hot-water funnel, Kirchner suggests 

 allowing the gelatine to cool slowly in the steam steriliser, 

 after turning out the flame ; it is then after a few hours 

 perfectly clear and can be filtered with facility. Instead 

 of the folded paper, a thin layer of cotton- or glass-wool may 

 be used for filtering. 



If a sample of the filtrate is taken in a test-tube and 

 heated until it boils, it must remain clear and should also 

 not become cloudy while cooling. Any turbidity, if such 

 occurs, may possibly be due to the gelatine having been 

 rendered too strongly alkaline in neutralising ; as in 

 heating such a solution the carbonic acid is driven off, 

 and then compounds are thrown down which cause the 

 turbidity. It need hardly be said that care must be 

 taken in such cases to neutralise exactly in order to 

 get an efficient gelatine. That perhaps other faults, such 

 as inferior paper, dirty vessels, &c., may be to blame, is 

 also evident, and such must be avoided in the preparation 

 of nutrient media. Cloudiness is most easily dealt with by 

 adding the white of a hen's egg to the lukewarm gelatine 

 while it is still fluid, and shaking so as to divide it finely, 

 after which the solution is again boiled and filtered in the 

 hot-water filter. Indeed it is the rule to add the white of 

 an egg to nutrient gelatine immediately after neutralising, 

 so as to ensure the avoidance of all faults of turbidity. 



The gelatine when ready should be clear and of an 

 amber-yellow colour, and should not become cloudy on 

 heating. Carefully cleaned test-tubes are filled with about 

 10 cubic centimetres each and plugged with cotton wool, or 

 Schill's double test-glasses may be used (see p. 16). The 



should be about eighteen inches square, and folding is begun by doubling it 

 down the centre. The creased paper is finally gathered up, inserted into 

 the funnel, and the superflous part cut off.] TB. 



