POTATO GELATINE 



41 



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equal halves, and place them in a weak solution of carbonate 

 of soda (i gramme to a litre of water). Allow the cut pieces 

 to remain in the solution for an hour, stirring 

 occasionally with a glass rod. Renew the solution 

 at least twice, or until all milkiness of the water 

 on stirring has disappeared. Pour off the water, 

 and, removing the superfluous moisture with a 

 piece of clean filter paper, place the sections 

 in tubes as figured, without touching them with 

 the bare fingers. The bulbs of the tubes should 

 have been previously filled to two-thirds of their 

 height with either alkaline bouillon, sterile milk, 

 distilled water, or a lO per cent, solution of gly- 

 cerine in water, in which, by inclining the tube, 

 the lower portion of the potato should be allowed 

 to bathe when cultural growth is taking place. 

 Sterilise in the autoclave at 1 1 5' for fifteen minutes, 

 or in the steam steriliser at \(xf for thirty minutes 

 on three successive days. After sterilisation, it is 

 well to lay the tubes, while cooling, with the plain 

 surface of the potato downwards in order to pre- 

 vent the curling up which takes place when cooled 

 in an upright position, but to prevent undue soften- 

 ing, care must be taken that at this stage the tubes are not 

 sloped at such an angle as will allow the fluid in the bulb to 

 reach the potato. When cool, a slight tap with the finger will cause 

 the potato to resume its proper position in the tube. 



22. Potato gelatine {Eisner's inediuvi) — 



Choose a sufficient number of sound well-grown potatoes. 

 Peel, remove eyes, and reduce to paste by means of a coarse 

 grater. Take 500 grammes of this paste and macerate in a litre 

 of water for twelve hours in a cool place. Strain through muslin, 

 make up to one litre, and bring slowly up to boiling point, 

 adding gradually, before this point is reached, 1 50 grammes of 

 best gelatine. Boil for five minutes, and neutralise to litmus 

 with caustic soda solution. Add a suflficient quantity of normal 

 HCl to leave the medium feebly and yet distinctly acid. Auto- 

 clave for twenty-five minutes at 115°, and filter hot through 

 " Chardin " paper. Run into tubes or flasks and sterilise in the 

 autoclave at 115" for fifteen minutes, or steam in the Koch 

 steriliser at lOO' for tAventy minutes on three consecutive days. 



Fig. 2. — Roux 

 potato tube. 



b 



