292 BIOLOGICAL AND SAXITARY MATTERS. 



cleared. Five c.c. is titrated after dilution with 5 c.c. of water with 



N 



r-- alkali solution, and fi'om the figure thus obtained the quantity 



of a strong caustic soda solution necessary to reduce the acidity 

 to 15 c.c, N acid per litre is calculated, and added. The liquid 

 is now filtered, the filter being kept warm, and the clear filtrate 

 is ready for use. Portions of 10 c.c. are placed in test tubes 

 which have been previously plugged with cotton wool and 

 sterilised by heating for half" an hour at 150° C. (350" F.). The 

 nutrient gelatine in these tubes is sterilised by heating to 100° C. 

 (212° F.) in steam for fifteen minutes on four successive days, 

 or by heating in an autoclave to 120° C. for fifteen minutes. 



M'Conkey's Bile-Salt Glucose Peptone Medium. — Weigh out 

 20 grammes of Witte's peptone, and dissolve in about 300 c.c. 

 of warm tap water, add 5 grammes sodium tauro-cholate, and 

 stir well till dissolved, adding a little more water to wash down 

 the sides of the vessel ; a porcelain double saucepan, in the 

 outside portion of which water is kept boiling, serves admirably 

 for the preparation of media. Add 5 grammes of glucose and 

 water to make altogether 900 c.c, and heat till the solution is 

 clear ; filter, and to the clear filtrate add 100 c.c. strong neutral 

 litmus solution. 



Prepare also some media of twice and some of three times 

 the above strength. 



Plug with cotton wool, and sterilise a number of test tubes, 

 each containing a small 2-inch X J-inch tube (Durham tube) ; 

 these tubes should be of different sizes, some of them holdmg 

 about 180 CO., others about 50 c.c, and the remainder about 

 30 c.c. The largest tubes should be marked at 150 c.c, and the 

 medium-sized ones at 20 c.c To the largest tubes add 50 c.c. 

 of the triple strength medium, to the next size 10 c.c. of double 

 strength medium, and to the others 10 c.c of the ordinary medium. 

 Sterilise these as directed for the gelatiiie. 



M'Conlcey's Bile-Salt Lactose Peptone Neutral Red Agar. — To 

 1 litre of tap water, 20 grammes Witte's peptone, 5 grammes of 

 peptone, 5 grammes of lactose, 15 grammes of powdered agar 

 are added, and dissolved by heating ; the solution is then centri- 

 fuged in wide tubes till as clear as possible, care being taken 

 that the temperature does not fall so low that the agar solidifies. 

 Pour off the clear liquid, add sufficient of a strong solution of 

 neutral red to colour the whole a deep red colour, place quantities 

 of 10 c.c. in plugged test tubes, and sterilise. 



Allow the agar to solidify so that a slanting surface is 

 obtained. 



Peptone Water. — This contains 1 per cent. Witte's peptone 

 and 0"5 per cent, salt ; the solution is heated till clear, filtered 



