"COEFFICIENT JELLY" 85 



It is important that the sodium citrate should be 

 neutral. Sodium citrate is inclined to become alkaline 

 when exposed for long to the air, owing to the deposit 

 of sodium carbonate. The jelly in the flask, therefore, 

 must be tested and neutralised to litmus with citric 

 acid. 



Previous to melting the jelly solution, a solution 

 containing 8 . 3 per cent of citric acid should have been 

 prepared; and now 1 cc. of that solution is added to 

 the 49 cc. of the molten agar solution in the flask. 

 This renders the whole of the jelly acid, the reason for 

 which will be given directly. 



A series of ten clean test-tubes should be ready, and 

 with a pipette 5 cc. of the acid jelly with its salts in 

 solution is measured into each test-tube. Each of the 

 ten test-tubes now contains 5 cc. of the jelly: total 

 50 cc. in all. The 10 test- tubes are placed in a stand 

 until the jelly is set, and a plug of wool is placed in 

 each; otherwise moulds may grow T on the jelly, as 

 it contains salts. 



Every one of the 10 test-tubes contains 5 cc. of 

 a 2-per-cent agar jelly, which is acid, because it contains 

 in solution 0.0083 gramme of citric acid. It also 

 contains . 1 gramme of sodium citrate and . 8 gramme 

 of sodium chloride ; and these tubes of jelly are known 

 for convenience as tubes of "coefficient jelly." 



To any one of these tubes we may add 5 more cc. 

 of any solution or solutions; and if the whole is boiled 

 and mixed by shaking, any portion of the 10 cc. of 

 jelly mixture now contained in the test-tube will 

 set on a slide as a firm jelly-film when it cools. 



