CH. XVI] ON ENZYMES. 307 



[Starch solution for these experiments should be very 

 carefully made. Rub *2 grs. of pure dry starch into a thin 

 cream with a little cold water and pour it into 200 c.c. 

 of actually boiling water ; continue boiling for two minutes 

 and allow it to cool. The solution should always be freshly 

 prepared immediately before using.] 



Put 10 c.c. of the above starch solution into each of 

 five test-tubes marked A, B, G, D, E. 

 To A add 1 c.c. extract + 4 c.c. water. 

 B 2 c.c. +3 c.c. 

 G 3 c.c. +2 c.c. 

 D 4 c.c. -f 1 c.c. 

 E 5 c.c. + 



Place the test-tubes in a water bath at 50 55 C., and 

 allow to stand for one hour. Take out the test-tubes and 

 cool thoroughly, then test a few drops from each (about 

 2 "3 c.c.) with a drop of iodine solution and note which 

 tube shews the reaction to be just complete (i.e. the first 

 which gives no blue or violet colour with iodine). 



If the reaction is not complete in any of the tubes, 

 more of the extract can be added in the same proportions 

 as at first and the heating continued for another hour 

 at 50 55C., if the same quantity was taken from each 

 for examination. 



If the reaction is complete in all the tubes more starch 

 solution can be added in the same way. A rough prelimi- 

 nary experiment will easily shew about what quantities of 

 starch solution and extract will be convenient. 



Repeat the determinations with the second extracts in 

 an exactly similar manner. 



202 



