510 DIGESTION. 



of caustic potash or soda, and boil for some time. If sugar is 

 present, the bismuth oxide will be reduced and become at first 

 gray, and lastly black. If only traces of sugar are present, a 

 mall quantity of bismuth must be used, or the whole will not 

 In- ivduced; if a first trial gives only a gray color, it should 

 be repeated with a smaller quantity of bismuth. 



/'. / mentation Test. A solution of grape sugar mixed with 

 yeast should at once ferment and give off carbonic acid. A 

 convenient apparatus for testing this is described by Bernard. 

 It consists of a test-tube, about three inches long, fitted with a 

 tight cork, through which a piece of small glass tubing passes 

 tcTthe bottom. The tube is to be completely filled with the 

 fluid to be tested, mixed with a little yeast, and then put in 

 the water-bath at 35 C. If sugar is present, carbonic acid is 

 given off, and as it cannot escape, it drives the fluid out through 

 the small tube. As the yeast may contain sugar itself, a similar 

 tube should be filled with yeast and water for comparison with 

 the first. The gas may be shown to be carbonic acid by shak- 

 ing it with baryta water. The fluid which escapes should be 

 collected by means of a piece of India-rubber tubing attached 

 to the upper end of the small tube, and tested for alcohol by 

 boiling it with a little potassium bichromate and sulphuric acid. 

 1 1' alcohol is present the fluid becomes green. 



** 156. Preparation of Glycogen. In order to obtain a 

 large amount of glycogen from a liver, the animal must be 

 healthy, and must be killed during digestion, as otherwise the 

 liver would contain but little glycogen. Conversion into sugar 

 after death must be prevented by rendering the ferment which 

 acts on it inactive, as quickly as possible ; this is done by heat- 

 ing the liver to 100 C. 



Kill a large and well-fed rabbit an hour or two after it has 

 had a full meal, by decapitation with a long knife. Open the 

 abdomen instantly, tear out the liver, chop it into pieces as 

 quickly as possible with a few strokes of the knife, and throw 

 it into a capacious capsule containing water, which is kept 

 briskly boiling by a large Bunsen's burner. The burner must 

 be large, because the liver cools the water into which it is 

 thrown, and unless ebullition be kept up briskly it may be some 

 time before the pieces of liver are heated to 100 C. throughout, 

 in which case the transformation of glycogen into sugar will 

 go on in those parts which are insufficiently heated. Let the 

 liver boil briskly for a short time ; then pour the liquid out of 

 the capsule into a large beaker, and put the liver into a mortar. 

 Return the liquid to the capsule, rub the liver to a fine pulp, 

 put it back into the capsule and boil it again. Then filter the 

 liquid and cool the filtrate rapidl}', by placing the vessel con- 

 taining it in ieed water. The filtrate contains a considerable 

 quantity of albuminous substances, which must be removed in 



