LABORATORY WORK 193 



surface. By adjusting the height, a position can be found at which 

 the weight will slide slowly down. This represents the course of a 

 reaction proceeding spontaneously at a slow rate. Apply next a 

 few drops of oil to the bottom of the weight and repeat. It slides 

 down rapidly. The oil represents a catalyst. Note that the energy 

 set free in the process, being given by the height from which the 

 weight falls, is not altered by the catalyst. The form of the energy 

 may, however, be changed. It will be noted that the weight arrives 

 at the bottom with more kinetic energy in the presence of the catalyst 

 than when it slides slowly down. In the latter case, there is more 

 heat produced by friction. Another fact to be taken note of is that 

 there is some loss of the oil " catalyst " by sticking to the glass. 

 This represents the disappearance of a catalyst by subsidiary 

 reactions, which often occurs. 



Various Digestive Enzymes 



Amylase. Add a little saliva to some starch paste. It is quickly 

 liquefied, and sugar will be found by boiling with alkaline copper 

 sulphate. The blue colour with iodine will disappear. 



Invertase. Add a little yeast to a solution of cane-sugar to 

 which a drop of chloroform has been added. This addition prevents 

 alcoholic fermentation. Cane-sugar does not reduce alkaline copper 

 sulphate, but the glucose and fructose resulting from its hydrolysis 

 by invertase do so. 



A similar experiment may be made with scrapings from the 

 inside of the small intestine of a mammal. 



Pepsin. Take scrapings from the inner lining (mucous mem- 

 brane) of the stomach. Add 0.5 per cent, hydrochloric acid and 

 filter. Add two or three little cubes of hard-boiled white of egg 

 and keep in a warm place. The egg white will be dissolved: 



Trypsin. Make a similar experiment with an extract of the 

 pancreas, made by rubbing in a mortar with sand and 0.2 per cent, 

 sodium bicarbonate. Add a scraping of the mucous membrane of 

 the small intestine (containing enterokinase) to activate the tryp- 

 sinogen into trypsin. Filter. 



Absorption 



Histological preparations of the mucous membrane of the various 

 parts of the alimentary canal can be bought. If made in the 

 laboratory the following method is employed : 



In order that thin sections may be cut, all tissues require to be 

 " fixed " or hardened by some means. There are many solutions 

 used for the purpose, and the appearance of the cells is not the same 



