CELL-CONTENTS AND CELL-WALLS. 77 



(i) The vapour of hydrochloric acid may be used to distinguish 

 between ethereal and fatty oils. Cement to a slide a large glass 

 ring, such as are used for hanging-drop cultures ( 18), and a 

 small glass ring shallower than the large one. Place hydrochloric 

 acid in the space between the two concentric rings ; place the 

 sections to be tested on a cover-glass in a drop of glycerine con- 

 taining strong sugar solution, then invert the cover and place it on 

 the larger ring. Note that in a short time any ethereal oil in the 

 sections takes the form of bright yellow drops which finally dis- 

 appear. Fatty oils do not form yellow drops on treatment in this 

 way with hydrochloric acid vapour. 



(j) Compare the reactions of oils and those of resins ( 88). 

 The alkannin test for oil is not decisive, since resins take the 

 same red colour ; suberised and cutinised walls also give a red 

 colour with alkannin. 



86. Digestion of Fatty Oils. The fats are hydro- 

 lysed into their constituent fatty acids and glycerine by 

 boiling with water or treatment with steam, and by boiling 

 with acids and alkalis. The alkali method of decomposing 

 fats is a special kind of hydrolysis, called saponification, 

 since it was first used in the making of soap. 



The fatty oils undergo hydrolysis during digestion. 

 They are decomposed by the enzyme lipase, present in the 

 pancreatic juice ( 57) and also in germinating oily seeds, 

 and hydrolysed into their constituents (fatty acids and 

 glycerine) . 



(a) Boil a small quantity of lard with about 20 c.c. of alcoholic 

 soda solution (1 gram sodium solution in 50 c.c. alcohol), or with 

 caustic soda solution, for about five minutes. The fat is converted 

 into soap (sodium stearate, etc.). Then pour the solution into an 

 evaporating basin to evaporate the alcohol, if alcoholic soda is 

 used. Add some water ; if oil drops are seen, saponification (hydro- 

 lysis by alkali) is incomplete and should be completed by boiling 

 with more soda. Acidify with dilute sulphuric acid. A precipi- 

 tate of fatty acid is formed from the soap. Filter this precipitate 

 off through a Wet filter paper, and wash it with water till free 

 from acid. Keep the filtrate to test for glycerine (see c and d below). 



(b) Prove that the precipitate from the preceding experiment 

 consists of fatty acid, as follows. (1) Dissolve some of it in ether, 

 and add some alcohol containing a drop of dilate soda and a drop 

 of phenolphthalein ; the red colour of the indicator disappears. 

 (2) Dissolve some precipitate in caustic soda and divide the solu- 

 tion into three parts : (i) shake up with warm water a soap 



