POTASSA. 289 



The potassa unites with the acid of the grease con- 



tained in the cloth, and thus makes it soluble in water. 



722. ACTION OF POTASSA ON ANIMAL 



What is the 



action of po- MATTER. Potassa is extremely destructive 

 ^al matter />" of animal matter. It readily dissolves the 

 skin, as may be proved by rubbing a little 

 between the fingers. If applied in sufficient quantity. 

 it destroys the vitality of the flesh. It is often used 

 for this purpose by surgeons. 



723. EFFECT ON VEGETABLE COLORS. 



How does po- 



tassa affect vc- Vegetable blues which have been pre- 

 getabie colors? viously re ddened by acid, are restored to 



their original color by the action of potash and other 

 alkalies. The blue pigment called litmus is the one 

 most readily obtained. In preparation for the exper- 

 iment it is infused in hot water The transformation 

 from blue to red, and vice versa, may be repeated as 

 often as desired, by the alternate addition of acid and 

 alkali. Paper soaked in the red and blue liquids 

 forms the test-paper of the chemist. It is used to in- 

 dicate the presence of smaller quantities of acid and 

 alkali than could be recognized by the taste. An extract 

 of purple cabbage leaves, or the leaf itself, may be 

 used in the above experiment. In this case, the change 

 of color by alkalies is from red to green. 



724. PROPERTIES OF SODA. The prop- 

 of erties of soda, are very similar to those of 



potassa, as above described. 

 13 



