i8o INTRODUCTION TO GENERAL PHYSIOLOGY 



solution of the free acid of congo-red and add to it a suspension of 

 aluminium hydroxide, which has been well washed. Metallic zinc 

 in powder (zinc dust) may be used, but the experiment is not so 

 striking. The colour of the mixture is at first that of the free acid, 

 deep blue. Allowing it to stand, chemical combination slowly 

 occurs, with the production of the aluminium or zinc salt. Com- 

 bination can be hastened by warming. The solution of the acid 

 is made by adding hydrochloric acid to a solution of congo-red. 

 The deposit is suspended in water, and dialysed until free from acid. 

 Contrast the rate of this chemical reaction in heterogeneous systems 

 with the immediate deposition of saponin in the bubble made in 

 the experiment on p. 168. 



