COMMON CHEMICAL SENSE 5 



is dipped into a weak solution of K4Fe(CN)6, which precipitates 

 the copper held by the mucus of the foot. A similar result was 

 obtained with copper acetate. Washing the foot with distilled 

 water does not lead to a cessation of the contractions, because, 

 after exposure of the foot to certain solutions (Loeb, '05), water 

 stimulates. 



2. After two to four stimulations of the frog's foot by relatively 

 strong solutions of either copper acetate or ferric sulphate, the 

 stimulated area was sectioned and tested microchemically for 

 the penetration of the copper or iron. The metals were found in 

 the mucus of the surface of the foot, and in several instances 

 doubtful traces were discovered between cells of the extreme 

 outer portion of the epidermis. No evidence was found of any 

 disruption of the germinative layer. 



3. As a stimulating agent whose penetration would rcadil} 

 be visible, the action of picric acid was studied. The outcome 

 of experiments with this sul)stance at several concentrations may 

 ho illustrated l)v the records here copied: 



Kxpcrimrttts ii unit L. Picric acid, f^. K. T. = 

 Successive tests at .5 min. intervals. 



Reaction time in seconds. 

 X. R. = \o reaction. 



* Not reactive to pinching beyond this point. 



From these and similar tests, it was concluded that the pene- 

 tration of the stimulant rapidily renders the frog's foot less re- 



