A FIXING FLUID FOR CARBON TRACINGS 231 



Most dry cells consist of a zinc cup or can enclosing ammonium 

 chloride usually mixed with plaster of Paris. In the midst of the 

 cell is a carbon plate surrounded by manganic dioxide. 



When the two plates (zinc and carbon) are joined in circuit outside 

 of the cell the NH 4 C1 attacks the zinc, forming ZnCl 2 and liberating 

 NH 4 and H at the carbon plate. This tends to polarize the cell 

 after it has been in use; but during the rest the H combines with O 

 liberated from MnO 2 . 



6. TO CURARIZE A FROG. 



On experiments of the irritability of muscle tissue it is necessary 

 to in some way suspend the activity of the irritable nerve fibres 

 which are supplied to every muscle. In certain other experiments 

 it may be advisable thus to remove the influence of the nervous 

 system. Curara (also spelled curare, curari, urari, woorara, woorari, 

 wourali, etc.), an arrow poison used by South American aborigines, 

 is the means usually employed to accomplish the end desired. The 

 way in which the curare exerts its influence is made the subject of 

 study in another place. Make a 1 per cent, solution by pulverizing 

 1 grm. of commercial curare and dissolve it in 100 c.c. of distilled 

 water. It need not be filtered unless intended for use with a hypo- 

 dermic syringe. If kept in a ground -glass-stoppered bottle, in a 

 cool place, it will retain its efficiency for months. 



The most convenient method of curarizing a frog is to inject with 

 a narrow-pointed pipette 1 to 3 drops of the solution, through a 

 minute, ventral, cutaneous incision. 



The drug will begin to take effect in a few minutes. The maximum 

 effect may be delayed some time. 



7. TO PREPARE THE KYMOGRAPH FOR WORK. 



Remove the cylinder, stretch a sheet of the prepared glazed paper 

 tightly upon the surface, place it upon such a stand as the one shown 

 in Fig. 33; set the drum to rotating and bring the triple gas flame 

 under the drum. In a few moments it will be evenly covered with 

 a film of carbon, which is as sensitive to touch as a photographer's 

 plate is to light. 



8. A FIXING FLUID FOR CARBON TRACINGS. 



Qum damar . . . . . . . . 160 grm. 



Benzole . . . . . q a. ad 2000 c.c. 



If this solution be kept in a large museum jar in the laboratory, 

 the removed sheet bearing the tracings may be dipped in toto or it may 



