THE FROG-HEART LEVER 233 



eighteen hours to remove any ZnSO 4 which has gained access to 

 the porous portion of the electrodes. 



If one has not the zinc rods at hand he may readily prepare 

 an efficient non-polarizable electrode as follows: (1) Take 5 cm. 

 of No. 16 copper wire; make one end perfectly clean and bright. 

 (2) Dip the bright end into molten chemically pure zinc. The zinc 

 adheres to the wire, and if the dipping be repeated two or three times 

 the lower 1 cm. of wire will have a thick coating of zinc. (3) Take 

 a glass tube 10 cm. long and with a 4 mm. lumen; draw it in the 

 middle to about two-thirds its original diameter; cut into two. 

 Before assembling the parts, that part of the copper wire not covered 

 by zinc, excepting the tip, must be painted with Brunswick black or 

 any varnish, and the zinc must be amalgamated. With this electrode, 

 as with the preceding, zinc sulphate, kaolin, and 0.6 per cent. NaCl 

 are used. 



10. THE FROG-HEART LEVER. 



A lever for recording upon the kymograph the movements of a 

 frog's heart may be constructed very simply from such materials as 

 a cork 2 cm. in diameter, a straw 30 cm. long, a piece of parchment 

 or celluloid for a tracing point, and a few pins. In the smaller end 

 of the cork cut a groove wide enough to receive the straw and leave 

 a space of 1 mm. on either side. The groove should have the sides 

 cut perpendicular to the end of the cork and should be 1 cm. deep. 

 Pass a pin or fine needle through the cork in such a way as to cross 

 the groove perpendicularly and about 2 or 3 mm. away from the end 

 of the cork. Partly withdraw the pin and pass it through the straw 

 or lever, ensuring free play of lever in the groove turning on the pin 

 as a fulcrum. Let 6 cm. of the straw be on the side of the fulcrum 

 and 24 cm. on the other. To the short arm a counterpoise may be 

 fastened, and to the long arm a slender cork foot may be fastened 

 about 4 cm. from the fulcrum and long enough to reach within 

 1 J cm. of the table when the cork fulcrum stands upon the table and 

 the lever is horizontal. To the distal end of the long arm fasten a 

 long, slender tracing point of parchment or celluloid. 



To adjust such an apparatus for tracing the movements of the 

 frog's heart one has only to place the cork fulcrum beside a frog 

 prepared as described on page 75, in such a position as to bring the 

 lower end of the cork foot into the required position upon the heart. 

 When adjusted fix in position by passing pins through the edges of 

 the cork fulcrum into the cork plate beneath. 



If one prefers to have permanent heart levers as a part of the 

 laboratory equipment they may be constructed as indicated in 

 Fig. 43. 



Prof. Porter, of Harvard, has devised a very fine heart lever which 

 may be preferred to either of the forms described above. 



