230 APPENDIX 



4. GALVANIC CELLS. 



For general use in the physiological laboratory there is probably 

 no galvanic element superior to the Daniell cell (named after Prof. 

 J. F. Daniell, of King^s College, London). Much the most con- 

 venient and economical size is the quart or litre cell, whose porous 

 cup measures 5 to 6 cm. in diameter and 10 to 12 cm. in height. If 

 more current is needed than is furnished by one of these cells it is 

 very easy to join two or more of them into a battery. 



In large laboratories it will be found expedient to devote a table 

 to the galvanic cells. This table should be provided with a supply 

 of copper sulphate and of 10 per cent, sulphuric acid in large siphon 

 bottles similar to the one suggested for normal salt solution, except 

 that instead of the short tube for equalizing pressure one may insert 

 a filter through which at the end of the laboratory period the student 

 may return the liquids. 



The accumulation of zinc sulphate in the acid makes the renewal 

 of acid necessary from time to time. The deposit of metallic copper 

 upon the copper plate reduces the copper sulphate solution in strength. 

 It may be kept replenished by an excess of crystals of that salt in 

 the large supply jar. A very practical method of amalgamating the 

 zinc plates is to have a jar containing 10 per cent, sulphuric acid 

 with mercury in the bottom; as the plate is immersed the acid attacks 

 it and cleans it so that the mercury readily clings to it and may be 

 rubbed over the surface with a cloth. Another method which is 

 preferred by some is as follows: 



Dissolve 75 grm. of mercury in a mixture of 150 c.c. strong 

 nitric acid and 300 c.c. strong hydrochloric acid. Keep this amal- 

 gamating solution in a ground-glass-stoppered jar. To amalgamate 

 a zinc plate one needs only to dip it for a few moments into the 

 solution, remove it, rinse under the spigot, and rub with a cloth. 



At the end of each laboratory period the cells should be emptied, 

 the zinc plates rinsed and drained, and the porous cups left in a 

 tray of running water, or at least in a considerable excess of water. 



5. DRY CELLS. 



For a part of the work in electro-physiology, particularly electric 

 stimulation with induction shocks, the common dry cell may be con- 

 veniently used. 



The dry cell becomes rather easily polarized and must, therefore, be 

 used on an open circuit only. Used in this way, it will maintain 

 its strength through a laboratory period and will recover its 

 original condition in the rest which intervenes between laboratory 

 periods. 



