THE ESSENTIALS 



OF 



EXPERIMENTAL PHYSIOLOGY 



CHAPTER I 



SOME PHYSICAL INSTRUMENTS IN CONSTANT USE IN 

 PHYSIOLOGICAL EXPERIMENTS 



Before undertaking any purely physiological experiments it is 

 necessary to understand the construction and mode of working of 

 certain pieces of physical apparatus which are in constant use ; such, 

 for instance, as batteries, induction coils, keys, &c. 



The Daniell's Element (fig. 1) is in very general use, on account 

 of the constancy of the current it yields. It consists of an outer 

 vessel of glass or glazed earthenware, in which is placed a cylinder 

 of copper open at both ends. Within the copper cylinder is a porous 

 pot, and within this is a roll of zinc. The outer vessel is filled with 

 a saturated solution of sulphate of copper, and an excess of the 

 crystals is kept in the solution. The porous pot is filled with dilute 

 sulphuric acid (1 to 5 of water). Connections are taken from the 

 copper and zinc cylinders. The positive pole of the battery is the 

 copper, the negative the zinc. To prevent local action the zinc 

 cylinder is previously thoroughly amalgamated by first cleaning its 

 surface with dilute sulphuric acid, and then rubbing metallic mercury 

 well over its surface with a piece of cloth dipped in the acid. When 

 in action the chemical changes in the battery are, solution of zinc and 

 formation of ZnS0 4 at the zinc plate, and decomposition of the C11SO4, 

 by the hydrogen appearing at the copper plate to form H 2 S0 4 and 

 metallic Cu, which latter is deposited on the copper surface. The 

 E.M.F. (electromotive force) of the battery is 1-072 volts. 



B 



