STIMULI AND THEIR ACTIONS 



405 



pole, or, as it is also said in order to express in words the direction 

 of the current, the copper ( + ) is the anode, the zinc ( — ) the 

 Imthode. 



This primitive form of galvanic element, upon which, slightly 

 modified, is based the very powerful chromic acid dip-element in 

 which carbon and zinc dip into dilute chromic acid, has proved 

 for many purposes unserviceable. If the circuit be kept closed 

 for a considerable time, i.e., if the metallic union between the two 

 ends of the metals be not interrupted, or, as is said, the current 

 be not " broken," it is found that the current is not so strong as at 

 the beginning. This depends upon the fact that certain substances, 

 the so-called polarisation-products, have be- 

 come formed by electrolytic decomposition, 

 have accumulated at the two ends of the 

 metals within the liquid, and by contact 

 with the liquid give rise themselves to a 

 galvanic current, which is opposed to the 

 original current and, therefore, gradually 

 weakens it. In order to prevent the appear- 

 ance of this polarisation-current and thus 

 to maintain the intensity of the original 

 current as constant as possible, the expedient 

 has been employed of dipping the two metals 

 into different liquids, which are separated 

 from one another by a porous partition-wall 

 of clay, and are so constituted that they 

 destroy the efficiency of the polarisation- 

 products at the very moment of their appear- 

 ance. Hence a polarisation-current cannot 

 develop, and the electromotive force of the 

 element remains constant. Such constant 

 elements are in use in various forms. The 

 best-known forms and those that are em- 

 ployed most in physiology are that of 

 Daniell, in which zinc dips into dilute sul- 

 phuric acid, and copper into a concentrated solution of copper 

 sulphate; that of Bunsen (Fig. 189).. in which zinc dips into 

 dilute sulphuric acid, and carbon into concentrated nitric acid ; 

 and that of Grove, in which zinc dips into dilute sulphuric acid, 

 and platinum into concentrated nitric acid. In all, the free zinc 

 pole is the kathode. 



These galvanic elements are sources of electricity ; from them 

 at any moment a galvanic current can be led off very conveniently 

 wherever desired. In order to stimulate galvanically a living 

 object, e.g., a nerve-muscle preparation of a frog, it is necessary 

 simply to cut the wire that joins the two metals of an element 

 and insert the preparation between its ends; the current then 



IG. 189. — Bunaen's element. 

 The carbon plate (+) stands 

 in a clay cylinder contain- 

 ing concentrated nitric acid. 

 The cylindrical zinc plate 

 ( - ) surrounds the clay 

 cylinder, and stands in a 

 vessel containing dilute sul- 

 phuric acid. 



