. General physiology; an outline of the science of life. 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, , if the metallic union between the two ends of the metals be not interrupted, or, a
. General physiology; an outline of the science of life. 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, , 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
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