82 ELEMENTS OF LABORATORY WORK 



quantity, nature, dimensions, or temperature of any portion of 

 that system. 



It will afterwards be shown that we may have an electric 

 circuit that is, a system of bodies exhibiting special proper- 

 ties without any permanent change of the kind about to be 

 investigated. For the changes taking place in the cell some 

 other kind of change may be substituted, so as to be correla- 

 tive with the electric change. This may be a change of tem- 

 perature or of position, as will be shown. 



The kind of change taking place in a cell may be illustrated 

 by the changes occurring in a zinc, copper, and hydrogen sul- 

 phate cell. When perfectly pure and homogeneous zinc is 

 placed in dilute hydrogen sulphate, no action is- observed. 

 The quantity of zinc, on weighing before and after immersion, 

 is found unchanged. Ordinary zinc, however, is acted upon by 

 hydrogen sulphate, and diminishes in quantity by entering 

 into solution, while the hydrogen sulphate itself becomes 

 changed, losing a gas hydrogen and yielding on evaporation 

 a white solid zinc sulphate. Precisely these changes take 

 place when pure zinc and copper are placed in 

 , hydrogen sulphate and connected electrically. 

 Since copper itself undergoes no change, nor 

 produces any change, in hydrogen sulphate, we 

 may consider that the conjunction of the zinc 

 and copper with hydrogen sulphate originates 

 Fi 27 certain changes of the special kind known as 



chemical, and also that the behaviour of impure 

 zinc is due to its impurities playing the part of copper in the 

 above arrangement. 



56. The Existence of Magnetic Stress Indicated by the 

 Simultaneous Movement of Two Magnets, When two 

 magnets, freely suspended, are brought sufficiently near to 

 one another, they will be found to mutually attract or mutually 

 repel, according to circumstances. Or if one is set oscillating, 

 the other will also oscillate. 



In addition, any freely suspended magnet will always set 

 itself in the same position with regard to the earth, lying 

 nearly north and south. If one of the ends of the magnet is 



