1871.] 



Action of Metals and Liquids. 



339 



flections, agreeing in direction with those of the other experiments, were 

 obtained. 



These results show the necessity (which I have already mentioned) of 

 excluding air-bubbles from beneath the copper dishes, and of not wetting 

 the sides of the dishes by the liquid above the level of their immersion. 



To ascertain the influence of difference of temperature of the air-contact 

 line I soldered two strips of perfectly similar sheet copper, each 12 inches 

 long and J inch wide, in the form of circular hoops 4 inches in diameter upon 

 the bottoms of two tin cups, and ground the edges of the strips perfectly 

 level, and soldered copper wires to them for connecting with the galvano- 

 meter. Two glass triangles were now put into the apparatus, fig. 1, one 

 in each dish, to support the cups, and a mixture of one measure of nitric 

 acid and 12 measures of distilled water poured in until it just touched the 

 edges all round of the perfectly horizontal copper rims resting on the tri- 

 angles. After the needles of the galvanometer had settled at zero, about 

 ten ounces of boiling water was poured into one of the cups ; a temporary 

 deflection of the value "0560, and a permanent one of value '0759, were 

 produced, the hot metal being negative. The direction of the current in 

 this experiment agrees with that obtained with the same mixture and the 

 copper dishes ; and the result indicates that a large proportion of the quan- 

 tity of the current obtained with copper dishes in dilute nitric acid was due 

 to the action of the air-contact line. 



The influence of the air-line is largely chemical. " A piece of copper 

 wire wholly submerged in the acid [dilute sulphuric] so as to entirely 

 exclude any portion of it coming into contact with the air, has remained 

 for many months without imparting the slightest tinge to the liquid." 

 " But on suffering the liquid to evaporate so as to bring the upper end of 

 the metal near to its surface, the instant the slightest portion becomes ex- 

 posed chemical action immediately begins." 



" Two equal portions of wire were similarly placed in acid, only that one 

 was fully exposed to the atmosphere in an open tube, while the other was 

 placed in a phial, the acid occupying half its height, and was kept closely 

 corked for several weeks — after which the fully exposed metal had lost in 

 weight two-fifths more than the one which had been excluded from contact 

 with fresh portions of air, showing that contact with the atmosphere in 

 bulk is necessary to the fullest action"*. 



Experiments with Liquids of unequal strength. 



To throw some light upon the questions, — 1st, Is the quantity of the 

 current simply a result of the difference of number of molecules of liquid 

 which touch the hot plate compared with those which touch the cold 

 plate ? and, 2nd, What amount of difference of strength of a liquid is equal 

 to the amount of difference of temperature employed ? — I brought the two 



* " On the Theory of the Voltaic Pile," Bridgman, Phil. Mag. Nov. 1869. 



