94 Mr. I, B. Cooke on the Measurement of Chemical Affinity, 



excessive facility with which nitric acid yields one out of its five 

 equivalents of oxygen, that Grove's nitric acid battery owes its 

 superiority in energy over Daniell's sulphate of copper batteiy. 



Since the coating actually exists on the surface of the negative 

 metal, and since the erosion of such a coating by oxygen suffices 

 to explain the polarization of the anode plate, it may be accepted 

 as the true explanation. If a platinum plate, thereforfe, previ- 

 ously purified from all adhering matter, be taken to serve as 

 cathode in combination with an electro-positive substance in 

 water, the quantity of current urged by them through the galvano- 

 metet*, multiplied by the number of divisions of the resistance tube 

 through which it is passed, may be taken as the measure of the 

 affinity of the electro-positive substance for oxygen. 



The measurements given in the table were made on this prin- 

 ciple in the following manner : — Two plates of platinum were 

 placed in a glass of clean rain-water, and were connected with 

 the poles of a two-celled Grove's battery. A portion of the sub- 

 stance whose affinity for oxygen was to be tested was placed in 

 the same glass, out of contact with the platinum plates. The 

 anode platinum plate was also permanently connected with the 

 resistance tube and the wires from the cathode platinum, and 

 from the substance to be tested, were so adjusted in mercury 

 cells on the table, that the connexion of the one with the bat- 

 tery could be destroyed and the other be instantly joined to the 

 free wire of the galvanometer. The first swing of the needle, 

 multiplied into the number of divisions + 4 of the tube below 

 the end of the wire, was recorded as the force of affinity. Each 

 experiment was repeated several times, and the result confirmed 

 by placing the wire of the tube at different altitudes, and com- 

 paring the deflection produced with that calculated. 



The measurements are far from the limit of minute accuracy, 

 of which the method is susceptible with superior instruments, but 

 are the best which the means and leisure of the writer enable 

 him to obtain. The current of zinc and copper in water is taken 

 as the standard unit of force, because, by simple arrangements 

 on the table, it is capable, at an instant's notice, of easy verifica- 

 tion. Frequent verification is indeed essential, in consequence 

 of changes to which the resistance tube is liable. After long- 

 continued action, the copper electrodes of the tube take on a 

 very slight amount of polarization. The homogeneity of the 

 solution in the jar and tube is apt to be slightly disturbed. 

 Changes of temperature, again, somewhat modify the conducting 

 capacity of the solution. Any of these sources of eiTor is de- 

 tected instantly by appeal to the standard electromotor, and 

 easily remedied or allowed for. The wire of the tube should, 

 when out of use, be thrust down into contact with the copper 



