THE ELECTRIC CONDUCTIVITY OF NITRIC ACID. 367 



nitric acid of small quantities of metallic nitrates. This work has, however, only an 

 indirect bearing upon the present investigation. 



PKELIMINABY EXPERIMENTS. 



In consequence of certain initial difficulties, experiments were made upon the 

 effects produced by (l) an added impurity of nitrous acid ; (2) sunlight; and (3) im- 

 perfect insulation of the electrolytic cell. The results obtained may serve to explain 

 certain discrepancies between our values and those of previous workers. 



(1.) Added Impurity of Nitrous Acid. The following experiment serves to illus- 

 trate this point, to which reference was made in the introductory section : The 

 resistance of a mixture of 1 vol. of acid (I '48 sp. gr.) and 4 vols. of water was found 

 to be 6'97 ohms ; a few bubbles of nitric oxide gas passed in so as to form a trace of 

 nitrous acid caused an increase of - 07 ohm, or 1 per cent, in round figures. 



(2.) Sunlight. It is a common matter of observation that the space above the 

 liquid in a partially-filled bottle of concentrated nitric acid contains coloured fumes, 

 though the liquid itself may be almost colourless. This would show that the vapour 

 rather than the liquid was decomposed by the action of light. In order to decide 

 this point a comparative experiment was made by taking two stoppered flasks of the 

 same capacity, and presumably of the same glass. One of these was completely, and 

 the other partially filled with acid of 85 per cent, concentration, which contained one 

 part per million of nitrous acid. Both flasks were exposed to the direct sunlight of 

 an April day for two hours thirty minutes. Though the amount of nitrous acid in the 

 former sample was unaltered, that in the latter was increased to 21 parts per million, 

 and it had become bright yellow in colour. 



As a further illustration, the following experiment may also be quoted : The 

 resistance of a certain length of acid (1 : 4 as above) was found to be 7'18 ohms, but 

 after exposure for six hours to the diffuse daylight of a dull and cloudy day in 

 October, the resistance had altered to 7'31 ohms, or an increase of 2 per cent, in 

 round numbers, both experiments being made under the same conditions of 

 temperature. 



(3.) Imperfect Insulation of Electrolytic Cell. As it appears from the diagrams 

 given in certain standard text-books on physical chemistry that the electrolytic cells 

 may be kept rigid by metallic clamps, it was thought desirable to ascertain if such an 

 arrangement might lead to incorrect determinations. Our experiment showed that 

 (l) winding the ends of a copper wire around the cell in the neighbourhood of the 

 electrodes, (2) wrapping tin-foil round the whole cell, produced a diminution of 

 resistance amounting to 0'3 and 1'G per cent, respectively. 



