224 



The Electric Conductivity of Nitric Acid. 



A rapidly revolving commutator was substituted for the usual 

 induction coil, as the latter was found to be unsatisfactory owing to 

 the susceptibility of nitric acid to polarisation. 



Various forms of electrolytic cells were used according to the con- 

 centration of the acid and the temperature of the observations ; these 

 were provided with movable electrodes, so as to throw into circuit 

 different lengths of acid. 



A special form of apparatus was devised to prepare nitric acid of 

 99*88 per cent., and another form to obtain acid of 99'97 per cent, 

 from the latter. As a considerable quantity of this practically 

 anhydrous acid was obtained, its chemical and certain physical pro- 

 perties were examined. It has no action on (i) copper, (ii) silver, 

 (iii) cadmium, and (iv) mercury, all of high degree of purity, and 

 (v) commercial magnesium, at ordinary temperatures ; purified iron 

 and commercial granulated tin were unaffected by the acid, even 

 when boiling. Purified zinc was slightly acted upon, but sodium 

 immediately caught fire. The acid has no action whatever on 

 calcium carbonate at ordinary temperatures or the boiling point. 

 Flowers of sulphur and iron pyrites dissolve quickly and completely 

 in the gently warmed acid. The following results were obtained 

 for the density of the 99'97 per cent, acid, corrected for weighings in 

 vacuo : — 



Density 4/4 = 1-54212; 14'2/4 = T52234; 24'2/4 = 1-50394, 



the mean values of two concordant observations. 



As a further check upon the measurements obtained by the 

 Kohlrausch method, certain other measurements were made by 

 Carey Foster's method for the comparison of resistances, and the 

 results obtained were found to be concordant within the limits of 

 experimental error. In a series of tables the values are given for 

 thirty-two samples of acid of the specific resistance in true ohms at 

 temperatures of 0°, 15°, and 30°, the temperature coefficients a 10* 

 and /310 6 deduced from the equation R t = ^(l-^at— /3f), as also for 

 K X10 8 , Ki 5 xl0 8 , and K 30 X 10 8 (the conductivity of mercury at 

 being taken as unity, and its specific resistance as 94"07 microhms 

 per 1 c.c). 



It is shown that the specific resistance decreases for percentage 

 concentrations from 1*30 to 30, at first more, then less rapidly 

 (thus confirming the previous observations of Kohlrausch) ; from 

 this point the resistance increases slowly up to 76 per cent., thence 

 more rapidly until a maximum is reached at 96"12 per cent., when a 

 sudden reversal takes place. 



Further, whereas nitric acid behaves as other electrolytes in pos- 

 sessing a positive temperature coefficient of conductivity for percent- 

 age concentrations from 1*3 to 96*12, yet from this point up to 



