394 MESSRS. V. H. VBLET AND J. J. MANLEY ON 



It will also be observed that whereas nitric acid behaves as other electrolytes, 

 namely, in possessing a positive temperature coefficient of conductivity for percentage 

 concentrations from 1'3 up to 96'12, yet from this point up to 99'27 per cent, it 

 behaves as a metallic conductor in possessing a negative temperature coefficient. 



Formerly, negative and positive coefficients were regarded as the essential point of 

 differentiation between metallic and electrolytic conductors respectively, until 

 ARRHENIUS,* led by theoretical considerations, based upon the ionic dissociation 

 hypothesis, observed negative coefficients in the cases of hypo-phosphorous acid (of 

 concentration = I '014 normal) and phosphoric acid (of concentration = 3 X '95 normal). 

 Upon this matter ARRHENIUS expressed himself as follows : " Es ist wohl kaum 

 denkbar dass man nach den bisherigen Ansichten ohne Zuhilfenahme der Dissociations- 

 theorie die Existenz von negativen Temperaturkoeffizienten erklaren konnte." 

 OSTWALD, and other writers of the same school, when quoting the results of 

 ARRHENIUS, have adopted the same view, with greater or less modification. But in 

 the case under discussion, nitric acid of concentration 96-99'97 per cent, would, 

 ex hypothesi, contain few, if any, free ions, and, therefore, the theory would lead to a 

 totally opposite conclusion. For the present it is not proposed to offer any expla- 

 nation of the phenomenon observed, but merely to call attention to the somewhat 

 analogous example of the density of sulphuric acid, which attains its maximum at 

 9878 per cent. (PICKERING, LUNGE, NEF, and ISLEE) and thence decreases up to 99 '8 f> 

 per cent. (PiCKERiNG).t 



In the paper quoted above, ARRHENIUS calculates, by interpolation, the temperature 

 of maximum conductivity of nitric acid as at 668, but to our minds it is idle to 

 speculate upon its properties at a temperature far above that at which it could have 

 any existence. 



HYDRATES OF NITRIC ACID. 



The existence of definite hydrates of nitric acid has been discussed by various 

 writers^ from the standpoints of (i.) direct experimental observations ; (ii.) analogy 

 of the composition of the metallic nitrates and of the phosphoric acids ; (iii.) differen- 

 tiations of observed values ; cumulative evidence from the first of the methods 

 appears the most cogent. The following resume gives briefly the conclusions of the 

 several writers : 



* ' Zeits. f. Physikal. Chem.,' vol. 4, p. 96. 



t Note added June 17, 1898. Since the above was written, the suggestion has been put forward that 

 in the case of nitric acid of 96-99'7 per cent., there is an initial decomposition into water and the 

 anhydride N 3 6 , precisely as sulphuric acid to a greater degree at a higher temperature, and to a less 

 degree at a lower temperature, decomposes into water and its anhydride SO 8 . 



J MBNDELEEFF, " Principles of Chemistry " ; W. H. PEEKIN, Sen., ' Journal of Chemical Society,' 

 Trans., 1889, p. 724, and 1893, p. 65 ; KOLB (vide supra) ; BEKTHELOT, ' Mecanique Chimique,' pp. 1, 397 ; 

 PICKERING, ' Journal of Chemical Society,' Trans., 1893, p. 436 ; CEOMPTON, ibid., 1888, p. 121 ; VELEY, 

 ' Ber. Deutsch. Chem. Ges,' vol. 28, p. 928. 



