376 Scientific Proceedings, Royal Dublin Society. 



acid isolated by Erdmann. This is the acid with maximum 

 viscosity in Graham's transpiration experiments. 



The transpiration times recorded by Graham are larger with 

 weak acids down to a dilution of 200 parts of water to 1 of HN0 3 , 

 the limit of his observations, than for pure water, or for pure 

 HN0 3 . This corresponds to an acid of 33*33 per cent. HN0 3 . 

 But Graham's times of transpiration become gradually diminishing 

 quantities with large dilutions, and a similar change may be seen in 

 the density curve drawn by Pickering from Kolb's Specific Gravity 

 Tables, and with the curve of heat evolution taken from Berthe- 

 lot's results ; nevertheless Pickering notices a break in his freezing 

 point curve corresponding to acid of the strength of 18 per cent. 

 HNO3 or HNO3 • 18H 2 0, which agrees with a break in the 

 density curve and in that drawn from Berthelot's heat of dilution. 

 With the absorption spectra, the transmission of rays is perfectly 

 definite to oscillation-frequency 3079 with all acids from 69 • 80 

 per cent, down to 20 ■ 31 per cent, strength, there being a feeble 

 extension to 1/A 3083 in the spectrum of the latter. This marks 

 the commencement of a change in the constitution of the acid, 

 and at this point we have the ratio HN0 3 : 14H 2 0, The acid 

 corresponding to this is N(OH) 5 ' 12H 2 0, and the hydrate indi- 

 cated by Pickering's curves is N(OH) 5 * 16H 2 0. Examined through 

 the same thickness, namely, 3 mm., the acid HN0 3 : 21H 2 or 

 N(OH) 5 • 19H 2 exhibits a profound change ; the spectrum is con- 

 tinuous to 1/A 3079 (A 3247), and extends weakly to 1/A 3157 

 (A 3167), from which point there is an absorption band 1/A 3157 

 to 3662 (A 3167 to 2730) ; beyond this the rays extend to 1/A 3947 

 (A 2533). 



The interest attached to the spectrum observations lies in the 

 recognition of four hydrates of the orthonitric acid. It remained 

 to be seen what compound would be formed by the absorption of 

 water vapour from the air under ordinary conditions of tempera- 

 ture and pressure. The first experiment was made with an acid of 

 1*420 sp. gr., containing 69*80 per cent, of HN0 3 , and consisting 

 almost entirely of the octobasic acid. There were 30 grs. of this 

 placed in a beaker under a bell-jar containing a dish with distilled 

 water in it. The acid remained without disturbance from Decem- 

 ber 16th, 1904, till January 18th, 1905 ; the gain in weight was 

 8*30 grs., making the total quantity of acid 38*30 grs. The com- 



