454 Mr. Smith on the Hydrates of Nitric Acid. 



As the sun descends beneath the horizon, the neutral point 

 of M. Arago separates from the antisolar point, and when this 

 point is first seen in the morning before sunrise, its distance 

 from the antisolar point is a maximum ; it gradually ap- 

 proaches that point till the sun rises, and also till the neutral 

 point itself reaches the horizon, when its distance from the 

 antisolar point is a minimum. 



When the altitude of the sun is 45°, the distance x of the 

 neutral point above the sun is about 13° 5', and the distance 

 a?' of the neutral point below the sun 6° 42'; at other altitudes 

 we have 



X = A cos A, 

 and j/=AcosA, 



tan Z, 

 A being 18|°, A the sun's altitude, and Z the zenith distance 

 of P', the neutral point below the sun. 



An interesting paper, entitled Delle Leggi della Polarizza- 

 zione della Luce Solare ?iella Atmosphera Seretia, communicato 

 con lettera al David Brewster, LL.D., F.R.S., Lond. et 

 Edin., membro delle Principali Academie di Europa, del 

 Prof. A. B. Francesco Zantedeschi, will be found in the Rac- 

 colta Fisico-chimica Italiana, torn. i. fascic. 10. 1846. The 

 details in this paper are chiefly historical. The results ob- 

 tained by M. Zantedeschi himself, which are of a general 

 nature, differ in several respects from mine ; but whether this 

 difference arises from a difference in the methods of observa- 

 tion, or from the different states of the atmosphere under which 

 the observations were made, I am not able to determine. 



In a Memoir on the Polarization of the Atmosphere, which, 

 I trust, will soon be published in the Transactions of the 

 Royal Irish Academy, I shall give a full account of my obser- 

 vations, and enter more deeply into the subject than would 

 have been proper in the preceding popular explanation of a 

 Map of the Lines of Equal Polarization. 



LXIX. On the Hydrates of Nitric Acid. By Mr. Arthur 

 Smith, Assistant in the Laboratory of University College, 

 London^. 



SOME doubt still hanging over the composition of the hy- 

 drates of nitric acid, especially of the first hydrate, I was 

 induced to try some experiments with a view of diminishing 

 this uncertainty. For this purpose a quantity of the red 

 fuming acid was procured, which I examined before com- 

 * Communicated by the Chemical Society; having been readjune 7. 

 1847. 



