1 76 Mr. Graham's Accoioit of M. Longchanip's Theory 



It is universally admitted that nitric, acid is not formed in 

 sheltered situations, unless a certain degi'ee of humidity pre- 

 vails, and the air circulates through all the parts ; for in places 

 whei'e the air cannot be renewed, there is no formation of 

 acid. Thus, Lavoisier observed at Roche Guyon, that in the 

 caverns or pits which were very deep and had but one issue, 

 nitric acid did not appear in the deep parts, but only at the 

 entrance. The same observation was made by that celebrated 

 philosopher in the tufa quarries of Touraine. The nitric acid 

 is formed only in places which contain porous rocks or light 

 soils, possessing carbonate of lime, moisture, and a constant 

 circulation of air. 



Tufa, light earths and chalk, act chiefly as absorbents. 

 Chevraud met with compact chalks which did not nitrify. 

 Hence we never find marble, whether in the quarry exposed 

 to the atmosphere, or in our houses, to exhibit any tendency 

 to the formation of nitre ; while tufa and chalk, which differ 

 from it only in porosity, nitrify with ease. 



It is upon water that chalk and tufas exert their absorbing 

 power. But these substances in contact with water, produce 

 no nitric acid when atmospheric air is withheld. But the wa- 

 ter brings air with it, and the nitrifiable materials, possessed 

 of humidity, continue to absorb air by means of that humidity. 



Chemists have long known that all kinds of water contain 

 air; but to MM. Gay-Lussac and Humboldt* we are in- 

 debted for a fact, which has more recently been confirmed 

 by the latter philosopher and M. Provencal f, that the air in 

 water contains more oxygen than atmospheric air does. The 

 mean of ten experiments made by Humboldt and Provencal 

 on air derived from water, gives the proportion of oxygen as 

 _3JLPJL. The previous researches of Gay-Lussac and Hum- 

 boldt made us acquamted with a still more interesting tact, 

 that if aerated water be exposed to heat, and if we divide the 

 air procured into any number of equal portions, the first por- 

 tions contain less oxygen than the last, as is exhibited in the 

 following table : 



Oxygen in 1000 parts of 1st portion of air 24*0 



2d 2b"-8 



3d 29-6 



4th 33-0 



5th 34.-8 



M. Lonw champ's application of this fact I shall give in his 

 own words, without abridgement, — the more so, as I consider 

 it not altogether correct. " According to M. Berzelius, prot- 



* Journ. de Phi^t. Ix. 129. t Mem. (TArcutil. ii. 2bO. 



oxide 



