152 Natural-Philosophical Collections. 
‘© Since nitric acid is formed in places where nothing is found but porous stones 
and light earths, containing chalk, moisture, and constantly renewed air, let -us 
inquire by what process this acid can be produced in circumstances so simple. 
<< Tufa, moveable soil, and chalk, act principally as absorbents : Chevrand, 
indeed, observed, that compact chalks would not nitrify, and no nitric acid was 
ever observed in any marble, either when exposed to the open air, or in’ the inte- 
rior of our houses. Thus we must chiefly attribute the easy nitrification of tufa and 
chalk to their porosity, since marble, which is also a carbonate of lime, cannot ni- 
trify. However, we know that the base must also perform a part in nitrification. 
_ “© Upon what bodies is it that chalk and tufa exercise their absorbing power ? 
Undoubtedly upon air and water, the latter of which will conduce to the ascribed 
effect in two ways. 1. By bringing air in a higher state of oxygenation, if I may 
use the expression, than that of the atmosphere, as all chemists agree; and 2. 
By being indispensable to the production of the acid, which could not exist with- 
out water. 
“¢ Since the times of Morozzo, Roupe, and de Saussure, we know the influence 
of the porosity of bodies in facilitating the combination of gases, which will only 
‘unite when in contact. If, then, to the action of the porosity of bodies, and to 
that of the water, the presence of a base be joined, we shall obtain the union of 
the oxygen and nitrogen of the air, in the proportions which constitute nitric 
acid, and this acid will be absorbed by the base, as it is produced by the pee 
body and the water. 
“‘ Every instance of nitrification, whether in quarries or in vaults, in caves or 
in cellars, under sheds or in artificial nitre beds, in sheep-folds or in stables, are 
simply and satisfactorily explained by the theory I propose. It is only necessary 
further to examine whether it affords an equally admissible explanation of the 
formation of saltpetre in India, Spain, and other similar places. We know that 
‘all the formations which afford saltpetre are very mobile. We know also, that 
in all warm countries, and particularly in India, the rains are extremely abun- 
dant, though rare. Here, then, are all the conditions requisite by my theory,— 
porosity of the soil, and deep humidity. When the heat has evaporated the 
“moisture, and rain does not soon fall, the saltpetre which has been conveyed to 
the surface by the water remains there, and may be collected, whilst in our cli- 
‘mates the constant rains do not permit the saltpetre to show itself; but an incon- 
testible proof that it exists in the soil is, that we find this salt in a great number 
of vegetables, and we have seen above that it is not a product of vegetation. 
But besides the nitrification which takes place in the soil of India and Spain, 
the saltpetre collected in these countries may have another source, as I have re- 
marked many years since, (Journ. de Phys. Ixix. p. 107.) In these countries 
the rains are often accompanied by thunder, which arises from the disengagement 
of electricity from the clouds ; and we have known since the days of Cavendish, 
that when an electric spark is made to cross a mixture of oxygen and nitrogen,’ 
nitric acid is formed ; indeed Marggraf has observed, that the water of rain which 
fails in a storm contains nitric acid. This acid combining with the base which 
the soil contains, forms saltpetre, which unites itself with that produced by nitri- 
fication.” 
Such is the theory of M. Longchamp, which certainly merits experimental ex- 
‘amination, although the commission of the Academy pronounced, in character- 
istic irony, the definitive conclusion, that ‘‘ it is only in a theoretical point of 
view that it might be useful to make the experiment proposed by M. Longchamp. 
It would, however, be very curious for science to know whether nitric acid could 
be formed in the circumstances which he describes.” 
On the Magnetic Influence of the Violet Ray.—When in 1812 Professor Mo- 
richini published his observations upon the magnetising influence of the violet 
ray, there was not an experimentalist in Europe who did not desire to repeat and 
‘vary them; but untortunately the attempts of the most expert men were not 
crowned with that success which they had a right to expect. Thus, it is not as- 
