Natural-Philosophical Collections, 227 
bands, in the shade of these wires, have remained equally invariable in intensity 
or dimension. 
From these experiments, M. Haldat thought that the explanation of diffraction, 
founded on the influence of an attractive force, or on the existence of certain 
atmospheres previously attributed to bodies, could not obtain the assent of philoso- 
phers when the attractive force and the atmospheres, submitted to the influence 
of agents so well calculated to alter them, had produced no change in the pheno- 
mena. These facts, doubtless, do not directly establish the theory of undulations, 
but they lead to it, by overthrowing the only explanation which could be opposed 
toit. The autiior moreover, does not conceal the difficulties which spring from 
these experiments with relation to the system of undulations, and he inquires 
how the movements of luminous waves, which must be so regular, are not affected 
by a fiux of subtle fluids which strike against them in their course. He remits 
the solution of these questions to the epoch when science shall have penetrated 
into the minute actions of these agents which are at present only known to us by 
their effects. Ibid. 
Fiesistance in Space to the Motion of Heavenly Ecdies.—In an account of the 
last appearance of Emncke’s comet in 1628, M. Gautier states, that the results 
then obtaimed accorded with those which Encke had previously procured, and 
which induced him, in 1823, to suppose the existence of a medium or ethereal 
fluid in space, of which the resistance, acting as a tangential force against the 
motion of the comet, would augment the power of the sun, and Shorten the pe- 
tiod of revolution. The most celebrated geometers, and even Newton himself, 
had already calculated the influence eiich such a resisting medium could exer- 
cise on the motions of comets and planets. They had found that its effect would 
be to diminish continually the eccentricity of their orbits, and to shorten the 
longer axes and the periods of their revolutions ; that the length of the perihelium 
would suffer only a periodical change ; and that the nodes and the inclination of 
the orbit would not be altered. In the case of Encke’s comet, the two first effects 
have been decidedly produced, and there are two circumstances to facilitate the 
calculation ; the first is, that this comet is always seen in the same point of its 
orbit and near to its perihelium; and the second, that its orbit is subjected only 
to very slow alterations. Both these circumstances permit the supposition that 
the times of revolution (at least for some periods) diminish by an equal quantity, 
so that their diminution may be considered as proportional to the square of the 
times; the periodical variation of the perihelium may also be neglected without 
inconvenience. M. Encke supposes, with Newton, that the ether, or resisting 
meditim, is diffused through all space ; that its density diminishes in the inverse 
ratio of the square of the sun’s distance, and that the resisting force is always 
proportional to the square of the actual linear Eevelocity of the comet.— Bib. Univ. 
May 1829. 
On the Production of Artificial Ultramarine.—The possibility of making ul- 
tramarine was first observed, when the blue matter found in a soda furnace was 
shown by Vauquelin to have the properties of that pigment ; and since then the 
experiments of Gmelin and Guimet have proved high ly satisfactory and success- 
ful; but as the processes published are still expensive, M. Kuhlman _ has been 
induced to publish an observation he has made, in hopes it may assist in simpli- 
fying them. Whilst repairing a reverberating furnace for the calcination of sul- 
phate of soda, he remarked that the brick bridge, separating the salt from the 
fire, was covered in different places with a coat of ultramarine. It appeared that 
previous to the formation of the ultramarine, a sulphuret of sodium was produced, 
for the blue places were surrounded by small, brilliant, reddish-brown crystals 
of this sulphuret. 
Whether the sulphate of soda is decomposed by the action of heat only, or by 
the simultaneous action of the heat and the fuel, or by the influence of the silica 
and alumina of the clay in the bricks, are questions M. Kuhlman could not an- 
