Natural-Philosophical Collections. 299 
General Conclusions. 
From what has been stated hitherto it is cbvious,—~ 
ls, That the laws which govern the retardation of bodies, gliding over each 
other, are as the nature of those bedies. 
2d, That with fibrous substances, such as cloth, &e. friction is increased by 
surface and time, and diminished by pressure and velocity. 
3d, That with harder substances, such as woods, metals, and stones, and 
within the limits of abrasion, the amount of friction is as the pressure directly, 
without regard to surface, time, or velocity. 
_ 4th, That with dissimilar substances gliding against each other, the measure 
of friction will be determined by the limit of abrasion of the softer substance. 
5th, That friction is greatest with soft, and least with hard substances. 
6th, That the diminution of friction by unguents is as the nature of the un- 
guents, without reference to the substances moving over them. 
_ The very soft woods, stones, and metals, approximate to the laws which go- 
yern the fibrous substances. 
In comparing the present experiments with those of Colomb, the discordances 
found to exist relate principally to time. The limited pressures (varying from 1 
to 45lbs. per square inch) under which his experiments were made, account in 
some degree for the anomaly. But in many of the minor, and in the general 
results, they will be found to coincide.—Vide Phil. Trans. 1829, p. 169. 
On the cold produced by the dilatation of air; by M. LEcranpD.—The ge- 
neral law that air is cooled by its dilatation was controverted by MM. Gay-Lus- 
sac and Welter, in the particular case where it is blown out of an aperture under 
a constant pressure. This strange result which was deduced from an experiment 
made with a fire engine at Chaillot, is published in the Ann. de Chim. tom. xix. 
p, 416. M. Legrand, Professor of Natural Philosophy at Besancon, has ob- 
tained very different results from the same engine. ‘The following were his ob- 
servations :— 
Distance from the aper- Temperature aR 
ture or cock. ? Gen i Cooling: 
10 millimeters. 22 uo 
30 25.5 4 
100 26.8 Qi, 
200 28.8 0.7 
250 29 0.5 
The temperature of the external air was 29°.5, and the third column is the differ» 
ence between this number and the temperature in the second column. When 
the cock was taken out altogether, and the bulb of the thermometer put in its 
place, the temperature oscillated between 12°.5 and 13°.5, so that the cooling was 
here about fifteen degrees centigrade, or twenty-seven dees of Fahrenheit. 
The experiments were repeated in June 1829 by M. Saigey, who obtained 
analogous results. Annales des Sciences d’ Observation, No. i. p. 45. 
Brown’s Active Molecules.—Mr. Holland, the inventor of a microscope sold 
by Carey, of the Strand, has inclosed some of the particles described by Mr. 
Brown, as active molecules, between glass and talc, closing the whole hermetically 
so as to exclude, as much as care could do, all interference of external causes. 
Notwithstanding this, the motion continued equally vivid, even after ten days. 
The lens used had a focus of the thirtieth of an inch ; and the particles were ob- 
tained, as we understand, from gamboge.—Brande’s Journal, Sept. 2829. 
