On Spontaneous Inflammations . 
Changing the experiment, and rubbing a cylinder of 
One of the kinds of wood between two tablets of the other, 
a cylinder of poplar for example between two tablets of 
mulberry wood, the increase of the rubbed surfaces which 
are in contact with the air produced a heat much more 
considerable, and almost the whole of the kinds of wood 
above enumerated took fire. 
The effect of friction still varies according as the woods 
employed, especially if they are of the same kind, are 
rubbed in the direction of the fibres, or when the fibres 
cross each other. In the first case, the friction and heat 
are much more considerable than in the second. 
In large machines where there is much friction, heat- 
ing may be prevented by continually directing a current 
of cold water on the rubbing surfaces : in common ma- 
chines, carriages, &c. it is diminished by covering the 
surfaces with an oily matter. There are many instances, 
during the great heats in summer, of carriages and other 
machines exposed to violent motion inflaming, because 
care has not been taken to grease them. Grease, by har- 
dening on the rubbing surfaces, instead of lessening the 
friction increases it ; and as this covering is highly com- 
bustible, it renders spontaneous inflammation more easy. 
In many cases, therefore, it is better to rub the machines 
with soap, talc, plumbago, or other substances, which 
without being oily are very unctuous to the touch. 
2, Action of the Sun. 
The strongest heat is produced, all sorts of combusti- 
bles are kindled, and the most refractory substances are 
fused by exposure to the sun’s rays concentrated by means 
of lenses or concave mirrors. It may happen that other 
bodies are in such a condition as to produce without our 
will the effects of lenses and of burning mirrors : though 
these effects are rather physical than chemical, it is of 
