New Outlines of Chemical Philosophy. 103 
Experiment 3. © If an ounce of strong nitrous acid be mixed 
with about half its weight of sulphuric acid, and poured into a 
little oil of turpentine, the whole will immediately burst inte 
flame.” 
Explanation. The thermogen and photogen contained in 
these fluids coming immediately into contact, flame is produced 
instantaneously, without any mechanical force or increase of 
temperature. 
Now, from this experiment it appears impossible that water 
should be a compound of oxygen and hydrogen ; for, if it were, 
these two principles of combustion would burst into flame like 
the acid and the oil. But no pressure, no increase of tempera- 
ture, nor any union with combustible matter, can produce a sin- 
gle spark of fire from water. In short, nothing combustible is 
found in its composition. 
Experiment 4. Combustion may also be generated by friction. 
Two pieces of hard wood, being smartly rubbed together with 
great pressure, will soon burst i into flame. 
-Explanation. The first increment of caloric, by friction, is 
excited by a portion of the thermogen of the air being forced inte 
union with the photogen in the wood; and by continuing the 
operation, the temperature of, the inflammable principle of the 
wood is increased till flame is produced. 
Experiment 5. When a rod of iron is laid upon an anvil and 
hammered with a quick succession of heavy strokes, it soon be- 
comes red hot; but it cannot be strongly heated a second time 
by the same means, unless it has been previously introduced into 
a fire. 
Explanation. ‘The first stroke of the hammer forces a small 
portion of the thermogen of the air into contact with the pho- 
togen of the metal, which generates the first increment of caloric; 
and by repeated strokes of the hammer the caloric is increased, 
till the rod becomes red hot. But the photogen of the metal 
becomes exhausted by this means, and cannot, therefore, pro- 
duce caloric a second time, till it has recovered its combustible 
property in the fire. 
The burning of iron wire in oxygen gas shows, in a very bril- 
liant manner, how iron loses its photogen and becomes oxidized. 
Experiment 6. “ Mr. Thomas Wedgwood has shown that it 
has never yet been explained how friction produces caloric. 
** He took a piece of common window glass, and held the edge 
of it against the edge of a revolving grit-stone, and that part in 
contact with the stone became red hot, and thréw off hot parti- 
cles which fired gun-powder. The stone and the glass being 
both incombustible substances, it remains to be explained how 
caloric was produced.”’— Philosophical Transactions for 1792. 
G4 Explanation 
