PRODUCED BY BODIES ACTING BY CONTACT. 7 
are formed, according to M. Dobereiner. It is readily intelligi- 
ble that, in this case, the condensed oxygen can easily combine 
with the hydrogen of the acid; nevertheless the affinity of the 
chlorine for the platinum necessarily intervenes also in this ac- 
tion. The affinity of gold for chlorine causes the decomposition 
of nitric acid by hydrochloric acid when gold-leaf is placed in 
nitro-muriatic acid, for this does not contain free chlorine until 
after being warmed, or left to itself for a long time. 
When a gas is in statu nascenti, a phenomenon analogous 
to those of which we have just spoken is presented: the gas 
often combines with a body, which, placed in contact with it, in 
other circumstances, would not be acted upon by it at all; thus, 
in cases where the chemical affinity of two gases is feeble, their 
condensation may cause them to enter into chemical combination. 
It however appears doubtful whether the combination of two 
bodies which, like hydrogen and oxygen, have so great a mutual 
affinity, should be attributed to the effects of condensation only ; 
although we are justified in admitting that, in the different phy- 
sical states of platinum, a condensation of gas always takes place 
on its surface. 
We know that platinum acts just as well in thin plates or wire 
as in the spongy or pulverulent state; but the combinations take 
place with a slowness proportioned to the smallness of the sur- 
faces employed. 
The laminz and sponge of platinum do not condense a very 
large quantity of oxygen; but on comparing their surface with 
that of the platinum black, we perceive that it must be so. The 
platinum black arising from the decomposition of the double 
chloride of platinum and potassium is formed at a temperature 
at which the mixture begins to agglomerate, and it assumes the 
form of lamella, which certainly cannot present a surface con- 
siderable enough to permit us to appreciate easily a condensa- 
tion of gases. 
An experiment, which was first made by M. Fusinieri, and 
which is very easy to repeat, proves that there are air and mois- 
ture condensed on the surface of glass. If some boiled mercury, 
which has been left to cool in vacuo, be poured into a glass tube, 
we notice that it yields air when it is warmed, even when the 
precaution has been taken to assure ourselves by the microscope 
that there remained not a trace of it adhering to the sides. 
But if into a glass tube, which has been exposed to a high tem- 
