_ the imperfection of contact. 
AFFINITY. 
gases, in equal measures, are introduced into a | time in that state without exhibiting any affinity | 
jar containing at the same time a large quantity 
of air, the formation of hydrochlorate of ammonia 
proceeds, the gases appearing to search out each 
other, till no portion of uncombined gas remains. 
The combination of two liquids, or of a liquid and 
a solid, is also facilitated in the same manner by 
the mobility of the fluid, and proceeds without 
| interruption, unless, perhaps, the product of the 
|| combination be solid, and by its formation inter- 
| pose an obstacle to the contact of the combining 
_ bodies. 
But the affinities of two solids which are 
not volatile, are rarely developed at all, owing to 
Even the action of 
very powerful affinities between a solid and a 
_ liquid or a gas, is often arrested in the outset from 
the physical condition of the former. Thus the 
affinity between oxygen and lead is certainly con- 
siderable, for the metal is rapidly converted into 
a white oxide, when ground to powder and agi- 
tated with water in its usual aerated condition ; 
and in the state of extreme division in which 
_ lead is obtained by calcining its tartrate in a glass 
| of several years: 
tube, the metal is a pyrophorus, and combines 
with.oxygen when cold, with so much avidity as 
to take fire and burn the moment it is exposed 
to the air. Jron also, in the spongy and divided 
state in which it is procured by reducing the 
peroxide by means of hydrogen gas at a low red 
heat, or by treating the oxalate in a tube, absorbs 
oxygen with equal avidity at the temperature of 
the air, and takes fire and burns. But notwith- 
| standing an affinity for oxygen of such intensity, 
_ these metals in mass oxidate very slowly in air, 
_ particularly lead, which is quickly tarnished in- 
_ deed, but the thin coating of oxide formed does 
not penetrate to a sensible depth in the course 
The suspension of the oxidation 
may be partly due to the comparatively small 
surface which a compact body exposes to air, and 
which becomes covered by a coat of oxide, and 
protected from farther change; but partly also 
to the effect of the conducting power of a consi- 
derable mass of metal in preventing the elevation 
of temperature consequent upon the oxidation of 
its surface ; for metals oxidate with increased 
facility at a high temperature, such as the lead 
pyrophorus quickly attains from the oxidation of 
the great surface which it exposes, compared 
with its weight. The heat from the oxidation of 
the superficial particles of the compact metal, 
however, is not accumulated, but carried off and 
dissipated by the conducting power of the con- 
tiguous particles, so that elevation of temperature 
is effectually repressed. It thus appears that the 
state of aggregation of a solid may oppose an in- 
superable bar to the action of a very powerful 
affinity. 
The affinity of two bodies, one or both of which 
are in the state of gas, is often promoted in an 
extraordinary manner by the contact of certain 
solid bodies. Thus oxygen and hydrogen gases 
may be mixed and retained for any length of 
for each other, and the gaseous mixture may, in- || 
deed, be heated in a glass vessel to any tempera- 
ture short of redness, without showing any dis- 
position to combme. But if a clean plate of 
platinum be introduced into the cold mixture, | 
the gases in contact with the metallic surface in- | 
stantly unite and form water; other portions of 
the mixture come then in contact with the pla- 
tinum and combine successively under its influ- 
ence, so that a large quantity of the gaseous mix- | 
ture may be quickly united. 
Chemical combination of two bodies seems | 
often to take place only by the co-affinity of other || 
bodies with each other. Thus nitrogen and hy- 
drogen gases do not form ammonia by direct || 
union, but by the oxidation of tin by nitric acid | 
or by nitric oxide and water; the metal abstracts | 
oxygen both from the water and nitrous body, || 
evolving nitrogen from the latter and hydrogen | 
from the water, which, in their then nascent 
state, combine to form ammonia. 
== 
Such com- || 
binations of bodies in their nascent state are | 
very numerous, and we are unable to produce a 
large number of them in any other way; such, | 
for example, are numberless artificial compounds, 
developed in the province of organic chemistry. 
There is another singular operation of affinity 
allied to the preceding, which is not well under- 
stood. It may be termed inducing, inductive, or 
imparted affinity. Attention was drawn to it by 
Liebig, in a general way, who classified some of | 
the facts by the law, that a body while in the act | 
of combining has the power of imparting the 
same action to another body in contact with it, 
inducing it to unite with a third body, when it | 
would not have done so under similar circum- | 
Nitrogen and | 
stances, without such contact. 
oxygen do not unite by heat, and indeed difficultly 
in any way ; but hydrogen burned in the air pro-— 
duces water containing nitric acid; platinum is 
wholly insoluble in and not oxidized by nitric 
acid when alone, but it is so when alloyed with | 
silver. This interesting subject is deserving of 
a full and extended investigation, which would | 
unquestionably lead to a much more thorough | 
knowledge of the operations of affinity than we 
at present possess.—Dooth’s and Boye’s Encyclo- | 
pedia of Chemistry. 
AFRICAN ALMOND,—botanically Brabejum. | 
A tender evergreen tree of the Protea family. It | 
attains a height of fifteen feet ; and its racemes | 
of splendid flowers have been compared in their | 
appearance to a sceptre. | 
AFRICAN LILY,—botanically Agapanthus. A 
genus of flowering bulbous-rooted plants of the 
hemerocallis or day-lily family. They are much 
esteemed by gardeners, and carry handsome blue 
flowers. The number of species is only three. 
AFRICAN MARIGOLD,—botanically Tagetes 
erecta. A well-known and favourite annual 
flowering plant of the composite tribe. It isa 
native of Mexico, but has long been naturalized 
