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screens the ground from the impressions of cold. 
But within the arctic circle, the surface of the 
earth is more effectually protected by the per- 
petual fogs which deform those dreary regions, 
and yet admit the light of day, while they ab- 
sorb the frigorific pulses vibrated from the higher 
atmosphere. Even the ancients had remarked 
that our clear nights are generally likewise cold. 
During the absence of the sun, the celestial im- 
pressions continue to accumulate ; and the ground 
becomes chilled to the utmost in the morning, at 
the very moment when that luminary again re- 
sumes its powerful sway. But neither cold nor 
heat has the same effect on a green sward as on 
a ploughed field, the action being nearly dissi- 
pated before it reaches the ground among the 
multiplied surfaces of the blades of grass. The 
lowest stratum of air, being chilled by contact 
with the exposed surface, deposits its moisture, 
which is either absorbed into the earth, or at- 
tracted to the projecting fibres of the plants, on 
which it settles in the form of dew or hoar-frost. 
Hence the utility, in this country, of spreading 
awnings at night, to screen the tender blossoms 
and the delicate fruits from the influence of a 
gelid sky ; and hence, likewise, the advantage of 
| covering walled trees with netting, of which the 
meshes not only detain the frigorific pulses, but 
intercept the minute icicles, that, in their forma- 
tion, rob the air of its cold.” See articles Ar- 
MOSPHERE, Duw, TEMPERATURE, CLIMATE, and 
WEATHER. 
AFFINITY. That variety of attraction which 
| is exerted between different kinds of matter, at 
insensible distances, uniting them into a new 
body, possessed of properties essentially different 
from those of its constituents. By acting only 
at insensible distances, it is sufficiently distin- 
guished from gravity ; exerted only between dif 
ferent kinds of matter, it is very different from 
cohesion ; but it is more difficult to draw a nar- 
row line of demarcation between affinity and 
that form of attraction which is usually placed 
under heterogeneous adhesion, viz., the attraction 
evinced in solution, for the latter seems to form 
a connecting link between chemical and other 
kinds of attraction. The latter part of the above 
definition, that “it forms bodies essentially dif- 
ferent,” will serve to distinguish it from attrac- 
tion in solution. 
Affinity is not always exerted between bodies ; 
thus fluorine does not combine with oxygen, but 
readily with metals. Carbon does not unite with 
mercury, but combines with oxygen, iron, Wc. 
It is therefore elective in its character, preferring 
one body to another. Such a view, however, is 
| | only in accordance with the present state of the 
| science, for it is possible that each element may 
combine with each of the others; but that 
| other forces interfere with the exercise of af- 
|| finity. Thus nitrogen may have a strong attrac- 
tion for the metals, and the reason why such 
compounds are difficult to make and easily 
AFFINITY. 
decomposed, may be the highly elastic state of 
nitrogen. 
As far as we know, it acts at insensible dis- 
tances, but it is possible that the distance to 
which its energy extends may be measured. 
Thus the union of some bodies by friction, when 
in a minute state of mechanical division, seems 
to hint at a sensible distance of action. 
Affinity must be sufficiently powerful to over- 
come the opposing forces of gravity, cohesion, 
and elasticity. The bodies must be in immediate 
contact, since the action is at insensible distances. 
It is rarely sufficient to pulverize two solids, how- 
ever fine, to induce their chemical union, for their 
particles do not admit freely of motion, but by 
constant and rapid trituration many points of 
surface are brought in contact nearly at one 
time, and combination may ensue ; in this man- 
ner sulphur and finely divided copper unite with 
the evolution of heat. The affinity of bodies is, 
therefore, promoted by everything which tends 
to their close approximation ; in solids, by their 
pulverization and intermixture, this attraction 
residing in the ultimate particles of bodies ; in 
gases, by their spontaneous diffusion through 
each other, which occasions a more complete in- 
termixture than is attainable by mechanical 
means ; and between liquids, or between a liquid 
and solid by the adhesive attraction which liquids 
possess, which must lead to perfect contact, and 
also by a disposition of liquid bodies to intermix, 
of the same physical character as gaseous diffu- 
sion. Hlevation of temperature has certainly often 
a specific action in increasing the affinity of two 
bodies ; but it also often acts by producing a 
perfect contact between them, from the diffusion 
or vaporization of one or both bodies. Hence, 
no practice is more general to promote the com- 
bination of bodies than to heat them together. 
Thus fused sulphur Coes not unite with carbon, 
but must be brought in the vaporous state, in 
contact with carbon: effloreseed carbonate of 
soda absorbs carbonic acid at first, slowly ; but 
more rapidly in proportion as absorption develops 
heat, until at length its absorption is violent. 
Carbon, iron, &c., must be heated in oxygen to de- 
velop their affinity for it. 
Light may frequently induce union like heat ; 
thus chlorine with hydrogen or carbonic oxide. 
Electricity may also unite gases by heat or com- 
pression, as hydrogen and oxygen, &c. The ex- 
pansion of a gas often assists affinity ; thus phos- 
phorus exhibits slow combustion in oxygen at a 
lower temperature, the more the gas is expanded. 
Condensation frequently effects union by the heat 
it develops, and by bringing the particles in closer 
contact, 
If the affinity between two gases is sufficiently 
great to begin combination, the process is never 
interrupted, but is continued from the diffusion 
of the gases through each other till complete, or 
at least till one of the gases is entirely consumed. 
Thus when hydrochloric acid and ammoniacal 
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