STUDY OP THE ATTRACTION OF COMPOSITION. 
m 
contact: 1. The Attraction of Aggregation, 
or Cohesion ; 2. The Attraction of Composi- 
tion. These powers both give way to the re- 
pulsive action of heat : but the former may 
also be overcome by mechanical force, which 
has no effect on the latter. 
The Attractionf)f Aggregation operates dif- 
ferently on different bodies, so as to produce 
the various degrees of cohesive force or consis- 
tence observable among them. The forms un- 
der which bodies appear are reducible to three 
classes, namely i Solids, Liquids, and Gases 
or Airs. These modifications of matter are 
influenced by the operation of mechanical 
pressure and the expansive force of heat, 
which seem to act as antagonist powers to 
each other. Some kinds of matter are capa- 
ble of existing either in a solid, liquid, or 
gaseous state, under different degrees of at- 
mospherical pressure and temperature. 
Thus, water, by the abstraction of heat, be- 
comes changed to ice ; by the addition of 
heat, it is, on the other hand, changed to va- 
pour ; and that change is facilitated or im- 
peded by lessening or increasing the atmo- 
spherical pressui'e. Mercury and several other 
substances exhibit analogous phenomena. 
Many bodies, however, commonly exist 
under only one or two forms of aggregation. 
Common air and other bodies, distinguished 
from vapours by the designation of permanent 
gases, were formerly supposed to retain the 
gaseous form under all circumstances ; but 
from the experiments of Dr. Faraday and Mr. 
Perkins, it appears that atmospheric air, car- 
buretted hydrogen, sulphuretted hydrogen, 
sulphurous acid, carbonic acid, protoxide of 
' chlorine, nitrous oxide, cyanogen, ammonia, 
muriatic acid, and chlorine, all which, under 
common temperatures and pressures, are 
permanent gases, may be condensed to the 
j liquid state by the joint operation of intense 
cold and powerful compression. However, 
oxygen, hydrogen, and some other gases 
have hitherto resisted all attempts to reduce 
them to the liquid state.* Several dense 
: solids, as lead and glass, are readily melted 
by heat ; but there are others, as wood, 
which, though speedily decomposed, when 
heated with access of air cannot be liquified. 
I The facts already stated, and othei s which 
ji might be adduced, still lead to the conclu- 
l| sion that the solid, liquid, and gaseous states 
; of bodies depend chiefly on their respective 
t’ relations to temperature and pressure; and, 
! therefore, the distinctions founded on those 
states or forms of matter do not furnish suf- 
ficient grounds for general arrangements of 
bodies in separate classes. 
Some writers have treated of the chemi- 
j cal properties of gases or airs as an inde- 
I pendent branch of science, under the appella- 
I tion of Aerology ; but the operation of bodies 
under their different forms are so intimately 
! blended, and the condensation of gases is so 
I commonly the effect of chemical combinations , 
I that it seems by far most convenient not to 
I separate Aerology from Chemistry, but to 
1 regard it as a subordinate section or subdi- 
i vision of chemical science. 
♦See Pneumatics, part ii. p. lO. 
In solid and liquid substances, the powers 
of attraction and repulsion counterbalance 
each other : while the latter predominates in 
gaseous bodies. Some have supposed soli- 
dity to result from the preponderance of the 
force of attraction over the force of repul- 
sion ; but this opinion seems incompatible 
with the well known fact of the expansion of 
water when it becomes solid. This, and 
some other phenomena of a similar descrip- 
tion, clearly show that the action of a 
repulsive force is not less obvious in solids 
than in fluids. They probably differ more 
in the arrangement of their particles, than in 
the manner in which those particles are 
united. 
The peculiar province of Chemistry, as 
might be inferred from the preeeding obser- 
vations, is the study of the Attraction of 
Composition, or the investigation of the 
properties of bodies, not as respects their 
organization, mechanical construction, form, 
or consistence, but with a view to the diseo- 
very of ther molecular composition, or the 
nature and mode of union of their component 
particles. Hence, there must necessarily be 
a wide distinction between mechanical and 
chemical combination. Any substances may 
be mechanically combined by mere mixture, 
which occasions no destruction or essential 
alteration of the sensible properties of the 
mixed bodies : for the compound formed by 
the union of two or more substances which 
have no chemical action on each other, will 
always exhibit their joint properties, modi- 
fied, perhaps, but not destroyed by their 
commixture. The effect of chemical combi- 
nation is very different, for bodies chemically 
united often become completely deprived of 
the peculiar properties they previously pos- 
sessed, and manifest new and sometimes 
very extraordinary powers, totally different 
from those of their constituent parts. 
“ If water be added to water, or salt, the 
effect is an increase of quantity, but no 
change of quality. In this case, the mu- 
tual action of the particles is entirely me- 
chanical. Again, if a blue powder and a 
yellow one, each perfectly dry, be mixed and 
well shaken together, a green powder will be 
produced ; but this is a mere effect arising 
in the eye, from the intimate mixture of the 
yellow and blue light separately and inde- 
pendently reflected from the minute parti- 
cles of each ; and the proof is had by examin- 
ing the mixture with a miscroscope, when 
the yellow and blue grains will be seen se- 
parate, and each quite unaltered. If the 
same experiment be tried with coloured li- 
quids, wich are siisceptible of mixing witho:ut 
chemical action, a compound colour is like- 
wise produced, but no examination with 
magnifiers is in that case sufficient to detect 
the ingredients ; the reason obviously being, 
the excessive minuteness of the parts, and 
their perfect intermixture, produced by agi- 
tating two liquids together. From the mix- 
ture of two powders, extreme patience would 
enable any one, by picking out with a magni- 
fier grain after grain, to separate the ingredi- 
ents. But when liquids are mixed, no mecha- 
