PHYSICS. 3 
to this subject. The manner in which atoms are combined, or their aggre- 
gation, is also deserving of mention, as the same atoms may be considered 
as combined under different forms and conditions; thus, ice, water, and 
steam, are all composed of oxygen and hydrogen, in the same proportions, 
yet all possess very different properties. Three conditions of aggregation 
are known, according to which bodies are divided into solid, liquid, and 
gaseous. 
By solid bodies are to be understood those which, apart from the changes 
produced by heat and mechanical agency, have an unchangeable volume, 
and an independent definite form. In these the single atoms are brought 
in the closest possible connexion. The connexion of atoms in liquid bodies 
is less intimate, possessing an almost unchangeable volume, even when a 
small quantity is exposed to great pressure ; they have, however, no definite 
form. In aeriform or gaseous bodies, the connexion of the atoms is exceed- 
ingly slight, there being neither an unchangeable volume nor a determinate 
form, both depending upon surrounding influences. All bodies, under cer- 
tain circumstances, may be transformed from one condition of aggregation 
to another, although the means to be employed, namely, change of tempera- 
ture and pressure, may not be applicable to a sufficient degree to effect this 
in certain cases. Thus, for example, mercury at a temperature of, and 
below —39° F., is a solid; at the ordinary temperature, it is a liquid; and 
by an increase of heat, it becomes converted into vapor. Inversely, watery 
vapor, by cooling, becomes a liquid: water—and a still further reduction of 
temperature turns this into a solid: ice. Mercury also can be converted 
from a vapor into a solid in the same way. Faraday, within a recent 
period, has succeeded in converting many gases into liquids and solids, for 
which great cold and pressure were both necessary. 
There must be a certain force which maintains the single atoms of a body 
in their mutual situations, giving to these bodies their structure and externa! 
form; another force again must cause the tendency to separation exhibited 
by these atoms, as among the gases. ‘These two molecular forces are the 
force of cohesion or attraction, and the force of expansion or repulsion ; and 
as heat converts solids into liquids, and liquids mto gases, it has been cus- 
tomary to consider heat and expansiveness as identical. The predominance 
of one or the other force determines the conditions of aggregation in a body. 
In solids, the former predominates; in gases, the latter; in liquids, the two 
are in equilibrium. 
Bodies may be considered under two conditions, namely, in a state of 
rest, and of motion; and this consideration brings us to another general 
property—that of inertia. Neither a part nor the whole of a body has in 
itself any tendency to change its present condition, that is, to pass from a 
state of rest to one of motion, or the contrary. The first case is illustrated 
daily ; the second, however, although true, is not so evident, as we see 
everything come to rest, after a time, from a state of motion. The cause 
of this cessation of motion, however, is not in the body itself, but in external 
influences operating upon it: if these latter be neutralized, the inotion con- 
tinues. The principal obstacles to a continuation of motion : >—friction, 
ICONOGRAPHIC ENCYCLOPADIA.—VOL. I. ig 177 
