Heat performs work in two ways. First, it expands bodies—that is, it draws their molecules farther apart against the force of cohesion which binds them together or keeps them moving in definite orbits at definite distances. It is as if it increased the velocity, and therefore the centrifugal force of a system of planets, and so caused them to revolve in wider orbits. The expansion of mercury in a thermometer affords a familiar instance of this effect of heat and the readiest measure of its amount. Secondly, it increases the energy of the molecular motions, so that they dart about, collide, and vibrate with greater force. Thus, as heat increases, evaporation increases, for molecules on the surface are projected with so much force as to get beyond the sphere of the cohesive attraction which binds them to the system, and they dart off like comets into space. Finally, as heat increases, and more and more work is done, against the centripetal force of cohesion, most substances, and doubtless all if we could get heat enough, are converted from solids into fluids, and ultimately into gases, in which latter state the molecules have got altogether beyond the sphere of their mutual attraction, and tend to dart off indefinitely in the direction of their own proper centrifugal motions, unless confined, in which case they dart about, collide, rebound, and exercise pressure on the containing surface. Conversely, if heat expands bodies, it is given out when they contract. Thus the enormous quantity of heat poured out for millions of years by the sun, is probably owing mainly to the mechanical force of contraction of the original cosmic matter condensing about the solar nucleus. Again, when gases suddenly expand, their temperature falls, which is the principle by which artificial ice is procured, and frozen beef and mutton are brought from America and Australia, producing, such are the complicated relations of modern society, agricultural depression, fall of rents, and a serious aggravation of the Irish question.