OCR Text |
Show 148 HEAT AND MOTION. tain range : though there are ca 3es 1· n w h"l Ch theh . melting and the passing into vapour follow. eacis other so very rtlpidly that the process of meltmg not usually observed. . . hat It is equally probable that m every motl?n. w 08 _ ever, even in the m~st gentle and cool that 1t IS P h sible for us to imagme, there are the elements bot of sengal heat and of visible light; but t~at these on~~ become apparent wben there are ?ertam degrees d resistance to the motion. Two p1eces of dry woo , rubbed against each other, soon become heated, ~nd the, are not very long in taking fire, a-r:d burmng with light. But they do not heat ~o soon 1f t?tY are wetted or covered with oil, or w1th any thmg ~lse that le~sens the resistance they have to .the motiOn. We feel the same truth in our own bodiCs. When all the systems of vessels in whi~h the blood and other fluids circulate or move are m a ~ealthy s.tate, we feel no sense of heat from the vanous motiOns, though all of them are continual, and many of them are rapid ; but when any part is so diseased as that the motion is resisted, we then feel ?eat as well as pain: and if the disease is only a whitlow, or something of an equally local nature, we fee~ the. part as hot as if it were burning; and t~e !ee~n~g IS not a merely inward feeling, like that of pam, It 1.s an actual increase of temperature, which we can discover by the healthy hand, or measure by the thermof!leter, just in the same way as if it had been commumca:ed by holding the part near to a c.ommon fire. In ca::sEs of fever, the sense of heat IS general all over the body and it too is discoverable by the touch of an-othe; person or by the .thc:>r~omete:r. . In all these cases it IS resistance to m~tiOn that causes the heat to appear; and the heat lS ~lways in proportion to the m~tion and the resistan?e jointly. Local inflam~atwn, such as that. of whitlows, is most common m young per~ons, m whom the circulation is quick; and fever IS more severe HEAT. 149 and ~mrning in the robust than in the weak. Some species of fevers in~eed have cold and shivering fits; but. th~se are occasiOned by the motion, as it were, shnn~mg bac~ from the. resista~ce, an~ the pulse langmshes d.urmg them, JUst .as It does m fainting. The .fire of hfe s~oulders, as 1t were, at those times; .an~ If they contmue too long the resistance is conS? bdal,ed and the. system will not react, but the pa~ Ient goes off m ~ fit." That part of the subject lS! howe.ver, very mce; and it requires to be treated With a little more of general philo-sophy than has yet been bestowed upon it, notwithstanding the number of able and eminent men by whom it has been treated. C . Whether it i~ in the living body or in any other kmd of matter, m any state where there is no resistance to motion:there is ~o production (as we call it) of h~at; ~h~t IS, there IS no heat which becomes sensible either ;to the touch of the hum~n hand or to any other test. Different kinds of matter resist differentl. y, aecm·ding to the nature of the cohesions by whtcll they are held together. Thus, some of the co~pound met~ls mel~ in the palm of a healthy person s hand, while platmum is stubborn in even the h?ttest c~mmon furnace. Some too, such as arsemc, pas~ mto vapour the instant that they are melted ; while others, such as gold, melt at a temperature . not very high, but if pure can hardly be changed mto. vapour by any ordinary heat. T~e coheswn of matter resists the motion of its particles .from each other, which is the effect that the heat mall cases tends to produce and which if urged far enough _it in every case act~ally does pro~ duce. As there Is no power of mere adhesion between mass and mass, mechanically united how sm~ll and ho~ close soever the masses m~y be, Which ~an resist the force of crystallization between the ulttmat~ atroms of the same kind of matter; so also there Is no power of crystallization that can N2 |