Heat – Its Nature and Effects
Our senses give us very inaccurate notions about heat; for instance, tepid water would be called hot by a person who had just been bathing in cold water, but it would be called cold by the same person if he had just taken a hot bath.
In order, therefore, to measure temperature accurately we must find some property of bodies which varies constantly with constant change of temperature. Now nearly all bodies expand when heated & contract when cooled, and with certain bodies this expansion or contraction is regular enough to be used as a measure of difference of temperature, e.g. the ordinary thermometer measures temperature by the expansion of the quicksilver inside its bulb.
By the application of heat, liquids expand to a far greater extent than do solids, gasses much more and with greater regularity than liquids. If we add equal weights of water and of mercury, each at 100 0 C, to separate equal quantities of cold water, the hot water is found to have much greater heating power than the mercury. The only rational explanation of this phenomenon is that though the mercury and hot water were at the same temperature and of the same weight, they contained different quantities of heat. We may thus measure the relative capacity for heat of any body by comparing the heating power of a given weight of the body, at a stated temperature with that of an equal weight of some standard body at the same temperature.
In order to melt 1lb of ice without raising its temperature it is found that as much heat is needed as will warm 1lb of water from 0 0 to 80 0 . The heat which is thus absorbed by any body in melting, and which does not raise the temperature of the body, is called the Latent Heat of Fusion of that body. Heat is also rendered latent when a liquid is vaporized. And this heat is of course given out when the vapour becomes liquid; hence the great heating power of steam.
Heat may be transferred from one place to another either by (1) Conduction, (2) Convection, (3) Radiation. In conduction heat passes from particle to particle of the bodies in which it acculates, [sic] as for instance when one end of a wire becomes hot owing to application of heat at the opposite end. Different bodies have very different powers of conducting heat.
In Convention hot particles of the body pass from point to point in the body itself, owing to their heated state, and thus give up their heat to the cooler portions of the body. From the nature of the case, convection can only take place in liquids and gasses.
In Radiation a hot body warms cooler bodies placed at a distance from itself; in this way, for example, the suns heat is transferred to the earth. Since radiant-heat and light obey the same laws they are probably due to similar causes.
The first hypothesis put forward as to the nature of heat, regarded heat as an invisible substance called Calorie. Now if heat be a substance it must differ from all other substances, in that it has no weight, for a given body weight the same whether hot or cold, and in that every body can be made by friction to yield an infinite quantity of it. Davy, by a series of experiments shewed the fallacy of the calorie hypothesis and since his time the researches of Joule, Hirw, and others have shown that Heat Mechanical work, Electricity, etc. are only mutually convertible forms of energy, and that a constant numerical relationship exists between them. The phenomena of heat are most easily explained upon the assumption that heat consists merely of a rapid vibration of the particles of the heated body, and that this vibration can be transmitted through space as radiation.
[A note added by styles to the envelope that contains these notes, reads ‘Mr. Easterfield’s Paper’]
[there are many similarities with the above notes and the paper reported below that was read by Easterfield on the 13th January, 1886, which appears in Minute Book One as a paper cutting, entitled “Heat, what it is and what it does”. It might be a first draught in the preparation of this paper.]
Mr. T.H. Easterfield scholar of Clare College, Cambridge, then read a paper on “Heat, what it is and what it does.”
Mr. Easterfield began his paper by pointing out that though all people knew the difference between hot and cold weather, no person’s senses could be relied upon to tell accurately how much hotter one day was than some other day, or one body than some other body.
He then showed by experiment that solids, liquids, and gasses all expand when heated and contracted when cooled. A red hot iron was fixed tightly in a cast-iron frame; upon cooling, the contraction of the bar broke the frame in which it was placed. He then briefly described how thermometers are made and graduated, and explained the differences between the thermometer scales in common use.
The lecturer then passed on to the subject of quantity of heat, and pointed out that two different bodies, though of the same size and temperature, might contain different quantities of heat. This he proved by placing bullets of lead and zinc at the same temperature upon a cake of wax. The zinc contained more heat than the lead and quickly dropped through the wax cake, but the lead did not posses heat enough to melt a hole through the wax.
Speaking of latent heat or the heat absorbed by a body upon changing its state, he showed that steam had a much greater heating effect than boiling water. He then explained that heat might be conveyed away from a body in three different ways, and that all bodies did not conduct heat at the same rate which was proved by experiments with bars of wood, iron, and copper.
Mr. Easterfield went on to say what heat was, and pointed out the errors in caloric theory, which was formerly so popular. He then alluded to experiments by Sir H. Davey, Joule, Hirn, and others which proved that mechanical work could be converted into heat, and the heat back again into the same amount of mechanical work.