136 



THE POPULAR EDUCATOR. 



HEAT. V. 



CONDUCTION SPHEROIDAL STATE CONVECTION RADIANT 



HEAT ABSORBING POWER POWER OF TRANSMITTING 



HEAT CONCLUSION. 



WHEN a heated body has to be handled, some non-conducting 1 

 material is usually interposed between it and the hand, so as to 

 guard against burns. Thus in most teapots an ivory ring is 

 let into the handle, for the sake of keeping it cool. Many 

 apparently strange phenomena may be explained iii this way. 

 A kettle, for instance, that has been used some time, and become 

 coated with fur outside, may be taken off the fire and placed 

 with impunity on the naked palm, even though the water be 

 boiling in it. The fur is a non- 

 conducting material, and protects 

 the hand from the heat. 



A red-hot poker likewise may 

 be safely struck with the hand. 

 This partly arises from the fact 

 that a quick blow does not allow 

 time for the metal to burn the 

 hand, and partly from the fact 

 that the moisture of the hand is 

 converted into vapour, and pre- 

 vents absolute contact with the 

 heated iron. Some remarkable 

 phenomena have been observed 

 which illustrate this fact. If we 

 take a silver vessel, and having 

 raised it to a temperature a little 

 above 212, immerse it in a vessel 

 of water, it will hiss from the 

 sudden conversion of the water 

 into steam, and will speedily be 

 cooled down. If, however, we 

 heat the vessel to redness, and 

 place it on the surface of water, 

 no effect will at first be produced. 

 It will quietly float for a time 

 without any sound being heard. 

 After a little while, however, a 

 cloud of steam will be suddenly 

 produced, and the usual hissing 

 noise will be heard. A similar 

 thing occurs if a highly heated 

 silver weight be dipped into a 

 vessel of water. 



The reason of these apparently 

 strange phenomena is that as 

 soon as the heated metal touches 

 the water, that portion which is 

 nearest to it becomes suddenly 

 converted into steam, and this 

 keeps the silver from contact with 

 the water. A layer of vapour is, 

 in fact, interposed, which pre- 

 vents actual contact. When, 

 however, the silver is cooled down 



nearly to the temperature of boiling water, the separation ceases 

 to exist, and the water comes in contact with the silver and 

 cools it. 



These effects were first observed by Leidenfrost, but have 

 since been carefully investigated by others. A simple way of 

 showing them is to take a platinum or silver dish, and having 

 placed a spirit-lamp under it so as to heat it to redness, drop 

 with a pipe a little water into it. The liquid does not spread 

 itself out and moisten the dish as it would at ordinary tempera- 

 tures, but at once assumes a globular form, and rotates rapidly. 

 Its evaporation, too, is very much less rapid than it would be if 

 it boiled, and its temperature appears only to be about 95 or 

 100. The liquid is said to have assumed the spheroidal state, 

 and will remain in this condition if the source of heat is kept 

 under the dish. If, however, it be removed, the heat will 

 gradually diminish, till it is no longer sufficient to maintain the 

 globule in the spheroidal state, and then the liquid will touch 

 the metal, and be immediately thrown into a state of violent 

 ebullition, a large amount of steam being given off. 



A remarkable experiment may be tried which will illustrate 



this fact very clearly. Procure a large melting-pot containing- 

 several pounds of lead, and place it over a fire until the lead is 

 not only melted, but quite red-hot. Having washed the hand so 

 as to free it from grease, dip it into a vessel containing strong 

 liquor ammonia, and it may then be plunged into the molten 

 metal, or the lead may be ladled out by it, without any danger. 

 The only sensation produced is one of cold. This experiment 

 is one which few have the courage to attempt, but it is per- 

 fectly safe. The heat of the metal evaporates the liquid and 

 drives out the amuioriiacal gas from it, and thus the hand is 

 entirely enveloped in a glove of vapour, which prevents contact, 

 with the lead. The cold felt arises from the rapid evaporation. 

 In performing this experiment, it is very important to have tha 

 lead red-hot, as otherwise it may 

 come into contact with the hand, 

 and a severe burn will then be 

 produced. 



After conduction, the next mode 

 in which heat is transmitted is by 

 convection, or the setting up of 

 currents in the liquid or gas to 

 be heated. By this means each 

 particle in succession is directly 

 exposed to the source of heat, 

 and thus has its temperature- 

 raised. 



There are several ways in which 

 convection may be illustrated : 

 one of the best is to take a glass 

 vessel filled with water (Fig. 25), 

 and having dropped in a few 

 fragments of litmus or a little 

 cochineal, place a spirit-lamp 

 under it, and watch the liquid. 

 A stream will begin to rise 

 directly over the lamp, its course- 

 being clearly shown by the 

 coloured particles. This stream 

 will rise to the top of the vessel, 

 where it will spread out and form 

 a down current at the sides, am! 

 in this way all the liquid will in 

 turn be exposed to the heat. 



Another way in which this cir- 

 culation may be shown is repre- 

 sented in Fig. 26. Two glass 

 tubes are bent as there shown r 

 the lower one being filled with 

 coloured water ; the other is filled 

 with clear water, and is inverted 

 into the funnel-shaped ends of the 

 first. A spirit-lamp is now held 

 to one side so as to warm the 

 liquid there, and the coloured 

 part of the liquid in that limb 

 will at once begin to rise, and to 

 descend in the other. 



On this principle the hot-water 



apparatus frequently employed for warming large buildings is 

 constructed (Fig. 27). A furnace and boiler are placed at the- 

 lower part of the building. From this a pipe, M, passes to 

 a cistern, Q, at the top, provided with a safety-valve, n; 

 and from it pipes lead to the stoves, a, b, c, d, e, f, in the 

 various rooms. The water traverses these on its way back to 

 the boiler, and gives up to them much of its heat. The water 

 heated by the furnace becomes, of course, specifically lighter, 

 and hence rises, while that which has been cooled by its pas- 

 sage through the pipes descends, and in this way a constant 

 circulation is maintained. 



Gases as well as liquids are heated by convection. The 

 trade-winds are grand natural illustrations of this fact : the 

 air having become heated by contact with the surface of the 

 earth in tropical regions, expands, and rises, making way for 

 the currents of colder air from the poles. In the higher regions. 

 of the atmosphere a current usually sets in the contrary direc- 

 tion to that on the earth's surface, and thus forms the return 

 current. Land and sea breezes are further exemplifications of 

 the same fact. 



