22 



HEAT. 



piece of metal, about one and a half inch 

 in diameter, and eight inches long; wrap- 

 ping a piece of clean writing paper round 

 the metal, so as to be in close contact with 

 its surface, and then holding the paper in 

 the flame of a spirit lamp : it may be 

 held there for a considerable time, with- 

 out being in the least affected. Wrap a 

 similar piece of paper round a cylindrical 

 piece of wood of the same diameter, and 

 hold it in the flame ; it will very speedily 

 burn. When the paper is in close con- 

 tact with the metal, the heat which is 

 applied to it in one particular part cannot 

 accumulate there; but enters into the 

 metal, and is equally diffused through 

 its substance, so that the paper cannot 

 be burned or scorched until the metal 

 becomes very hot: but when paper is 

 wrapped round wood, the heat that is 

 applied in one particular part, not being 

 able to enter into the wood with facility, 

 accumulates, in a short time, in suffi- 

 cient quantity to burn the paper. 



Sand conducts heat so slowly, that the 

 red hot balls used at Gibraltar in re- 

 pelling the attack of the Spaniards, were 

 conveyed from the furnaces to the bas- 

 tions, in wooden wheelbarrows, having 

 only a layer of sand between them and 

 the balls. 



Solid substances conduct heat in all 

 directions, upwards, downwards, and 

 sideways, with nearly equal facility. 



A set of experiments was made by 

 Richman, with a view to ascertain if 

 any relation existed between the con- 

 ducting powers of bodies and their other 

 properties. He took hollow balls of the 

 metals, equal in size to each other, and 

 having the bulb of a mercurial ther- 

 mometer inclosed in each. The balls 

 having been immersed in boiling water 

 until each thermometer attained the 

 same temperature, they were then ex-, 

 posed to the air, and the times of their 

 cooling observed : the differences in this 

 respect were considered as marking 

 their differences of conducting power. 

 The metals which appeared to have the 

 greatest power of retaining heat were 

 brass and copper ; then iron, tin ; and 

 lead the least of all. The decrements of 

 temperature in a given time, in the me- 

 tals above mentioned, being as follows : 

 lead, 25 ; tin, 1 7 ; iron, 1 1 ; copper, 1 ; 

 brass, 10, he considered himself jus- 

 tified in inferring, from his experiments, 

 that the increments and decrements (or 

 increases and decreases) of temperature 

 in the bodies upon which he experiment- 

 ed, are not in the inverse ratio of their 



density, of their hardness, of their cohe- 

 sion, nor in any compound ratio of these. 



Rods of different metals, of the same 

 length and diameter, were dipped by In- 

 genhpuz into melted wax, by which they 

 acquired a coating of that substance. 

 When cold, they were plunged to the 

 depth of about two inches into heated 

 oil, and the conducting power was in- 

 ferred from the length of wax coating 

 melted in a given time. Silver, accord- 

 ing to these experiments, is the best 

 conductor ; then gold, tin, copper, pla- 

 tinum, steel, iron, and lead. These ex- 

 periments, however, are not considered 

 as perfectly accurate. The experiments 

 of Meyer, of Erlangen, by whieh he 

 endeavoured to ascertain the conducting 

 powers of different kinds of wood, ap- 

 pear to be subject to so many causes of 

 error, that the results obtained by them 

 can scarcely be depended upon. 



The following TABLE gives the results 

 which he obtained; the conducting 

 power of water being made the standard. 



Conducting Specific 

 Power. Gravity. 



Water ,10 1 . 000 



Ebony wood 21.7 1.054 



Apple tree 27.4 0.639 



Ash 30.8 0.631 



Beech 32.1 0.692 



Hornbeam 82 . 3 . 690 



Plum tree 32 . 5 . 687 



Elm 32 . 5 . 646 



Oak 32 . 6 0.668 



Pear tree 33.2 0.603 



Birch 34.1 0.608 



Silver fir 37 . 5 . 495 



Alder 38.4 0.484 



Scotch fir 38.6 0.408 



Norway spruce 38.9 0.447 



Lime 39.0 0.408 



Experiments were made by Count 

 Rumford, for the purpose of inves- 

 tigating the fitness of various sub- 

 stances, as articles of warm clothing. 

 That philosopher suspended a ther- 

 mometer in a cylindrical glass tube, the 

 end of which had been blown into a 

 bulb, l^ 3 inch in diameter, placing the 

 bulb of the thermometer in the centre 

 of the larger bulb, surrounded with the 

 substance, the conducting power of 

 which was to be ascertained. Prepared in 

 this way, the apparatus was heated by 

 being plunged into boiling water, and 

 afterwards cooled by being plunged in a 

 mixture of pounded ice and water ; and 

 the number of seconds was accurately 

 marked, which the thermometer required 

 in each experiment to cool from 70 to 

 1 of Reaumur. 



