HEAT. 



21 



Tt has been asserted by Guyton de 

 Morveau*, that the indications of 

 Wedgewood's pyrometer are not so high 

 as they are made to appear; but he 

 has certainly erred, in supposing that 

 the red heat, at which Wedgewood com- 

 mences hjs scale, is no higher than 5 1 7 

 of Fahrenheit ; since oil and mercury 

 are both capable of indicating higher 

 degrees of heat, without exhibiting" the 

 least appearance of redness. 



In Guyton de Morveau' s pyrometert, 

 platina is used to measure high degrees 

 of heat. The instrument alluded to, is 

 formed of a mass of highly-baked white 

 clay, having a groove in it for the pur- 

 pose of receiving a rod or plate of pla- 

 tina, which, resting on the clay at one 

 end, at the other presses against it the 

 end of a bended lever, the longest arm 

 of which is made, by the expansion oc- 

 casioned by increased heat, to traverse 

 a graduated arc, and thus indicates the 

 rise of temperature. 



Dr. Ure (Chemical Dictionary, p. 

 657) is of opinion that high degrees of 

 heat may be measured by the expansion 

 of air. " Since dry air augments in vo- 

 lume 3-8ths for 180 degrees, and since 

 its progressive rale of expansion is pro- 

 bably uniform by uniform increments of 

 heat, a pyrometer might easily be con- 

 structed on this principle. Form a bulb 

 and tube of platinum of exactly the same 

 form as a thermometer, and connect, 

 with the extremity of the stem at right 

 angles, a glass tube of uniform calibre, 

 filled with mercury, and terminating be- 

 low in a recurved bulb, like that of the 

 Italian barometer. Graduate the glass 

 tube into a series of spaces equivalent 

 to 3-8ths of the total volume of the pla- 

 tina bulb, with 3-4ths of its stem. The 

 other fourth may be supposed to be little 

 influenced by the source of heat. On 

 plunging the bulb, and 2-3rds of the stem 

 into a furnace, the depression of the 

 mercury will indicate the degree of heat. 

 As the movement of the column will be 

 very considerable, it will be scarcely 

 worth while to introduce any correction 

 for the change of the initial volume by 

 barometric variation. Or the instrument 

 might be made with the recurved bulb 

 sealed, as in Professor Leslie's differ- 

 ential thermometers. The glass tube 

 may be joined by fusion to the platinum 

 tube. Care must be taken to let no 



* Guyton on Wedgewood's Thermometer, Ann. de 

 Chimic, xxxi. 171. 



f Guyton's Metalline Thermometer of Platina, 

 Repertory, ii. III. 459. 



mercury enter the bulb. Should there 

 be a mechanical difficulty in making a 

 bulb of this metal, then a hollow cylin- 

 der half an inch in diameter, with a pla- 

 tinum stem, like that of a tobacco pipe, 

 screwed into it, will suit equally well." 



Having considered the expansion of 

 bodies by heat, and the various means 

 of measuring that expansion, it seems to 

 be required, in order to give a complete 

 view of the subject, that the effects ca- 

 pable of being produced by reducing 

 bodies below their usual temperatures, 

 and the artificial modes by which this is 

 effected, should be noticed in this place ; 

 but it is believed that a still more appro-, 

 priate opportunity of entering into this 

 discussion will be found in a more ad- 

 vanced part of this treatise. 



CHAPTER V. 



Of the different powers of bodies in con- 

 ducting heat. 



To prove in a simple and convincing 

 way that heat passes through different 

 bodies with very different degrees of ve- 

 locity, it is only necessary to take slen- 

 der cylinders of different substances, as, 

 for example, silver, glass, and charcoal, 

 and while holding one end of each in the 

 hand, let the other end be held in the 

 flame of a candle ; the silver will soon 

 become too hot to hold, the glass will be 

 much longer in being heated, and the 

 charcoal will be ignited (or red-hot) at 

 one end, long before any sensation of 

 heat is felt at the other. The substances 

 that become hot soonest at the end 

 farthest from the flame, are said to be 

 the best conductors of caloric. 



The densest bodies are generally the 

 best conductors ; but there is no inva- 

 riable relation existing between the den- 

 sity of a body and its conducting power ; 

 as the densest of the metals, platinum, 

 is one of the worst of metallic con- 

 ductors. Earthy substances are much 

 inferior to metals in their conducting 

 power ; wood is still more so ; but the 

 solid substances that have the least 

 conducting power, are those which con- 

 stitute the coverings of animals, as wool, 

 hair, and feathers. Hence the great use 

 of even small portions of such substances 

 in preventing the heat of animals from 

 being carried off by the cold ah' ; in 

 other words, keeping them warm. 



Ex. The difference between the con- 

 ducting powers of metal and wood may 

 be strikingly shown, by taking a smooth 

 cylindrical tube, or still better a solid 



