32 



HEAT. 



different circumstances, illustrative of 

 the radiation of heat. 



Count Rumford found that a thermo- 

 meter suspended in a Torricellian vacu- 

 um, required ten minutes twelve seconds 

 to cool from 190 of Fahrenheit to 68. 

 When surrounded with atmospheric 

 air confined in a vessel, having the 

 same extent of space as the vacuum, 

 it required only six minutes eleven 

 seconds : proving that the air facili- 

 tates refrigeration. 



Two hollow tin globes, one of which 

 was painted with lamp-black, and the 

 other left bright, were filled with warm 

 water and exposed by Mr. Leslie to 

 the influence of air, in different states 

 of motion. A gentle breeze occasioned 

 the bright globe to lose half its heat 

 in 44 minutes, the painted one lost the 

 same quantity in 35 minutes. Exposed 

 to a strong breeze, the times of cooling 

 were 23 minutes and 20! minutes ; and 

 a vehement breeze occasioned them to 

 cool in 9 minutes and 9 minutes. 

 The influence of currents of air in ac- 

 celerating the cooling of bodies, is 

 clearly shown by these experiments. 



Mr. Leslie filled a hollow globe of 

 tin, having a thermometer inserted in 

 it with warm water, and marked its 

 time of cooling, from 35 of the centi- 

 grade scale to 25 of the same scale, 

 to be 150 minutes; when the globe 

 was covered with lamp-black, it re- 

 quired only 81 minutes to cool the 

 same number of degrees. 



A tin vessel, covered with a thin 

 coating of isinglass, lost its heat by 

 radiation much more rapidly, and the 

 rapidity of the cooling was greater in 

 proportion, as the coating of isinglass 

 was thicker. 



When heated bodies are immersed 

 in water, the nature of their surfaces 

 does not affect their rates of cooling, 

 because radiation does not take place 

 under the surface of that fluid. 



The times of cooling heated bodies 

 was found to be different in different 

 gases. Mr. Dalton shows that a ther- 

 mometer immersed in 



Seconds. 



Carbonic acid gas, (cooled in) 112 



Sulphuretted hydrogen and ni-~) lnn 



trous oxide J 



Olefiant gas 100 



Common air, azotic, and oxy-1 inn 



gengas . . J 



Nitrous gas 90 



Coal gas 70 



Hydrogen ...,,,.,.., 40 



The times of cooling are different, 

 also, according to the degree of density 

 of the gas in which a body is immersed, 

 the cooling proceeding most slowly 

 when the density is least. Mr. Leslie 

 ascribes the different rates of cooling 

 in the different gases, to the different 

 conducting powers of these gases. 



The radiating power of different 

 substances in atmospheric air, are 

 stated by the same philosopher as 

 follows. 



Seconds. 



Lamp-black ]00 



Writing-paper 98 



Sealing-wax 95 



Crown-glass 90 



China-ink 88 



Ice 85 



Red-lead 80 



Plumbago 75 



Isinglass 75 



Tarnished lead 45 



Clean lead 19 



Iron, polished 15 



Tin-plate 12 



Gold, silver, and copper 12 



Several important suggestions arise 

 from a review of the doctrines of radiant 

 heat. Whenever it is necessary to the 

 complete success of any operation that 

 the heat of a fluid should be retained 

 for a considerable length of time, the 

 vessel containing that fluid should have 

 bright metallic surfaces, as such sur- 

 faces have least radiating power : thus, 

 water in bright metallic coffee or tea 

 pots will be more effectual in extracting 

 the strength of tea and coffee than it 

 would be if contained in vessels of any 

 other kind. 



In heating an apartment with 

 steam, it would be absurd to use black 

 pipes for conveying the steam ; be- 

 cause, in that case, much of its heat would 

 escape by radiation, before it arrived 

 at its place of destination: the pipes 

 should be of bright metal. It would 

 be equally absurd to make the pipes, 

 intended to distribute heat to the air 

 of the apartment, bright, because such 

 pipes would defeat the object in view 

 by retaining the heat ; black pipes 

 would here answer the best. 



Vessels intended to receive heat in the 

 operation of cookery and in those of the 

 arts, should not be bright, because 

 bright surfaces reflect and do not ab- 

 sorb heat ; and it may be considered as 

 a useful property of the fuel which we 

 generally use, that it blackens the sur- 

 faces of metallic vessels in heating 

 them. 



