HEAT OR CALORIC. 07 



serve our animal heat from escaping. Loose garments are warmer 

 than those that are tight, because they imprison the air, and the same 

 weight of clothing, in two or more thicknesses, is warmer than in one ; 

 hence the advantage of lining and quilting, as in comfortables,* down 

 coverlets, &c. 



The finer the fibres, the more effectual they are ; therefore an- 

 imals are provided with fur which is finest in the coldest countries, 

 and in winter it is finer than in summer ; in aquatic birds and am- 

 phibia, the fur and feathers are finer than in the terrestrial races. 

 Fine wooled sheep would, in torrid climates, become coarse wooled. 

 Some covering of this nature is necessary even in hot climates, to 

 protect animals from the copious dews and rains, and other atmos- 

 pherical changes. 



(k.) Ice is a bad conductor and snow still worse. Hence ice 

 retards the congelation of the water below; snow protects the 

 grass and grain from destruction by severe cold ; it differs from ice 

 because it imprisons air in its cavities. When the air in Siberia was 

 70, the earth under the snow was only 32^. Snow huts or holes 

 so often used by travellers in cold countries, as in the north western 

 regions of America,-)- are very warm. 



(L.) FLUIDS^ ARE WORSE CONDUCTORS THAN ANY SOLIDS. 



The common impressions on this subject are erroneous ; fluids 

 are usually heated at bottom, and the change of specific gravity throws 

 them into currents ; warm currents flow upward and cold down- 

 ward, and thus the heat is soon diffused. " If a thermometer be 

 placed at the bottom and another at the top of a tall jar, the heat be- 

 ing applied below, the upper one will begin to rise almost as soon as 

 the lower." Turner. 



Heat, applied at the surface, travels downward very slowly. Mr. 

 Murray provided a cylindrical vessel of ice ; he froze a thermometer 

 in at right angles to the side, and near the top, filled the vessel with 

 oil and applied heat on the surface ; there was no conducting pow- 

 er in the sides, || but the thermometer proved that the heat did travel 

 down, although with extreme tardiness : therefore fluids are not non 

 conductors, but only very bad conductors. 



In solids, the particles are stationary or only recede from each 

 other, and the heat travels ; in fluids, their own particles travel and 

 transport the heat. 



(M.) GASES, AIR, VAPORS, AND ALL AERIFORM FLUIDS, ARE THE 



WORST CONDUCTORS KNOWN. 



* A name given in this country, to a bed covering made in the manner described in 

 the text. t Captain Parry, and Am. Jour. Vol. X11I, p. 391. 



t Except mercury and melted metals generally. 



Nicholson's Journal, 8vo. Series, Vol. I, p. 241. 



|j Ice is a conductor, although a bad one, at all temperatures below 32 ; as it melts 

 at that degree, it follows that in this experiment, any heat derived from the hot fluid, 

 would go only to melt the ice, but would not travel down its sides. 



