HEAT. 



Common Salt 6.25 + water 98.75 



Epsom Salt 41. 



do. 

 do. 

 do. 

 do. 



58 4 

 87.5 

 95.84 

 58.4 



Nitre 12.5 + 



Common Salt 4.16 + 



Copperas 41.6 + 



Vinegar 



Sulphate of Zinc 53.3 + water 46.7 



Milk 



Water . . 



25. 5 



25.5 



26 



27.5 



28 



28 



28.6 



SO 



30 



The rays of the sun constitute the 

 most important natural source of heat. 

 Various opinions have been entertained 

 respecting the cause of this kind of 

 heat. Sir Isaac Newton, influenced by 

 the obvious analogy existing between 

 the sun and terrestrial bodies rendered 

 luminous by heat, believed the sun to 

 be an intensely hot body, having the 

 power of projecting hot particles from 

 its surface, which when they come into 

 contact with other bodies impart heat 

 to them. Another hypothesis was main- 

 tained by Descartes and Huygens, and 

 adopted by many other philosophers, 

 which supposes that luminous bodies 

 have the power of propagating vibra- 

 tions through an extremely rare and 

 elastic fluid that is diffused through all 

 space ; and that heat is occasioned by 

 these vibrations. 



Several circumstances influence the 

 degree of heat communicated to diffe- 

 rent parts of the earth's surface by the 

 solar rays ; the chief of these are eleva- 

 tion, distance from the sea, and more 

 particularly latitude. The extremes of 

 temperature over ihe globe, are com- 

 prehended within a range of about 160 

 of Fahrenheit: the heat in the shade 



having been observed in tropical climates 

 to attain 110, as at Pekin; at Pondi- 

 cherry it has risen to 115; while at 

 Hudson's Bay the spirit thermometer 

 has sunk to 50, ten degrees lower than 

 the freezing point of quicksilver. 



The range of temperature capable 

 of being produced by art is much 

 more extensive. By reference to the 

 preceding tables, it will be seen that 

 the greatest degree of cold, hitherto 



E reduced, is 91 below zero of Fahren- 

 eit ; while the highest degree of heat 

 attempted to be measured, is that of 

 Mr. Wedgwood's small air-furnace, 

 which he considered equal to 21,877 

 of the same scale. More intense heat 

 may be produced by the agency of a 

 powerful voltaic combination, and by 

 enflaming a condensed mixture of 

 oxygen and hydrogen gases, issuing from 

 a small aperture. 



Heat may be artificially produced 

 by electricity, galvanism, condensation, 

 friction, percussion, and chemical action, 

 but the full consideration of these, to- 

 gether with ignition, and the economy 

 of fuel, will afford abundant useful 

 and interesting matter for another trea- 

 tise. 



CHAPTER XI. 



Books relating to Heat. 



The books which have been written on this subject are very numerous, and of very 

 various degrees of merit. The following list contains a selection of the most useful. 



The Philosophical Transactions contain many important Essays by Black, Blagden, 

 Cavendish, Darwin, Delisle, Count Rumford, Smeaton, Walker, Wedgwood, and others. 



The Annales de Chimie are rich in original communications on heat ; and many are 

 contained in Nicholson's Journal, the Philosophical Magazine, the Manchester Me- 

 moirs, the Annals of Philosophy, the Transactions of the Royal Society of Edinburgh, 

 Gilbert's Journal der Physik, Rozier's Journal de Physique, and the Journal of Science. 



Martine on Heat and Thermometers. 



Dr. Crawford on Animal Heat. 



De Luc's Recherches sur les Modifications del' Atmosphere. 



Dalton's New System of Chemical Philosophy. 



Count Rumford's Essays. 



Dr. Black's Lectures, by Robison. 



Chemical Statics, by Berthollet. 



Scheele's Treatise on Air and Fire. 



