POLMATSE METHOD OF HEATING HOTHOUSES. 



53 



of fuel, but by the quantity of atmospheric air enabling the fuel 

 to burn ; but the question with which I am concerned is one of 

 diffusion, and this is all important ! We have three forms of 

 matter, each having properties enabling them to diffuse heat : 

 solids, liquids, and gases. Which of these instruments of diffu- 

 sion does science teach us to choose ? Heat is diffused through 

 solids only in one way, namely, by the contact of one particle 

 with another ; this is called conduction. This property is pos- 

 sessed by all solid bodies ; but in some it is very considerable, in 

 others slight, giving rise to the term conductors and non-con- 

 ductors ; it is a comparatively slow process. Heat is diffused 

 through liquids slightly as in solids, from the contact of their 

 particles ; but there exists another means of diffusion. The 

 form of matter depends on the cohesion of its particles : if this is 

 great they are solid, if less they are liquid, and where wanting 

 they are gaseous ; in these two last forms of matter the particles 

 or molecules are so far removed as to be enabled to circulate 

 among each other, so that when set in motion currents are pro- 

 duced in liquids, and drafts in air ; thus, when heat is applied to 

 the upper surface of a liquid, it heats it very slowly, only by 

 contact from particle to particle, but when ap'plied to the lower 

 surface, it heats it with great rapidity, because those particles 

 which become heated first necessarily expand ; expanding, they 

 become specifically lighter; becoming lighter they ascend, while 

 the heavier particles descend ; for these currents are unceasing 

 till the entire liquid becomes of the same temperature through- 

 out ; and it is in consideration of this principle that man has 

 taken a liquid for the purpose of diffusing heat ; it is this which 

 enables the hot water to perform its office. Hot water is essen- 

 tially a go-between — something to take the heat absorbed by the 

 solid body (namely, the bottom of the boiler) to the air. Heat 

 is diffused through gases, perhaps not at all from particle to par- 

 ticle, as these are so far removed from each other in this form of 

 matter, but it is diffused through them with extraordinary rapi- 

 dity; for owing to their very slight cohesion, their particles are 

 free to move with a rapidity unknown to the liquid form, a 

 rapidity we acknowledge in our expression " as swift as the 

 wind." Gases also, being transparent, allow the free transmission 

 of radiant caloric. Which of these forms of matter, then, shall 

 we employ for the diffusion of artificial heat ? Shall we take the 

 solid, with one portion in the fire, the other in the place we 

 desire to warm, allowing the heat gradually to progress, particle 

 by particle, by the slow process of conduction? It is folly, ex- 

 claims the philosopher ! Certainly, cries the advocate of hct 

 water ! for a liquid will diffuse the heat by the internal move- 

 ments of its particles with great rapidity. But the philosopher 



