314 FRUIT CULTURE UNDER GLASS. 



the effects of heat and the power of gravitation on water, 

 it would be next to impossible to commit the blunders, 

 and resort to the unnecessary and expensive precautionary 

 measures, one so often meets ynth. and has to deal with. It 

 is no part of my intention to pretend to deal with that im- 

 ponderable and powerful agent called by men of science 

 caloric, but which I shall call heat — ^hypotheticaUy regarded 

 as a subtle fluid, the particles of which are to each other 

 repellent, but attractive to all substances, though in various 

 degrees. But the effect of heat upon water, an element com- 

 posed of minute and distinct particles that are supposed not 

 to have the quality or power of transmitting heat the one to 

 the other, as in the case of solid bodies, is one of the matters 

 concerning which some knowledge is indispensable in the 

 case of aU who have anything to do with heating by means 

 of heated water circulating in pipes. 



The particles of which water consists, it need scarcely be 

 said, have a capacity for heat from different sources, but most 

 manifestly so to us in this case from combustion in the fire- 

 place. Now the expansion of bodies is one of the most 

 universal effects of increasing their heat. This expansion 

 takes place to a greater degree in some bodies than in others. 

 Liquids expand much more by the same increase of heat 

 than solid bodies, and air more than either. With the 

 expansion of the individual particles of water, their specific 

 gravity becomes less ; in other words, they become Hghter in 

 proportion to their size. Here lies the whole secret of hot- 

 water circulation in pipes and boilers, and the well-known 

 law which should regulate their relative positions. The 

 heated particles of water bound upwards, and, as "nature 

 abhors a vacuum," their place is taken up by a rush of colder 

 and heavier particles. It is of very little practical use to 

 cavil about the question as to whether heat or the greater 

 specific gravity of the cold water which jostles up the warmer 

 and lighter plays the greater part in sending up and away 



