3IO FRUIT CULTURE UNDER GLASS. 



would be next to impossible to commit the blunders, and 

 resort to the unnecessary and expensive precautionary- 

 measures, one so often meets with 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 — hypothetically regarded 

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

 repellent, but attractive to aU 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 be- 

 comes less ; in other words, they become lighter in propor- 

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

 circulation in pipes and boUers, 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 greatest part in sending up and away the 

 stream of hot water. Both have a hand in it, no doubt. 

 This influence of heat upon water can be very manifestly 

 shown by filling a tumbler with cold water, and mixing with 



