VAUIOUS METHODS OF HEATING DESCRIBEl IN DETAIL. 213 



over one third or half full ; the less water that is in them the 

 better, compatible with safety. To work well, the water ought 

 never to boil into them, otherwise the force of the steam will 

 expand the metal, and, if the heat continue, it is liable to burst 

 Fig. 44 represents a house heated with a tank made of galvan- 

 ized zinc. Next to the boiler there is a short piece of cast-iron 

 pipe which prevents the zinc from being affected by the imme- 

 diate action of the fire. The house from which this sketch was 

 taken has been in use for some years, and has given perfect sat- 

 isfaction, while the original cost was very small. The principal 

 objection to the use of this material for heating purposes, is, as 

 I have already stated, the rapidity with which it is heated, and 

 the rapidity with which it parts again with its heat. This cir- 

 cumstance renders it a good conductor, but a bad retainer, of 

 heat; useful where speedy and immediate action is required, but 

 useless where a slow and long-continued radiation is necessary 

 at a very low temperature, as, for instance, for bottom-heat, 

 for propagating-beds, and for plant-stoves. In such circum- 

 stances, we should decidedly prefer wood, particularly for the 

 first-mentioned purposes. In green-houses, and even in forcing- 

 houses, it may answer well ; for it must be admitted that the 

 source of heat must ever be looked for at the boiler, not in the 

 material of which the tanks or pipes are made. And, although 

 the advantage of employing a material that will absorb the heat 

 given off from the source to any extent, and part with it gradu- 

 ally, must be apparent, at least, when it is an object to take 

 advantage of the heat so absorbed, store it up, so to speak, with 

 the view of employing it when the action of the apparatus 

 becomes enfeebled. The law by which this is effected is the 

 same as that by which the two bodies of water become equal- 

 ized in temperature by admixture as described on page 210. 

 This is the law of equalization, which constantly tends to bring 

 all bodies to an equal temperature. If, for instance, the hot water 

 from a boiler be admitted into two separate tanks, one of wood 

 and the other of zinc, then, by placing the palm of the hand upon 

 the wooden tank, it will feel agreeably warm, while the zinc oi 

 tin one would be quite unbearable, if not burn, and this while 

 the temperature of the water in both tanks was the same. The 



