The bursting of the kitchen boiler is an accident resulting from disregard of the phenomena of heated water. It sometimes happens that the hot-water supply of the various taps in the house fails. If the boiler supplying the water is a hand-fed one some one whose duty it was to fill it has neglected that duty. An empty boiler with a removable lid will do no harm, but it is not advisable to leave it empty, as the heat of the fire will destroy the iron of which it is made. No attempt, however, should be made to fill the boiler while it is hot, as the result of pouring cold water into it will be the sudden and violent conversion of the water into steam, and the person pouring in the water will assuredly be scalded. If the boiler be one that is filled automatically, one of two things has probably occurred: either the pipes are blocked by fur--that is to say by sediment from the boiled water--or the supply-pipe is frozen. In neither case is it safe to light the fire. If the pipes are blocked by fur steam will be formed in the boiler and it will burst; if the supply-pipe is frozen the heat may thaw the ice, and the inrush of cold water will at any rate crack the boiler. [Illustration: FIG. 3.] When water expands with heating convection currents are formed in it, and the hot water rises to any height we please if cold water be available to take its place. This law of convection is applied to maintain a circulation of hot water in pipes used for warming a house. The general arrangement of such a system is shown in Fig. 3. The furnace heats a boiler in the basement or on the lowest storey of the house; HB and HL' are parallel vertical pipes connected with a horizontal pipe H'H at the top of the house; C is a small cold-water cistern which is furnished with a ball-tap to maintain the supply of cold water to the pipe H'L if any water is drawn off at any part of the circuit. The short pipe A acts as a valve for the escape of air from the pipes. The pipes H'L, H'H, and HB are filled with water. When the fire is lighted in the furnace, hot water is driven up the pipe HB by cold water descending through H'L, and this circulation goes on so long as a difference of temperature is maintained in the pipes; that is, so long as the fire is burning. Any number of coils of pipes may be introduced into the circuit between the boiler and the top of the pipe HB. In filling the pipes with water allowance is made in these coils for the expansion of the water with heat and for the air which we have seen escapes from heated water, and a tap is fixed in each coil for letting out any air that may have lodged in it. If free air remains in the pipes the circulation of the water will be hindered and the boiler may become dangerously overheated. It is therefore necessary when the heating apparatus is in use to examine these taps and see that water and not air escapes from them.