Let us consider first the effect of heating water. If water at the ordinary temperature be poured into a vessel which is placed on a fire or other source of heat the water at the bottom of the vessel will be warmed and will expand; it will therefore be lighter, bulk for bulk, than the water nearer the top of the vessel. The cold water will therefore descend, and the warm water will rise. All ordinary water contains air; presently the air in the water will become visible as small bubbles which rise to the surface of the water and escape noiselessly into the atmosphere. As more heat is applied some of the water in the bottom of the vessel will be formed into steam, and bubbles of steam will expand and rise into the cooler water above and collapse there with a rattling noise which is characteristic of the state known as simmering. These bubbles of steam rising and bursting aid the convection currents in stirring and mixing the water so that it presently becomes of even temperature throughout. When this occurs the bubbles of steam rise to the surface and burst explosively into the atmosphere, throwing the water violently about; the water is then boiling. It is an important point to remember in cookery that boiling water will not 96 become any hotter with the application of more heat, but it will “boil away;” that is, it will be completely converted into steam. The steam resulting from any volume of water occupies a space 1700 times that of the water from which it is produced, but what concerns the housewife most seriously is that the change of water into steam is accompanied with the evolution of tremendous mechanical force that will burst any vessel in which the water is enclosed. It is the fact of this tremendous exercise of mechanical force that has led to serious accidents when hot-water bottles have been put into the oven to keep warm. It has been assumed by some people that if the hot-water bottle be not completely filled, that if what they consider to be sufficient room is left for the expansion of the water, no harm can result from putting the bottle into the oven, but no arrangement can make such a course safe.
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 97 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.
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. 99 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.
The installation of a heating apparatus in middle-class houses is fairly common, and where one is not found many persons use gas or oil stoves in the passages in the winter, for it is now realised that it is not possible to heat rooms by means of open fires without creating cold draughts in them from the cold passages into which they open. And, moreover, the constant change of temperature encountered in passing from one warm room to another through cold passages is not only disagreeable, but is not found to be conducive to health.
Let us turn to the cooling of water. Water expands about one-eleventh of its volume on becoming ice. This change of state, like that of change into steam, is accompanied by the evolution of tremendous mechanical force. If water freezes in pipes it bursts the pipes, and on a thaw taking place the pipes are found to leak. The appropriate remedy for this state of things is to protect the pipes from cold or to empty them when a frost is apprehended. In all properly built houses there is a tap by means of which the water supply can be cut off from the house, thus allowing the pipes to be emptied on a frosty night. The custom of leaving the taps dripping is effective, because the pipe is generally liable to freeze at some particular point where it is in immediate contact with the cold air, probably in the unclosed 100 chink where the pipe passes through the wall; keeping the water moving in the pipe prevents any part of it getting cold enough to freeze, but the practice should not be resorted to, as it wastes water.