In the course of a recent lecture by Dr. Tanner before the Louisville Board of Underwriters, the subject of fires caused by steam pipes and hot-air flues was discussett at considerable length. In the course of his address, Dr. Tanner spoke as follows:

James Braidwood, who was for many years chief of the London Fire Brigade, made the startling statement in 1846 that it was his belief that " by long exposure to heat not exceeding 212° F. timber is brought into such a condition that it will fire without the application of light. The time during Which this process will go on until it ends in spontaneous combustion is 8-10 years, so that a fire might be hatching in a man's premises during the whole time of his lease without making any sign/' Among the many instances cited by Braidwood in support of this statement, is one to the effect that a fire in the Bank of England was traced to a stove which was resting on a cast-iron plate 1 in. thick, this in turn resting on concrete 2 1/2 in: thick, which was supported by wooden joists, the joists under the stove igniting. If this is a cause of fire, then the majority of houses heated by means of steam, hot water, and hot air are in constant danger of fire from spontaneous combustion, since the general impression prevails that the pipes and flues for heating can with impunity be placed in contact with timber.

In examining this cause of fires, the first question is whether wood will char at as low a temperature as 212° F. In tearing down houses for the purpose of rebuilding, the timber in contact with the heating pipes anJ flues has often been found charred. Charcoal is made for certain purposes in the arts at 300° F. As the result of experiments performed in the laboratory, small pieces of white pine heated a few hours in an air bath at a temperature of 300° F., were partially converted into charcoal. Considering these facts, it must be admitted the temperature of 212° F. is sufficient, if applied for a long time, to convert wood into a partially burned charcoal. Accepting this as a fact, the next point to consider is the degree of heat at which charcoal will ignite. Made from the same wood at different temperatures, the product ignites accordingly; that is, if made at a low heat, it fires from a correspondingly low temperature. It has been determined experimentally that charcoal for making powder, when made at 500° F., would fire spontaneously at 680° F., and when wood has been carbonised at 260° F., a temperature of 340° F. only was required for spontaneous ignition.

Under certain circumstances, charcoal at a temperature of 500° F. even will ignite when heated to 212° F.

So far the discussion of heating pipes and flues as a cause of spontaneous fires has been upon the false idea that they are never heated beyond 212° F. Under the ordinary pressure of the atmosphere, as when water is heated in the open air, it boils at 212° F., but if it is heated under pressure, the boiling temperature increases accordingly; for instance, water boiling at a temperature of 212° F. is under a pressure of 147 lb., equal to a column of water 1 in. square and about 30 ft. high; if the pressure is increased to two atmospheres, the temperature required will increase to 249° F., and so on, so that when a steam gauge registers 60 the actual pressure is 75 lb., and the temperature at which the water is boiling as high as 307° F. The higher the house, the greater must be the pressure, and hence the higher the temperature at which the water boils, and it follows that the pipes must heat accordingly, and it is stated that in some systems of hot water heating the pipes have the water started through them at a temperature of 350° F.

By the system of low-pressure steam heating, which is far the most generally used, the pressure is only 5-7 lb. above that of the atmosphere, with a corresponding temperature of 228° to 235° F. Then, when furnaces are used for heating, the temperature in a flue has been found to be 300° F., at a distance of 50 ft. from the fire. Couple these figures with those given in reference to the heat necessary to produce charcoal and cause its ignition, and it must be admitted that these pipes and flues for heating are responsible for many fires. The application of these facts is as follows: After long exposure, the wood in contact with the heating pipes and flues is changed on the surface to charcoal. During the warm season this charred surface absorbs moisture from the air; then in the fall comes a cold spell and heat is turned on, when the moisture is driven from the pores of the charcoal, leaving it in a condition to readily absorb gases. The cold abates and the heat is lowered; fresh air in abundance then passes into the confined spaces where the pipes are generally placed, rapid absorption of oxygen from the air by the charcoal follows, with heating and spontaneous firing, as already explained.

The body of the timber is heated, and this heat prevents too rapid cooling of the charred surface when the fresh air passes in, otherwise the charcoal would be placed under circumstances unfavourable to ignition. The experiment of burning iron filings in the flame of a spirit lamp illustrates the influence of division upon the igniting point; now, if the iron is in a pulverulent slate, as when made by hydrogen, it will, when freshly made, ignite to a red heat when shaken into the air. Then, if it is true, as stated by an English scientist, that the oxide of iron, if placed in contact with timber and excluded from the air, and aided by a slightly increased temperature, will part with its oxygen and be converted into very finely divided particles of metallic iron, here is another cause of fires from heating pipes. For during the summer the pipes rust, and then when heated the rust is i educed, leaving the metallic iron in the same condition as that made by hydrogen.