IXPLOSIOXOFSTEAM-BOILEBS.] UNDULATORY FORCES. HEAT. 



at a low level, any source of heat. Much has been 

 said and written in respect to the best modes of ven- 

 tilation ; and whilst the necessity of a supply of pure, 

 and the removal of foul air, is universally recognised, 

 the different means of obtaining these results have been 

 the subjects of a large number of patented inventions. 



In many of these, the projectors have lost sight of the 

 principles, in obedience to which success can only be 

 insured. In some instances, the supply of fresh, and the 

 removal of foul air, are proposed to be effected by open- 

 ings at the upper part of a room. Despite of the 

 ingenious modes by which these arrangements are carried 

 out, their success is but partial, and can never go beyond 

 a dilution of the vitiated air contained in the room. We 

 know of no experiment which more completely illustrates 

 the true principles which should govern attempts to effect 

 proper ventilation than the following ; and a careful 

 study of its facts will be a better guide to success than a 

 volume of precepts : 



Experiment 15. Place a lighted candle on the floor 

 of a room at the open door ; and, if there is no other 

 access for air into the apartment, it will be observed 

 that a current of cold, and therefore dense, air, on 

 entering, will drive the name of the candle inwards. By 

 holding the candle midway between the top and bottom 

 of the doorway, the flame will be quite still, because no 

 current of any kind exists in that potation. If the can- 

 dle is held so as that its flame shall be near the top of 

 the door, it will be observed to be driven outwards, be- 

 cause thence the hot and lighter air makes its escape. 



FoP ring these principles, it is evident that a supply 

 of pure air must be made at the lower part of an apart- 

 ment ; and an escape for hot air should be arranged fur 

 at the top, or near the ceiling, of a room . Within the 

 last few years, openings, concealed by ornamental work, 

 have been made in most of the rooms of new houses ; 

 and, by communicating with the open air, they permit 

 the escape of noxious products, and so ensure, to some 

 extent, an amount of ventilation. 



It is impossible here to trace the variety of interfering 

 causes which oppose the application of such principles in 

 general practice ; but, in most cases, such maybe overcome 

 by attention to the rules we have laid down ; and just so 

 far as natural effects are imitated, so success will attend 

 endeavours of this kind. 



A very interesting series of effects has been lately 

 brought to light ; all of which relate, more or less, to the 

 laws of the conduction of heat by liquids and vapours. 

 We refer to the peculiar state which a liquid acquires 

 when placed in contact with a surface heated considerably 

 beyond its boiling point, and which has, to some extent, 

 explained the reason of boiler explosions in instances 

 wherein no other cause could be assigned. 



If cold water is allowed to come in contact with metal 

 heated to 2CKI 3 or 300, the liquid is speedily converted 

 into vapour, because the heat from the metal is rapidly 

 communicated to it. If, however, the metal is heated to 

 a temperature approaching red-heat, the same result does 

 not take place the liquid, in fact, will run off the sur- 

 face, just as mercury leaves a sheet of glass on being 

 poured thereon. The effect is termed the spheroidal state, 

 and only occurs when a stratum of steam or air exists 

 between a surface and a globule of liquid. Something 

 of an analogous kind occurs when water is cast on to the 

 feathers of a bird ; or, as may be often observed, in the 

 case of the dew on leaves ; which, running over them, is 

 nevertheless not in contact with them. 



If a vessel of copper, <tc., is made red-hot, and cold 

 water is poured into it, some time will elapse before the 

 liquid will boil, because a film of steam is formed, which 

 nts the conduction of heat between the metal and 

 the liquid. If, however, the vessel is allowed to cool, 

 the liquid will flash into steam so soon as a lower tem- 

 perature is arrived at. It will be seen, therefore, that 

 vapour is a worse conductor of heat than the liquid from 

 which it has been formed. 



Now, steam-boiler explosions may, in many cases, be 

 explained on the principle here named. In almost every 

 instance, the water supplied to steam-boilers contains 



earthy matter, such as lime, <fec. ; and, in the process of 

 time, this forms a " fur" on the shell of the boiler. The 

 fur being a bad conductor of heat, permits the metal of 

 the boiler to become red-hot, and this often causes the 

 earthy matter to " chip" off, and thus leaves exposed the 

 red-hot metal, apparently in contact with the water. For 

 some time no extraordinary effect results ; but if a supply 

 of cold water is sent into the boiler, the metal gradually 

 cools down, and the stratum of steam, which had kept 

 the liquid from touching the metal, being removed, an 

 immense evolution of steam takes place, and an explo- 

 sion of course at once ensues. An inexperienced person 

 would at once suggest the extinguishing of the fire as the 

 best course in such circumstances ; whereas, strange to 

 say, the only safe plan is to increase the heat until all the 

 water can be removed from the boiler by means of a 

 valve, &c. By such a course the intermediate stratum of 

 steam is maintained, and risk of explosion is diminished. 



By forming a stratum of vapour by means of sulphurous 

 acid, M. Bontigny was the first to show that water could 

 be frozen in a red-hot platina crucible. As the repetition 

 of this experiment is by no means easy to many persons, 

 we suggest one which may easily be tried, and which fully 

 illustrates the principles to which we here allude. It is 

 that of handling red-hot lead, without risk of injury to 

 the operator. 



To do tliis safely, the hand must be perfectly clean, 

 and must be coated with a " skin" of steam. This is best 

 effected by washing the hand in some strong liquid 

 ammonia, which, by its alkaline action, removes all grease, 

 <fcc., that would otherwise prevent the moisture from 

 completely wetting its surface. The lead should be red- 

 hot; as, if used at a lower temperature, severe injury 

 might be sustained. The best manner of trying this 

 singular experiment, is to pour the liquid ammonia (liq. 

 ammon. forts, of the shops) into a basin, whilst an assis- 

 tant presents the lead in a ladle. The hand is first to be 

 carefully washed in the liquid, and may then be intro- 

 duced into the red-hot metal. Any quantity may thus 

 be cast into a pail of water ; and, having repeatedly tried 

 the experiment, we may say, that the only inconvenience 

 which we have sustained is, that the hand becomes gradu- 

 ally cold, and sometimes painfully so, if the experiment 

 in much extended. 



Few persons have nerve enough to try this ; but there 

 is no danger, providing the precautions we have named 

 are fully attended to. If, however, the lead is not very 

 hot, it may solidify round the fingers, and so produce 

 great inconvenience. 



A similar experiment to the above is that of walking 

 on red-hot iron the feet being prepared in a similar 

 manner ; or they may be coated with a skin of glue and 

 sand, which, as a non-conductor, will effectually prevent 

 injury to them. 



Daily life presents us with innumerable instances 

 wherein the conduction or non-conduction of heat is taken 

 advantage of. Metallic vessels, because of their good 

 conducting powers, are employed for the purpose of heat- 

 ing liquids for manufacturing and domestic purposes ; 

 whilst the interposition of non-conductors such as the 

 glass or wooden handles of teapots, the cover of felt to 

 the cylinder and boiler of the steam-engine, and numerous 

 similar contrivances, are used for an opposite purpose. 

 In vegetable life, we have the bark of the tree ; in animal 

 life, the skin or wool over the flesh each of which, 

 as a comparatively bad conductor, retains the vital heat 

 of either system. The thick coating of snow wliich 

 falls during winter, has the effect of preventing the 

 destruction of vegetable life, through being a bad con- 

 ductor of heat. The cells of the plant, which would 

 otherwise be burst by the expansion of the ice, are 

 thus preserved uninjured, and their delicate organisa- 

 tion maintained till the returning spring. Thus, in this, 

 as in every other instance of natural laws, we find one 

 cause having a universal and beneficial operation in its 

 varied relationships. 



Having thus spoken of the effects of sensible heat, we 

 next proceed to a subject of great interest, wherein caloric 

 will be considered in its latent or hidden state. 



