July 6, 1888.] 



SCIENTIFIC NEWS. 



19 



tablet eighty years before his accession, and cannot, 

 therefore, have been invented by him. The Babylonian 

 name of this coin, darikii, is probably derived from the 

 Greek dont, a lance, on account of its having on it the 

 representation of an archer. 



THE CHEMISTRY OF FIRE AND FIRE 

 PREVENTION. 



Abstract of a Lecture delivered by Mr. W. Lasceixes- 

 Scott, before the Balloon Society, on June 8th. 



FIRE, or, to use a more appropriate term, combustion, 

 is simply the result or accompaniment of chemical 

 action, be it combination or the reverse, of so intense a 

 kind that the substances immediately concerned are 

 raised and maintained for a time at an exceedingly high 

 temperature — the temperature of flame, or, popularly, 

 that of a white heat. 



" Combustion," however, may be very slow or ex- 

 tremely rapid according to circumstances, the former 

 being well illustrated by the rusting, or oxidation of 

 masses of iron, lead, zinc, etc., in the air, and the latter 

 by the oxidation of the same metals when reduced to a 

 state of fine division, as by powdering or filing, and then 

 subjected to heat. 



If a piece of sheet lead be scraped clean we know per- 

 fectly well that the surface will soon become dull again 

 on account of the formation of a film of oxide thereon, and 

 the same thing happens with iron, zinc, and many other 

 substances ; combination takes place with a little of the 

 oxygen of the air (or of water, as the case may be), an 

 oxide is formed, and " combustion " occurs. Of course 

 the heat produced is quite inappreciable, but it is pro- 

 duced all the same, and we have only to accelerate the 

 oxidation in order to render this quite evident. It is 

 difficult to oxidise a bar of zinc or an iron rod with suffi- 

 cient rapidity in the flame of a candle or spirit lamp to 

 cause it to burn, because the heat communicated to any 

 one particle of metal is conducted away by the mass so 

 quickly that that particle does not rise to the temperature 

 at which the oxidation can proceed with sufficient rapidity 

 to produce light and heat. If, however, this cooling be 

 prevented by exposing to heat and oxygen enormously 

 increased surfaces of these same metals, with compara- 

 tively little mass, rapid oxidation, even to the extent of 

 energetic combustion, takes place. This may be well 

 shown by sifting iron filings into the flame of a spirit lamp. 



This question of relative " surface " is worth a little 

 more consideration ; it is, of course, quite impossible that 

 a block or sheet of bright metallic lead when exposed to 

 the air should oxidise so quickly as to become heated to 

 to the point of ignition, and the same remark applies to 

 carbon or charcoal, but if either or both of these 

 elements be so prepared as to exhibit, so to speak, an 

 infinitely large surface and an infinitely small mass, ex- 

 posure to oxygen, or even to air, at the normal tempera- 

 ture, causes immediate ignition of the whole. Phos- 

 phorus, again, when exposed to the air, at low tempera- 

 tures, although it oxidises more quickly than the sub- 

 stances we have just been considering, seldom accumu- 

 lates so much heat as to inflame; but by dissolving it 

 in a certain manner and applying the solution to a porous 

 substance — such as a bit of blotting paper, for instance — 

 the menstruum would quickly evaporate and leave the 

 phosphorus so minutely divided that the heat resulting 

 from its rapid oxidation speedily causes it to burn. It is 



perhaps to such causes that some of the " suspicious 

 fires " referred to lately by Mr. Peter McLagan are due. 



The lecturer then referred to the modifications in the 

 properties of bodies by the substitution of some other 

 element or group of elements for one of the constituent 

 elements of the compound, and instanced the case of 

 cellulose and its three nitro-derivatives. He said 

 nothing done in this direction had succeeded in making 

 fabrics uninflammable, but it has long been known that by 

 impregnating timber, textile fabrics, and other inflammable 

 substances superficially, or throughout their substance, 

 with certain chemical salts, they could be rendered less 

 easy to burn, and a variety of saltshave been recommended 

 for this purpose. Hitherto these compounds have not 

 proved very successful in practice. These chemical salts 

 render canvas, muslin, etc., less inflammable in several 

 ways dependent either upon the water they contain, the 

 vapours they give off when heated, or the manner in which 

 they surround the particles or encrust the fibres with in- 

 combustible mineral matter. As typical examples of salts 

 of this kind he cited silicate of sodium (the so-called 

 " water glass "), tungstate of sodium, phosphates and car- 

 bonates of sodium and ammonium, borax, and ordinary 

 alum. Some of these — and particularly certain mixtures 

 of two or more — give fairly good results — for a time. 

 Then comes the reckoning. Mr. Scott said probably 

 many of his audience had noticed that if a piece of 

 damp linen be hung out of doors during a sharp frost 

 it speedily gets stiff, and is so extremely brittle that it 

 may be easily broken in pieces like a sheet of glass, 

 or rather like the sheet of ice or crystallised 

 water it really is. Well, something very like this 

 happened when fabrics were impregnated with some 

 of the chemical salts he had mentioned ; if enough 

 be used to make the stuff what is called 'fireproof, 

 the salt is deposited in the pores and between the 

 fibres of the material in the crystalline form. These tiny 

 crystals have very sharp angles, and they act just as 

 myriads of little knives or saws would do, speedily 

 weakening the fabric and rendering it as brittle as the 

 frozen linen mentioned before. Other varieties of these 

 salts are prone to absorb moisture from the air, so that 

 a lady's dress prepared with any of them, and got up, as 

 the laundresses say, to be " very nice and stiff," would, 

 in the course of a very few hours — the time varying with 

 the material, and with the state of the weather — become 

 quite limp and unwearable. Again, some salts are un- 

 suitable by reason of their caustic properties, which, 

 whether they be of the acid or of the alkaline class, tend 

 to destroy the substance of the linen or cotton fabric, and 

 to render it rotten. Many compounds of this class, even 

 if giving apparently good results at the outset, develop 

 acidity throughout the interstices, and, as it were, in the 

 heart of the fabric, when a little time has elapsed. 

 Increase of temperature greatly facilitates the decomposi- 

 tion of these salts, and enhances their destructive influ- 

 ences. Therefore in selecting a compound intended for 

 the treatment of theatrical scenery or properties excep- 

 tional caution should be taken. In theatres and other 

 places of public amusement the warmer regions are 

 always near the top. In such hot places some salts — 

 like phosphate of ammonium, and like common alum, 

 as examples of a class — soon develop acidity, with the in- 

 evitable result that any fabric impregnated with them 

 gets very tender after a time, and actually falls to pieces 

 sooner or later. 



In conclusion, Mr. Scott said : I believe I have made 



