158 



SCIENTIFIC NEV\^S. 



[Sept. 1st, 1 5 



Fig. 7. Reis's Receiver. 



Fig. 8. Reis's Receiver. 



to have made with an armature consisted of a horse-shoe 

 electro-magnet, attracting a light iron armature carried on a 

 thin wide vertical wooden bar, suspended from a metal 

 upright by means of a stretched wire running across it. A 

 screw served to adjust the position of the armature, and a 

 spiral spring opposed the magnetic pull. The instrument 

 is much like a telegraph sounder laid on its side. The idea 

 evidently was that the vibrations of the armature would be 

 communicated to the air by means of the broad wooden bar. 

 It does not seem that this receiver worked very well. 

 Probably the suspended armature behaved like a pendulum, 

 and was only sensitive to waves corresponding to its own 

 vibration rate, or to multiples of that rate. 



(To be coyitimtcd.) 



SPONTANEOUS COMBUSTION. 



THAT the outbreak of fire in dwellings, warehouses, 

 manufactories, etc., is, in most cases, due to human 

 carelessness or malice needs no demonstration. Still it is a 

 matter of experience that certain substances are capable, 

 under well-known circumstances, of becoming spontaneously 

 heated until they ignite and communicate fire to any com- 

 bustible matter which may be near them. 



The simplest illustration of this fact may be found in the 

 pneumatic tinder-box — an instrument for obtaining fire and 

 light far too inconvenient for practical use, but curious and 

 instructive in its principle. A well-fitting piston is made 

 to work in a brass cylinder, at the bottom of which is laid 

 a little tinder. The outside of the cylinder is covered 

 with some non-conducting material, and if the piston is 

 then forced down with sufficient rapidity the tinder is 

 ignited by the liberation of heat from the air on com- 

 pression. 



But a condensation of air can be effected without any 

 mechanical force being employed. Many kinds of organic 



bodies, if in the state of thin films, fibres, or fine powder, 

 condense air upon the surface of each particle. It need not 

 be said that this is a very favourable condition for oxidation, 

 and in case of any combustible body combustion is merely 

 oxidation rapidly effected, so that the heat generated is 

 manifest to our senses. 



Perhaps the most familiar instance of the spontaneous 

 combustion of fibrous organic matter is that occasionally 

 seen in hay-ricks. In rainy summers, when the sunshine 

 and wind have not sufficed to dry the hay, and when the 

 farmer has been too " penny wise and pound foolish " to 

 accept the safety offered by the Gillwell drying machine, it 

 is often noticed that a rick begins to steam and gives off a 

 peculiar smell. If these symptoms are neglected the steam 

 and the smell increase, and ultimately the mass bursts out 

 into flame, or at least smoulders away, leaving only a 

 charred and blackened residue. Heaps of tanners' bark fire 

 in a similar manner, and if precautions are taken for regu- 

 lating and limiting their heat, they serve, like horse-dung, 

 for supplying " bottom-heat " in forcing-pits, cucumber and 

 melon frames, etc. 



Ground dye-woods — especially logwood — if stored up in 

 bulk in a damp condition at dye works, drysaltery ware- 

 houses, etc., are liable to " heat," as it is technically called. 

 Here a temperature far below visible redness renders the 

 wood utterly useless. It is further noteworthy that log- 

 wood, when once it has begun to heat, can scarcely ever be 

 cooled again. It may be spread out to the air and the 

 mischief to all appearance may be stopped. But as soon 

 as it is collected together again in a heap the heating 

 returns. 



Now, in all these cases we have certain points in common: 

 organic matter in a fibrous or powdery state and a large 

 proportion of moisture. Blocks of logwood and heaps of 

 unground tanners' bark will not heat however long they 

 may be exposed to damp air ; they simply moulder away. 

 Neither does dry hay, dry bark, or dry logwood, though 

 finely ground, rise in temperature. Thus, paradoxical as it 

 may sound, water is here a cause of fire. Where there is 

 not fine division little surface is exposed to the action of the 

 air, and where there is little or no moisture chemical changes 

 are suspended. 



Another cause 01 spontaneous combustion, far from rare 

 in manufacturing establishments, depends on the changes 

 which oils and fats of animal'or vegetable nature undergo 

 when exposed to the air. This also depends on the absorp- 

 tion of oxygen. If such oils and fats are present in masses, 

 this absorption is slow ; no heat is felt, and we perceive 

 merely the gradual production of rancidity. But if the oils 

 are spread out in thin layers over large surfaces the con- 

 ditions for combustion in the popular sense of the term are 

 at hand. These conditions are perhaps most fully realised 

 in cotton waste, as it is used for wiping the working parts 

 of machinery. A handful of such waste, saturated with 

 grease, is carelessly thrown aside, and happens to fall upon 

 or close to shavings, raw cotton, paper, or other matter 

 easily liable to ignition. The film of oil coating every fibre 

 of the waste absorbs oxygen, heats, and finally inflames the 

 cotton, and there is the tiny seed of a mighty conflagration. 



There is, perhaps, no material known so suitable for 

 fitters, engine-minders, etc., to use in cleaning machinery 

 as cotton-waste, and if due care were always taken to throw 

 it, when done with, upon some non-combustible substance 

 no harm would result. 



Even wool and woollen rags, though much less inflam- 

 mable than cotton, are capable of spontaneous combustion if 

 soaked with oil. 



We must here glance at a popular error. Petroleum and 

 the mineral oils generally are much more readily kindled 



