February 3, 1916] 



NATURE 



637 



Explosion. — When gas or vapour is released so 

 suddenly as to cause a loud noise an explosion is said 

 to occur, as, for instance, the explosion of a steam 

 boiler or a cylinder of compressed gas. Great and 

 increasing use is made of explosive processes in gas, 

 jjetrol, and oil engines for driving machinery of all 

 kinds. In these engines the material that explodes 

 is a mixture of air with combustible gas, vapour, or 

 finely-comminuted liquid, and in the explosion these 

 are suddenly converted into water vapour and the 

 oxides of carbon, which latter are gases. Although all 

 these things are liable to explode, none of them are 

 called explosives ; this term is confined to liquid and 

 solid substances, which produce much more violent 

 effects than exploding gaseous mixtures, because they 

 occupy much smaller volumes originally. 



Explosive. — .-\n explosive is a solid or liquid sub- 

 stance or mixture of substances which is liable, on 

 the application of heat or a blow to a small portion 

 of the mass, to be converted in a very short interval 

 of time into other more stable substances largely or 

 entirely gaseous. A considerable amount of heat is 

 also invariably evolved, and consequently there is a 

 flame. 



Gas Evolution. — That evolution of gas (or vapour) 

 is essential in an explosion is rendered evident by 

 considering thermit. This consists of a mixture of 

 a metallic oxide, generally oxide of iron, with 

 aluminium powder. When suitably ignited the 

 XO. 2414, VOL. 96] 



tical value of the forecasts to be established. This can t 

 only be done by trial, and both sides are waiting for 

 the other to demonstrate, beyond cavil, the value of i 

 the information to be supplied. The fundamental diflR- j 

 culty seems to be that the farmer has made his own \ 

 study of the weather, and uses it in his own way with- I 

 out committing the results to writing, while the 

 Meteorological Office prints large masses of data 

 without knowing precisely in what directions to dis--j 

 cuss them in relation to agricultural problems. | 



Appendix 4 deals with proposals for the establish- | 

 ment of a "Central Observatory for the Investigation 

 of the Upper Air," in which it is pointed out that since 

 1905 the small sum of 450/. a year has been available 

 for the purpose of upper air research. Having in view 

 the great and rapidly growing importance of the aero- 

 nautical and aerological aspects of the work, especially 

 in relation to aviation, it is to be hoped that this j 

 scheme will go through. The services rendered by 

 Mr. W. H. Dines, F.R.S., in the past are so well 

 known that the mere mention of them is an ample 

 guarantee that the annual appropriation of some loooZ. 

 to 1500/. proposed would be money well expended. 

 The site suggested is at Benson, in Oxfordshire, which 

 has many advantages to recommend it as a position 

 for the central aeronautical observatory contemplated. 



THE NATURE OF EXPLOSIVES. 



IT was suggested in the review of Mr. A. Marshall's 

 important work on "Explosives" in Nature 

 of June 3, 1915 (vol. xcv., p. 366) that the book would 

 be improved if it had an introductory chapter dealing 

 with the general principles on which the composition 

 and action of explosives depend. Mr. Marshall, writ- j 

 ing from Naini Tal, India, says that he had prepared j 

 a chapter on the lines suggested for another shorter | 

 work of a less technical character than that which 

 was the subject of our review. Unfortunately, 

 through pressure of other work, he has been obliged 

 to postpone for the present the completion of this book, 

 but he sends us the chapter ; and we are glad to 

 publish it as a separate article, as the subject is of 

 particular interest at the present time. 



aluminium is converted into oxide and the iron or 

 other metal is set free in a very short interval of time 

 with the evolution of an enormous quantity of heat, 

 but there is no explosion. It is indeed because no gas 

 is evolved that thermit can be used, as it is, for local 

 heating and welding. 



Heat Liberation, — It is also an essential condition 

 that heat should be evolved in an explosive reaction, 

 otherwise the absorption of energy due to the work 

 done by the explosion would cool the explosive and 

 consequently slow down the reaction until it ceased^ 

 unless heat were supplied from without. Ammonium 

 carbonate, for instance, readily decomposes into carbon 

 dioxide, ammonia, and water, but in so doing it 

 absorbs heat ; consequently the reaction is much too- 

 slow to be explosive. Ammonium nitrate, on the 

 other hand, is decomposed into oxygen, nitrogen, and 

 water, with the evolution of heat, and is consequently 

 liable to explode. A violent impulse is required tcv 

 start the explosion, but once it is started the energy 

 (or heat) liberated suffices to propagate the explosion, 

 unless the conditions be such that the energy is dissi- 

 pated more rapidly than it is liberated. 



Sensitiveness. — Another essential for an explosive 

 is that the reaction shall not set in until an impulse 

 is applied. If the reaction set in spontaneously, it is 

 obvious that its energy cannot be utilised in the form 

 of an explosion. A mixture of sodium and water 

 evolves hydrogen with the liberation of heat, but 

 reaction sets in immediately the two substances come 

 in contact with one another. Different explosives re- 

 quire impulses of very different strengths to cause them 

 to explode. Some, such as diazobenzene nitrate, are 

 exploded by a slight touch ; these explosives are of no 

 practical utility as they are too unsafe. Others, such 

 as fulminate of mercury, are exploded by a moderate 

 blow or a small flame ; these are used principally for 

 charging caps and detonators, a small quantity serving 

 to explode a large amount of some other less sensitive> 

 explosive. Most of the explosives now used can be* 

 exploded by a blow only if it be extremely violent, and 

 many of them cannot be exploded by a flame in the 

 open in ordinary circumstances. The tendency is to 

 use less sensitive explosives because they are safer to 

 handle, but it should never be forgotten that the term 

 " safe," when applied to an explosive, is only a com- 

 parative one. The duty of an explosive is to explode, 

 and if it is not treated with proper respect it will, 

 sooner or later, explode at the wrong time with ex- 

 tremely unpleasant results. 



Before the subject of explosives was understood so- 

 well as it is now, inventors were very liable to think 

 an explosive was very powerful, and therefore valuable 

 merely because it was very sensitive, whereas too great 

 a degree of sensitiveness is really a most objectionable 

 feature. In the middle of tTie nineteenth century many 

 such mixtures as potassium chlorate and picric acid 

 were proposed through this want of comprehension of 

 a fundamental condition. 



Constituents of Explosives. — The explosive 

 gaseous mixtures used in gas and oil engines to which 

 reference has been made are composed of a combustible 

 material, consisting largely of carbon and hydrogen, 

 and air, the useful constituent of which is oxygen. 

 Similarly, nearly all commercial explosives are com- , 

 posed partly of combustible elements, of which carbon 

 and hvdrogen are the most important, and partly of 

 oxygen combined, but not directly with the hydrogen 

 and carbon. On explosion the oxygen combines with 

 the hydrocren to form water, and with the carbon to- 

 form carbon monoxide or dioxide, or a mixture of the 

 two. It is the heat set free in this combustion that is 

 the main or entire cause of the rise of temperature. 

 The formation of these two oxides of carbon liberates- 

 very different quantities of heat; 12 grams of carborr 



