548 



NATURE 



[April 5, 1900 



(9-6) and 48 (9-1] 

 of the variable :— 



the following are the observed magnitudes 



1900. Jan. 16 ... 8-8 



19 ... 87 



Feb. 20 ... 9-0 



March 14 ... 9*5 



Solar Eclipses of the 2oth Century.— In a reprint from 

 the Btilletin de la SocitH^ Astronomique de France ior November, 

 M. Camille Flammarion brings together the local particulars for 

 the eclipses of the sun which will be visible in Paris during the 

 20lh century. Forty-three eclipses will be visible, but only 

 thirty-three under good observing conditions. Special attention 

 is drawn to the eclipses of April 17, 1912, and August 11, 1999, 

 as although Paris is not included in the path of totality, in each 

 case the central line of eclipse is only a short distance away 

 from the capital. Maps are given of the paths of the shadow 

 for both dates. These are also reproduced in the last number 

 of the Bulletin (Maich). 



A Brilliant Fireball.— On March 28, 8h. 31m., a very 

 large meteor, giving several flashes like vivid lightning, was ob- 

 served from the south-eastern parts of England. At Bishops 

 Stortford, Herts, the light was so great that it illuminated the 

 country, and three distinct explosions were observed. A sound 

 like that of the roar of a distant cannon followed the disappear- 

 ance of the meteor, and would indicate that 'it was 24 miles 

 distant, but this is probably much underestimated. The meteor 

 descended from the constellation Leo in the south. In Berk- 

 shire it was seen falUng in Virgo, and it flashed out very brilli- 

 antly just prior to its disruption. The head of the meteor was 

 very much brighter than Venus, and it travelled rather swiftly. 

 Two vivid flashes were observed here as at Reading, where the 

 terminal point of the flight was noted as being near e Virginis. 

 At the latter place the phenomenon ended in a cloud of sparks, 

 and for a moment the sky and landscape were flooded in light. 

 At Blackheath the meteor was seen by Mr. Crommelin, of the 

 Greenwich Observatory. He estimated it as three times as 

 brilliant as Venus at her brightest, and describes the terminal 

 point as 1° N. of ;3 Leonis. Many reports of this brilliant object 

 are available for discussion, and it will be possible to determine 

 its real path satisfactorily. Many large fireballs are directed in 

 very slow flights from westerly radiants, but in this case the 

 object moved swiftly, and probably had a radiant not far from 

 the star e in Ursa Major. Its position was over the east coast of 

 Kent, and its height, when it finally burst and disappeared, 

 about 52 miles. 



MODERN EXPLOSIVES. 1 

 T^HE subject of explosives is one which never fails to excite 

 interest even under the most ordinary conditions, doubt- 

 less owing to the enormous potentiality of these substances, 

 whilst at the present time more than u.sual attention is directed 

 to them, it being scarcely possible to read a daily paper without 

 finding some reference to the behaviour of various modern 

 explosives in the theatre of war. 



Explosion may be defined as chemical action causing 

 extremely rapid formation of a very great volume of highly 

 expanded gas, this large volume of gas being generally due to 

 the direct liberation by chemical action and the further 

 enormous expansion by the heat generated. Explosion itself 

 may therefore be regarded as extremely rapid combustion, 

 whilst the effect is obtained by the enormous pressure produced 

 owing to the products of combustion occupying probably many 

 thousand times the volume of the original body. The effect of 

 high temperature is seen in the well-known ca.se of explosion of 

 a mixture of hydrogen and oxygen, where if the original mix- 

 ture and the products of explosion are each measured at the 

 same tempeiature above the boiling point of water, a less 

 volume of gas (water vapour) is actually found. The explosion 

 can only have been produced by the enormous expansion of 

 this vapour in the first place by the heat of the reaction. Such 

 an explosion when carried out in a closed bomb with the mixed 

 gaes under ordinary conditions of measurement produces a 

 pressure of about 240 lbs. to the .square inch. A more practical 

 illustration is seen with nitroglycerine, which Nobel found 

 yielded about 1200 times its own volume of gas calculated at 



i A lecture delivered at the London Institution «on February 12, by Mr. 

 J. S. S. Brame. 



NO. 1588. VOL. 61] 



ordinary temperatures and pressures, whilst the heat liberated 

 expands the gas to nearly eight times this volume. 



Clearly, then, a substance for use as an explosive must be 

 ' 'e of undergoing rapid decomposition or combination 

 with the production of large volumes of gas, and further pro- 

 duce sufficient heat to greatly expand these gases ; the ratio of 

 the volume of gases at the moment of explosion to the volume 

 of the original body largely determining the efficiency of the 

 explosive. 



Explosives-may be divided into two great classes — mechanical 

 mixtures and chemical compounds. In the former the com- 

 bustible substances are intimately mixed with some oxygen- 

 supplying material, as in the case of gunpowder, where carbon 

 and sulphur are intimately mixed with potassium nitrate ; while 

 gun-cotton and nitroglycerine are examples of the latter class, 

 where each molecule of the substance contains the necessary 

 oxygen for the oxidation of the carbon and hydrogen present, 

 the oxygen being in feeble combination with nitrogen. Many 

 explosives are, however, mechanical mixtures of compounds 

 which are themselves explo.Mve, e.g. cordite, which is mainly 

 composed of gun-cotton and nitroglycerine. 



Two methods are in common use for bringing about 

 explosions — ignition by heat, thus bringing about ordinary but 

 rapid combustion, molecule after molecule undergoing decom- 

 position ; and detonation, where the effect is infinitely more 

 rapid than in the first case ; in fact, it may be regarded as prac- 

 tically instantaneous. The result may be looked upon as 

 brought about by an initial shock imparted to the explosive by 

 a sub.stance — the detonating mateiial — which is capable of 

 starting decomposition in the adjacent layers of the explosive, 

 thus causing a shock to the next layer and so on with infinite 

 rapidity. That the results are not entirely due to the 

 mechanical energy of the liberated gas particles is shown by the 

 fact that the most powerful explosive is not the most powerful 

 detonator; neither is it entirely due to heat, since wet substances 

 undergo detonation. The probability is that the result is 

 brought about by vibrations of particular velocity which vary 

 for different substances, the decomposition being caused by the 

 conversion of the mechanical force into heat in the explosive, 

 thus bringing about a change in the atomic arrangement of the 

 molecule. According to Sir Frederick Abel's theory of detona- 

 tion, the vibrations caused by the firing of the detonator are 

 capable of setting up similar vibrations in the explosive, thus 

 determining its almost instantaneous decomposition. 



The most common and familiar of explosives is undoubtedly 

 gunpowder, and although for military purposes it has been 

 largely super-eded by smokeless powders, yet it has played such 

 an important part in the history of the world during the last 

 few centuries that apart from military uses it is even now of 

 sufficient importance to demand more than a passing notice. 



Its origin, although somewhat obscure, was in all probability 

 with the Chinese. I\.oger Bacon and Berthold Schwartz appear 

 to have rediscovered it in the latter years of the thirteenth and 

 earlier part of the fourteenth centuries. It was undoubtedly 

 used at the battle of Crecy. The mixture then adopted appears 

 to have consisted of equal parts of the three ingredients — sul- 

 phur, charcoal and nitre ; but some time later the proportions, 

 even now taken for all ordinary purposes, were introduced, 

 namely — 



Potassium nitrate... 

 Charcoal ... 

 Sulphur 



75 parts 

 15 .. 

 10 ,, 



Since gunpowder is a mechanical mixture, it is clear that the 

 first aim of the maker must be to obtain perfect incorporation, 

 and necessarily in order to obtain this, the materials must be in 

 a very finely divided state. Moreover, in order that uniformity 

 of effect may be obtained, purity of the original substances, the 

 percentage of moisture present, and the density of the finished 

 powder are of importance. 



The weighed quantities of the ingredients are first mixed in 

 gun-metal or copper drums, having blades in the interior capable 

 of working in the opposite direction to that in which the drum 

 itself is travelling. After passing through a sieve, the mixture 

 (green charge) is passed on to the incorporating mills, where it is 

 thoroughly ground under heavy metal rollers, a small quantity of 

 water being added to prevent dust and facilitating incorporation, 

 and during this process the risk of explosion is greater possibly 



