Sunday 21 April 2013


When I were a nipper (that's before the First War), I used to browse the Encyclopedia Britannica in the school library.  I'd be there for a reason - possibly consulting the biography of a 19th century general, or a technical article on pistons - and a phrase or word would give me pause . . . and I'd be off down the rabbit hole.  A hour later, I'd surface like a desperate whale gasping for air and wonder just how I'd got from General McClellan (the American Civil War general with the least facial hair) to the Northamptonshire boot industry.  It's easier nowadays, and so more time-consuming, since they invented the hypertext link but you don't get the exercise of hefting 2.5kg books off shelves.

So I was resting on my oars in the blogosphere waiting for something to happen when over the horizon loomed a (British) Independent link to a compendium of fact(oid)s about 42.  Facts like: "The atomic bomb that devastated Nagasaki, Japan, contained the destructive power of 42 million sticks of dynamite."  That made me reasonably confident that Fat Man released energy equivalent to either 42 or more likely 21 kilotons of TNT (this is the standard comparative measure of explosive power) and the usual sources revealed this to be so.  The same train of reasoning circled on the fact that a stick of dynamite weighs a kilogram or more likely 500g.

But this confidence hinged significantly on the assumption that the journo who wrote the article had been educated in the Arts Block.  Because TNT isn't the same as dynamite.  It's much less powerful: 2.8 megajoules/kilogram compared to dynamite's 7.5MJ/kg. What he should have said was "...destructive power of 14 million sticks..." and then gone off back to google to find a different 42-fact because the Nagasaki line was a bust.

So TNT (tri-nitro-toluene) is interesting. Its chemical formula
C7H5N3Oshows that it has proportionately much less oxygen than the related compound tri-nitroxy-propane C3H5N3O9 aka nitroglycerine which is the active principle in dynamite.  This means that combustion is less efficient - hence its lower MJ/kg.  Chemical reactions are usually complex but chemists like to summarise them with formulae - and insist that budding chemists add up the atoms on each side of the arrow to ensure they have accounted for everything. Here's what they think happens when TNT goes up:
2 C7H5N3O6 → 3 N2 + 5 H2O + 7 CO + 7 C and/or
2 C7H5N3O6 → 3 N2 + 5 H2 + 12 CO + 2 C
Note all the surplus carbon on the right-hand, result, side of the equation: that's the enormous cloud of sooty black smoke that is characteristic of TNT explosions.  Here's a video from Operation Sailor Hat - a cunning plan by the US navy to simulate the effect of a massive blast on their ships.  They commandeered Kaho'olawe, the smallest Hawaiian island, built a stack of TNT weighing 500 tons (that's 300 cu.m. - just about the size of our modest house); moored some surplus-to-requirements ships and ... baDOOM.  As well as sending a dirty black plume up into the air (three times in the Spring of 1965), and knocking chunks off their floating Tonka-toys, the USN created the only non-volcanic crater in the state of Hawaii.  Kaho'olawe hasn't had any surface water since shortly after the Hawaiians arrived around 1000CE and stripped off the forest. But Operation Sailor Hat has created a brackish anchialine pool - a landlocked body of water connected to the ocean by a subterranean passage - which is home to some unique shrimps.

Thar she blows!!  One minute you're safe in a warm bath of truthiness on a Douglas Adams tribute site and an hour later you bob up with hearing loss in a remote pond in Hawaii wondering if Halocaridina rubra is edible.

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