Out for an early morning bicycle ride on Saturday, August 10, I was among the first to come across the bizarre sight of Hinksey swimming pool turned purple. I was quickly on the phone to our news desk. My first thought as I surveyed the weirdly coloured water was of Lady Macbeth: "This my hand will rather/The multitudinous seas incarnadine,/Making the green one red." But I knew, even as I recited the line, that it was not a bloody hand that had caused this overnight transformation but the bloody silly throwing in of a handful of potassium permanganate. I shared my thought with members of the pool staff who were standing at the entrance.

This was hardly an aperçu of the greatest acuity. Look on the internet and you will find such observations as "KMnO4 that's the chemical name for potassium permanganate is often used in pranks, in which a small amount is added to a swimming pool or other similar body of water, thus turning the water a deep purple." Council officials, too, suspected that the chemical had been used in this costly and stupid act of vandalism. Astonishingly, however, they seemed unable to confirm this.

Ten days after the pool changed colour (by which time it had long been restored to its original hue), the water remained closed to swimmers as tests continued. The city council's leisure operations manager Steve Hold told a reporter from the Oxford Mail: "We are still waiting for the test results to come back from the laboratory. Before we can reopen the pool we have a duty to find out what was put in the water to colour it, and we need to make sure that even small traces of the substance will not be harmful to customers." (What, not more harmful than the sweat, urine, mucus, saliva, faecal matter, dead skin, sun cream and cosmetics, which a report this week revealed to be major ingredients of some swimming pool 'water'?) Last week, The Oxford Times reported: "Scientists have said the water turned purple at open-air Hinksey Pool, in Oxford, because of a chemical called potassium permanganate. Oxford City Council this week received final confirmation of the cause of the discolouring and the all clear from health and safety inspectors to reopen the popular Hinksey Par facility." (Notice the involvement of those inveterate interferers 'health and safety' here. And I don't care how often that plausible-sounding bloke turns up on the Today programme to tell us they are a misunderstood breed; they emphatically are not.) The pool eventually opened last Saturday, a full two weeks after it shut.

What kind of laboratory was it, I wondered, that could take so long to identify a chemical that any schoolboy - or at any rate anyone who was a schoolboy in the 1960s - could discern in one glance to be the cause of the discolouration? And is this a laboratory that any local authority with an ambition to save council tax payers' money ought to be employing? For, of course, every day that the pool stayed shut - while these seemingly highly complicated tests continued - was costing the authority (and therefore all of us) a considerable sum in lost £4 admission fees.

My recognition of potassium permanganate, incidentally, dates from the time, fully 40 years ago, when I was shown its effect on water by chemistry master Mr Marsh (known, of course, with the lack of invention that is traditional with schoolboys, as 'Boggy'). It was the sort of thing we did in those days - like watching pieces of phosphorus whizzing about on water beneath a bell-jar, or playing chase the silver globules with flasks of (costly) mercury.

Boggy taught us so well that I still find I can recall the rather complicated equation for what results when potassium permanganate is combined with hydrochloric acid as a means of making chlorine. Would the GCSE pupils - sorry, students - of today be able to recite: "2KMnO4 + 16HCl = 2KCl + 2MnCl2 + 8H2O + 5Cl2"?