Bernard O’Donoghue’s poem “What’s the Time, Mr Wolf?” in his collection Gunpowder (1995) opens with a snatch of hearsay about the amorphous inorganic material that casts its lustre on our lives:  Glass, someone once told me, is a liquid Of such density that its sluggish Downward seep takes centuries to work, So medieval windows are thicker At the bottom than the top.  To imagine that old glass flows imperceptibly, much as we imagine time itself flowing, is a beautiful conceit; alas, like most poetic ideas, it isn’t true. It’s like the belief, which persisted into the twentieth century, that as…