Richard Coles visits the Treasures of Catherine the Great, the first exhibition at the Hermitage Rooms in Somerset House, in which jewels, silver and miniature portraits offer an insight not just into Russian art but the country's history. And A.C. Grayling, William Hazlitt's latest biographer, argues his importance as a philosopher and critic. Hazlitt is regarded as the finest essayist in English but has his stylistic prowess been overshadowed?