HALF A MILLION people are admitted to psychiatric hospitals each year in the UK - insane, isn't it? One in four of us will have a mental health problem. Like the poor, the mad, as Catharine Arnold reminds us - just after dedicating her new book on London's insane to her husband - “are always with us”. So it is not so much that we might allow ourselves a “slightly voyeuristic interest in madness” akin to that of our ancestors who gawped at the afflicted in the Bedlam of popular and Hogarthian imagining, but more that we are honour-bound to look at madness as the mirror of ourselves.

London and its Mad is the apposite subtitle to Catharine Arnold's catch-all history of Bedlam. The central question it begs, but ultimately ducks, is why urban life seems to damage, define and defile the human psyche.

The mad are particularly with us in London - as anyone who has travelled the Northern Line will know - and Bedlam might seem the ideal prism with which to view the history of madness in the city where, as Shakespeare's gravediggers tell Hamlet, “they are all as mad as he”.

The broadness of Bedlam is both the strength and weakness of Catharine Arnold's follow-up to her magisterially creepy Necropolis: London and its Dead. Bedlam needs the wider context; the social history of the city it served and the changing concepts of “madness” over the centuries. Yet the building - or rather buildings - themselves and the evolving institution are worthy of more space than this fascinating but slim volume can afford. At the same time Bedlam meanders into the wider social and literary landscape - Mrs Rochester and Miss Havisham both feature.

Bedlam history takes us from the ghoulish and ghastly early history of mental illness all the way to Care in the Community. There are unexpected cameos - the last “Bishop” of “Bethlem Hospital” before the Reformation was George Boleyn, benighted brother of Anne.

There is a beautifully handled overview of the madness of George III, though the relevance to 18th-century attitudes to insanity is better expressed with the parallel case studies of the aspiring regicides Margaret Nicholson and James Hadfield, who actually ended up in Bedlam. Arnold is also very good on syphilis, which nearly overwhelmed the mental institutions and scientific establishment in the 19th century, and on the shell shock of the First World War.

Yet the tides of influence crash over the unfortunate “lunatics” of Bedlam as if properly pulled by lunar cycles. Disentangling cause and effect, wax from wane, symptom from disease via a morass of evidence and exquisitely eccentric case studies is an ambitious task.

Understandably Arnold struggles to find the straitjacket for her unruly patient. A number of chapters begin or end with “as we have seen” and “let us now examine”, which hints at the dead hand of unsympathetic editing when the author's style tends instead towards the Gothic (Arnold catalogues “the angel of death that spread his wings to the blast of some battlefield in China and breathed ... over England plague, famine and disaster”: she means the Black Death) But the book is as crowded as any asylum ward with the bizarre, the heartbreaking, the disturbed and disturbing.

Arnold, like the mental patient Siegfried Sassoon, knows poetry is in the pity, and finds both solace (she is open about her own brushes with mental-health issues) and also historical insight in poems, quoted often at length.

I am not sure that Blake's London or Sassoon's The General can justify their page-space here, but Arnold closes with Keats's Ode on Melancholy with its incitement to embrace the “wakeful anguish of the soul” and know that to understand life is to understand suffering; something our ancestors, gawping at Bedlam's inmates, surely sensed.

Bedlam: London and its Mad by Catharine Arnold
Simon & Schuster, £14.99; 320pp
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Ian Kelly appears at Edinburgh International Book Festival on Sunday, August 10. (0845 3735888; www.edbookfest.co.uk)

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