The letter begins as an intimate billet-doux. ‘Oh Harry, my own precious darling, your letter today is one long yearning cry for your little love.’ But within a few lines, a more sinister story begins to emerge. ‘Yesterday, I administered the powder you left me . . . the result? Nil.’ The powder – arsenic – had not worked. The writer of the letter was an Edwardian housewife named Augusta Fullam, who lived in Agra in central north India. Her ‘precious darling’ was Lieutenant Henry Clark, a surgeon. Together, in 1911, the two lovers decided to poison Augusta’s husband Edward. Once he was dead, they would then kill Mrs Clark, Henry’s wife. And then they would get married.

But they found themselves with a significant problem when they tried to kill Edward: he stubbornly refused to die. Each day, Augusta sprinkled arsenic powder onto his supper or slipped it into his tea, but all to no avail. ‘My hubby returned the whole jug of tea saying it tasted bad,’ she wrote in one letter to her lover.

On Friday 16 June 1911, Augusta managed to administer a massive dose to her husband. This time he ate the lot. But once again, it failed to kill him. ‘Since 4pm [he’s] vomited eight times . . . vomited ten times at a quarter to nine . . . vomited 12 times at ten pm.’

Augusta began to fear that he was indestructible. ‘I give him half a tonic powder every day in his Sanatogen, lovie darling, because it lays on the top of the white powder quite unsuspiciously.’

For month after month, Augusta fed her husband arsenic. And for month after month Edward clung to life, despite vomiting many times each day. But eventually he fell seriously ill. As he lay in bed with a raging fever, Lieutenant Clark decided to finish him off with a huge dose of poison, administering it himself. Within hours, Edward Fullam was dead.

In his capacity as surgeon, Lieutenant Clark was able to sign the death certificate: it recorded the cause of death as heart failure.

The lovers were halfway to their goal. All they now had to do was murder Mrs Clark. This time, they decided not to waste months in administrating arsenic. Instead, Lieutenant Clark hired four assassins who broke into the house and struck Louisa Clark with a sword, smashing her skull. The noise of the brutal attack woke the Clarks’ daughter, Maud, who screamed, causing the robbers to flee.

Agra police began their investigations that same day and their suspicions immediately fell on Lieutenant Clark and Augusta Fullam. Their love affair had not gone unnoticed in the local community and they clearly had a motive for both murders. But the detectives assigned to the case could find no conclusive proof.

None, that is, until Inspector Smith called at Augusta Fullam’s house and noticed a large metal box hidden under the bed. When he asked what was inside, Augusta turned bright red ‘and fell like a heap into a chair’.

Inspector Smith had the box prised open: inside there were 370 love letters that set down in great detail how Augusta and Lieutenant Clark had planned their terrible crime.

The ensuing trial proved a sensation: colonial India had never before seen such a spectacular double murder. Every sordid detail was splashed across both the Indian and British newspapers.

The two lovers were tried separately and both were convicted. Lieutenant Clark was hanged on Wednesday 26 March 1913. Augusta Fullam, who was pregnant at the time of the trial, was sentenced to life. She served just fifteen months before dying of heatstroke the following year.

‘My very own precious lovie,’ she had written when she and Clark first started administering the arsenic, ‘don’t you think our correspondence rather risky?’

But Lieutenant Clark assured her it was fine. He said they would never be caught.

This excerpt is from Fascinating Footnotes From History by Giles Milton, published by John Murray.