‘I never saw the point about the letter,’ said Sir Henry. ‘That’s a most important point, of course.’

‘Rather a bold step to take,’ said Miss Marple, ‘because Mrs Pritchard might have recognized her in spite of the disguise—though of course if she had, the nurse could have pretended it was a joke.’

‘What did you mean,’ said Sir Henry, ‘when you said that if you were a certain person you would not have trusted to fright?’

‘One couldn’t be sure that way,’ said Miss Marple. ‘No, I think that the warnings and the blue flowers were, if I may use a military term,’ she laughed self-consciously—‘just camouflage.’

‘And the real thing?’

‘I know,’ said Miss Marple apologetically, ‘that I’ve got wasps on the brain. Poor things, destroyed in their thousands—and usually on such a beautiful summer’s day. But I remember thinking, when I saw the gardener shaking up the cyanide of potassium in a bottle with water, how like smelling-salts it looked. And if it were put in a smelling-salt bottle and substituted for the real one—well, the poor lady was in the habit of using her smelling-salts. Indeed you said they were found by her hand. Then, of course, while Mr Pritchard went to telephone to the doctor, the nurse would change it for the real bottle, and she’d just turn on the gas a little bit to mask any smell of almonds and in case anyone felt queer, and I always have heard that cyanide leaves no trace if you wait long enough. But, of course I may be wrong, and it may have been something entirely different in the bottle; but that doesn’t really matter, does it?’

Miss Marple paused, a little out of breath.

Jane Helier leant forward and said, ‘But the blue geranium, and the other flowers?’

‘Nurses alwayshave litmus paper, don’t they?’ said Miss Marple, ‘for—well, for testing. Not a very pleasant subject. We won’t dwell on it. I have done a little nursing myself.’ She grew delicately pink. ‘Blue turns red with acids, and red turns blue with alkalis. So easy to paste some red litmus over a red flower—near the bed, of course. And then, when the poor lady used her smelling-salts, the strong ammonia fumes would turn it blue. Really most ingenious. Of course, the geranium wasn’t blue when they first broke into the room—nobody noticed it till afterwards. When nurse changed the bottles, she held the Sal Ammoniac against the wallpaper for a minute, I expect.’

‘You might have been there, Miss Marple,’ said Sir Henry.

‘What worries me,’ said Miss Marple, ‘is poor Mr Pritchard and that nice girl, Miss Instow. Probably both suspecting each other and keeping apart—and life so very short.’

She shook her head.

‘You needn’t worry,’ said Sir Henry. ‘As a matter of fact I have something up my sleeve. A nurse has been arrested on a charge of murdering an elderly patient who had left her a legacy. It was done with cyanide of potassium substituted for smelling-salts. Nurse Copling trying the same trick again. Miss Instow and Mr Pritchard need have no doubts as to the truth.’