He'd thought so.

An orally administered poison that affected the lungs? Hemoptysis (coughing up blood), dyspnea (difficult and painful breathing), and respiratory acidosis (light-headedness, fast heartbeat, losing consciousness) were caused by differing chemicals; poisoning was not a café where one could tell the cook, "I'll have anaphylaxis and Cheyne-Stokes respiration, but hold the convulsions and vomiting, please. Additionally, would you mind working quickly so that I can't be incriminated?"

Few poisons, except for the more corrosive acids, worked instantly. A more likely theory was that Moriarty had poisoned her over a period of several weeks. Months, even.

Using the records of Moriarty's purchases, he'd figured out how much of each chemical the professor had purchased from various suppliers around London, mixed up a batch of instant tuberculosis (tea-matching jasmine scent optional), then tested it on twenty grayish-brown rats that a pigtailed Irregular had cheerfully nabbed from London's slums.

"At least you'll have something to keep you occupied while we're visiting Mary's mother in the country." Watson observed, hugging him farewell. "Don't drink any of the poison by accident, all right?"

The experiment had proved his theory correct. The first dose of reddish liquid, daubed on a piece of Mrs. Hudson's orange-cranberry bread, had not seemed to affect the scurrying creatures. By the fifteenth, however, they had lost their fight. The struggling, feral rodents with gnashing teeth and anger flaring in their yellow eyes had melted into pliable invalids who froze in fear at the faintest blare of a kazoo and struggled to cross the cage. Occasionally, they seemed to forget what they had intended to do halfway across the torn-up newspaper lining, and they frequently stopped for breath. (Which explained the rather disconcerting sight of a bound and gagged Irene handcuffed to the conveyor belt of a pigs' meat-processing plant- judging by his knowledge of the men in Lord Blackwood's employ, she would never have been subdued if not for a sudden dizzy spell.)

He wrote:

Day 15. Subjects exhibit the following: Weakness. Increased startle reflex. Shortness of breath and general malaise.

Irene would have written the symptoms off as a mere cold.

Holmes had undertaken the experiment in an effort to absolve himself of some of his guilt. "Leave my sight and you'll be dead within the hour," he'd warned her. She proved him right.

Yet if the poison already inhabited her body, an irreversible clock ticking towards death, he had little reason to brood on allowing her to meet with Moriarty. He detested having emotions cloud his judgment; this bias towards sentimentality was a flaw that required dealing with.

Now, however, he pondered a different question.

Why the bloody hell hadn't he noticed?

Day 16. Shortly after feeding, the smallest rat adopted an unsteady, weaving gait. It dragged itself to the cage's corner, where it curled up into a heap of bedding, coughed up a larger-than-expected amount of blood, and, after struggling to breathe for approximately three minutes and fourty-two seconds, ceased to move.

I could not detect its pulse.

Day 17, 20, 31. Each day brought additional deaths. Some rats struggled to the cage's edge, pressing themselves against the bars, or made their way to their sleeping places. Others, too weak to move, simply collapsed where they stood, their small legs flopping out from under him, before the fatal coughing began.

He watched the last rat expire and recorded the fact in his journal.

Silence permeated the flat.

In her will, Irene had left the contents her second husband's wine cellar to "Mr. Basil Sigerson, a dear friend of many years' standing." Assuming the appropriate alias, Holmes had dragged himself down to the relevant country house. (She barely used it except in the winter, and evidently intended to turn it into an orphanage.)

Tearing his bleary-eyed gaze away from the cage of dead rats- and feeling rather like a dead rat himself, what with the stiffness in his limbs and all- he stumbled to one of the crates of wine, picking a bottle up at random. Absentmindedly kicking away crumpled-up chemistry notes, he read the label. Hmm. A decent vintage, although a brief drought had altered its taste somewhat. More than adequate for his purposes. He uncorked the bottle and took an enormous gulp.

The room lurched.

What in the…

Attempting to retain his balance, he staggered forward, and something cracked. Holmes hopped backwards with exaggerated care. The cork had split into two parts, and a small scrap of paper revealed itself from within. Curling, ladylike script spelled out:

Consider this my gift to the attendees of any dinner parties you might host. Letting wine breathe really improves the taste considerably.

Yours, I.A.

She'd meant it as a chastisement, but Holmes welcomed the opportunity to achieve the maximum of unconsciousness with the minimum of inconvenient hangover. "A toast," he said, hoisting the bottle high. "To a dear friend of many years' standing." He drank as much as he could before toppling to the ground.

Light permeated Holmes' eyelids. He winced. "Bloody hell…" The faint squeaking noises from the rats' cage couldn't be more annoying. Every tiny footfall seemed too loud. Lurching to his feet, he peered blearily into the cage, wondering if it was worth it to kill the idiotic things again…

Wait.

Rats.

The rats were dead-

The rats were alive-

Of course. How could he not have realized it?
Holmes grabbed a piece of paper and a pen from the desk and started scribbling, his hand moving so fast that he ripped the paper in half. He didn't care. For England's sake, brainless self, the chemical subreaction had been common knowledge since Shakespeare's time-

Jamming a hat on his unkempt head, he ran out of the building and into the street, where he was nearly mowed down by a passing carriage. "WATSON!"

Several blocks away from where the newly resurrected rats scurried happily around their cage, Mary Watson (nee Morstan) lifted her head from her husband's shoulder. "Did you hear something?" she mumbled, blinking sleepily.

"Yes, I thought I did," Watson replied with a yawn. "Almost like-"

The door burst open, letting morning light into the room. Watson squinted into the sudden illumination to see-

Sherlock Holmes, disheveled, visibly hungover, yet with an almost manic grin, the spark of life more vibrant in his eyes than it had seemed in the past several months. "Grab your bag, Watson," he ordered, excitement bursting in his voice. "We're going to rescue Irene!"

(Author's Note: This story was written with the help of "Deadly Doses: A Writer's Guide to Poisons." Love that book!)