Dr. van Heerden's surgery occupied one of the four shops which formed the ground floor of the Krooman Chambers. This edifice had been erected by a wealthy philanthropist to provide small model flats for the professional classes who needed limited accommodation and a good address (they were in the vicinity of Oxford Street) at a moderate rental. Like many philanthropists, the owner had wearied of his hobby and had sold the block to a syndicate, whose management on more occasions than one had been the subject of police inquiry.

They had then fallen into the hands of an intelligent woman, who had turned out the undesirable tenants, furnished the flats plainly, but comfortably, and had let them to tenants who might be described as solvent, but honest. Krooman Chambers had gradually rehabilitated itself in the eyes of the neighbourhood.

Dr. van Heerden had had his surgery in the building for six years. During the war he was temporarily under suspicion for sympathies with the enemy, but no proof was adduced of his enmity and, though he had undoubtedly been born on the wrong side of the Border at Cranenburg, which is the Prussian frontier station on the Rotterdam-Cologne line, his name was undoubtedly van Heerden, which was Dutch. Change the "van" to "von," said the carping critics, and he was a Hun, and undoubtedly Germany was full of von Heerens and von Heerdens.

The doctor lived down criticism, lived down suspicion, and got together a remunerative practice. He had the largest flat in the building, one room of which was fitted up as a laboratory, for he had a passion for research. The mysterious murder of John Millinborn had given him a certain advertisement which had not been without its advantages. The fact that he had been in attendance on the millionaire had brought him a larger fame.

His theories as to how the murder had been committed by some one who had got through the open window whilst the two men were out of the room had been generally accepted, for the police had found footmarks on the flowerbeds, over which the murderer must have passed. They had not, however, traced the seedy-looking personage whom Mr. Kitson had seen. This person had disappeared as mysteriously as he had arrived.

Three months after the murder the doctor stood on the steps of the broad entrance-hall which led to the flats, watching the stream of pedestrians passing. It was six o'clock in the evening and the streets were alive with shop-girls and workers on their way home from business.

He smoked a cigarette and his interest was, perhaps, more apparent than real. He had attended his last surgery case and the door of the "shop," with its sage-green windows, had been locked for the night.

His eyes wandered idly to the Oxford Street end of the thoroughfare, and suddenly he started. A girl was walking toward him. At this hour there was very little wheeled traffic, for Lattice Street is almost a cul-de-sac, and she had taken the middle of the road. She was dressed with that effective neatness which brings the wealthy and the work-girl to a baffling level, in a blue serge costume of severe cut; a plain white linen coat-collar and a small hat, which covered, but did not hide, a mass of hair which, against the slanting sunlight at her back, lent the illusion of a golden nimbus about her head.

The eyes were deep-set and wise with the wisdom which is found alike in those who have suffered and those who have watched suffering. The nose was straight, the lips scarlet and full. You might catalogue every feature of Oliva Cresswell and yet arrive at no satisfactory explanation for her charm.

Not in the clear ivory pallor of complexion did her charm lie. Nor in the trim figure with its promising lines, nor in the poise of head nor pride of carriage, nor in the ready laughter that came to those quiet eyes. In no one particular quality of attraction did she excel. Rather was her charm the charm of the perfect agglomeration of all those characteristics which men find alluring and challenging.

She raised her hand with a free unaffected gesture, and greeted the doctor with a flashing smile.

"Well, Miss Cresswell, I haven't seen you for quite a long time."

"Two days," she said solemnly, "but I suppose doctors who know all the secrets of nature have some very special drug to sustain them in trials like that."

"Don't be unkind to the profession," he laughed, "and don't be sarcastic, to one so young. By the way, I have never asked you did you get your flat changed?"

She shook her head and frowned.

"Miss Millit says she cannot move me."

"Abominable," he said, and was annoyed. "Did you tell her about Beale?"

She nodded vigorously.

"I said to her, says I," she had a trick of mimicry and dropped easily into the southern English accent, "'Miss Millit, are you aware that the gentleman who lives opposite to me has been, to my knowledge, consistently drunk for two months--ever since he came to live at Kroomans?' 'Does he annoy you?' says she. 'Drunken people always annoy me,' says I. 'Mr. Beale arrives home every evening in a condition which I can only describe as deplorable.'"

"What did she say?"

The girl made a little grimace and became serious.

"She said if he did not speak to me or interfere with me or frighten me it was none of my business, or something to that effect." She laughed helplessly. "Really, the flat is so wonderful and so cheap that one cannot afford to get out--you don't know how grateful I am to you, doctor, for having got diggings here at all--Miss Millit isn't keen on single young ladies."

She sniffed and laughed.

"Why do you laugh?" he asked.

"I was thinking how queerly you and I met."

The circumstances of their meeting had indeed been curious. She was employed as a cashier at one of the great West End stores. He had made some sort of purchase and made payment in a five-pound note which had proved to be counterfeit. It was a sad moment for the girl when the forgery was discovered, for she had to make up the loss from her own pocket and that was no small matter.

Then the miracle had happened. The doctor had arrived full of apologies, had presented his card and explained. The note was one which he had been keeping as a curiosity. It has been passed on him and was such an excellent specimen that he intended having it framed but it had got mixed up with his other money.

"You started by being the villain of the piece and ended by being my good fairy," she said. "I should never have known there was a vacancy here but for you. I should not have been admitted by the proper Miss Millit but for the terror of your name."

She dropped her little hand lightly on his shoulder. It was a gesture of good-comradeship.

She half-turned to go when an angry exclamation held her.

"What is it? Oh, I see--No. 4!"

She drew a little closer to the doctor's side and watched with narrowing lids the approaching figure.

"Why does he do it--oh, why does he do it?" she demanded impatiently. "How can a man be so weak, so wretchedly weak? There's nothing justifies that!"

"That" was apparently trying to walk the opposite kerb as though it were a tight-rope. Save for a certain disorder of attire, a protruding necktie and a muddy hat, he was respectable enough. He was young and, under other conditions, passably good looking. But with his fair hair streaming over his forehead and his hat at the back of his head he lacked fascination. His attempt, aided by a walking-stick used as a balancing-pole, to keep his equilibrium on six inches of kerbing, might have been funny to a less sensitive soul than Oliva's.

He slipped, recovered himself with a little whoop, slipped again, and finally gave up the attempt, crossing the road to his home.

He recognized the doctor with a flourish of his hat.

"Glorious weather, my Escu-escu-lapius," he said, with a little slur in his voice but a merry smile in his eye; "simply wonderful weather for bacteria trypanosomes (got it) an' all the jolly little microbes."

He smiled at the doctor blandly, ignoring the other's significant glance at the girl, who had drawn back so that she might not find herself included in the conversation.

"I'm goin' to leave you, doctor," he went on, "goin' top floor, away from the evil smells of science an' fatal lure of beauty. Top floor jolly stiff climb when a fellow's all lit up like the Hotel Doodledum--per arduis ad astra--through labour to the stars--fine motto. Flying Corps' motto--my motto. Goo' night!"

Off came his hat again and he staggered up the broad stone stairs and disappeared round a turn. Later they heard his door slam.

"Awful--and yet----"

"And yet?" echoed the doctor.

"I thought he was funny. I nearly laughed. But how terrible! He's so young and he has had a decent education."

She shook her head sadly.

Presently she took leave of the doctor and made her way upstairs. Three doors opened from the landing. Numbers 4, 6 and 8.

She glanced a little apprehensively at No. 4 as she passed, but there was no sound or sign of the reveller, and she passed into No. 6 and closed the door.

The accommodation consisted of two rooms, a bed-and a sitting-room, a bath-room and a tiny kitchen. The rent was remarkably low, less than a quarter of her weekly earnings, and she managed to live comfortably.

She lit the gas-stove and put on the kettle and began to lay the table. There was a "tin of something" in the diminutive pantry, a small loaf and a jug of milk, a tomato or two and a bottle of dressing--the high tea to which she sat down (a little flushed of the face and quite happy) was seasoned with content. She thought of the doctor and accounted herself lucky to have so good a friend. He was so sensible, there was no "nonsense" about him. He never tried to hold her hand as the stupid buyers did, nor make clumsy attempts to kiss her as one of the partners had done.

The doctor was different from them all. She could not imagine him sitting by the side of a girl in a bus pressing her foot with his, or accosting her in the street with a "Haven't we met before?"

She ate her meal slowly, reading the evening newspaper and dreaming at intervals. It was dusk when she had finished and she switched on the electric light. There was a shilling-in-the-slot meter in the bath-room that acted eccentrically. Sometimes one shilling would supply light for a week, at other times after two days the lights would flicker spasmodically and expire.

She remembered that it was a perilous long time since she had bribed the meter and searched her purse for a shilling. She found that she had half-crowns, florins and sixpences, but she had no shillings. This, of course, is the chronic condition of all users of the slot-meters, and she accepted the discovery with the calm of the fatalist. She considered. Should she go out and get change from the obliging tobacconist at the corner or should she take a chance?

"If I don't go out you will," she said addressing the light, and it winked ominously.

She opened the door and stepped into the passage, and as she did so the lights behind her went out. There was one small lamp on the landing, a plutocratic affair independent of shilling meters. She closed the door behind her and walked to the head of the stairs. As she passed No. 4, she noted the door was ajar and she stopped. She did not wish to risk meeting the drunkard, and she turned back.

Then she remembered the doctor, he lived in No. 8. Usually when he was at home there was a light in his hall which showed through the fanlight. Now, however, the place was in darkness. She saw a card on the door and walking closer she read it in the dim light.

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                |        BACK AT 12. WAIT.        |
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He was out and was evidently expecting a caller. So there was nothing for it but to risk meeting the exuberant Mr. Beale. She flew down the stairs and gained the street with a feeling of relief.

The obliging tobacconist, who was loquacious on the subject of Germans and Germany, detained her until her stock of patience was exhausted; but at last she made her escape. Half-way across the street she saw the figure of a man standing in the dark hallway of the chambers, and her heart sank.

"Matilda, you're a fool," she said to herself.

Her name was not Matilda, but in moments of self-depreciation she was wont to address herself as such.

She walked boldly up to the entrance and passed through. The man she saw out of the corner of her eye but did not recognize. He seemed as little desirous of attracting attention as she. She thought he was rather stout and short, but as to this she was not sure. She raced up the stairs and turned on the landing to her room. The door of No. 4 was still ajar--but what was much more important, so was her door. There was no doubt about it, between the edge of the door and the jamb there was a good two inches of space, and she distinctly remembered not only closing it, but also pushing it to make sure that it was fast. What should she do? To her annoyance she felt a cold little feeling inside her and her hands were trembling.

"If the lights were only on I'd take the risk," she thought; but the lights were not on and it was necessary to pass into the dark interior and into a darker bath-room--a room which is notoriously adaptable for murder--before she could reach the meter.

"Rubbish, Matilda!" she scoffed quaveringly, "go in, you frightened little rabbit--you forgot to shut the door, that's all."

She pushed the door open and with a shiver stepped inside.

Then a sound made her stop dead. It was a shuffle and a creak such as a dog might make if he brushed against the chair.

"Who's there?" she demanded.

There was no reply.

"Who's there?"

She took one step forward and then something reached out at her. A big hand gripped her by the sleeve of her blouse and she heard a deep breathing.

She bit her lips to stop the scream that arose, and with a wrench tore herself free, leaving a portion of a sleeve in the hands of the unknown.

She darted backward, slamming the door behind her. In two flying strides she was at the door of No. 4, hammering with both her fists.

"Drunk or sober he is a man! Drunk or sober he is a man!" she muttered incoherently.

Only twice she beat upon the door when it opened suddenly and Mr. Beale stood in the doorway.

"What is it?"

She hardly noticed his tone.

"A man--a man, in my flat," she gasped, and showed her torn sleeve, "a man...!"

He pushed her aside and made for the door.

"The key?" he said quickly.

With trembling fingers she extracted it from her pocket.

"One moment."

He disappeared into his own flat and presently came out holding an electric torch. He snapped back the lock, put the key in his pocket and then, to her amazement, he slipped a short-barrelled revolver from his hip-pocket.

With his foot he pushed open the door and she watched him vanish into the gloomy interior.

Presently came his voice, sharp and menacing:

"Hands up!"

A voice jabbered something excitedly and then she heard Mr. Beale speak.

"Is your light working?--you can come in, I have him in the dining-room."

She stepped into the bath-room, the shilling dropped through the aperture, the screw grated as she turned it and the lights sprang to life.

In one corner of the room was a man, a white-faced, sickly looking man with a head too big for his body. His hands were above his head, his lower lip trembled in terror.

Mr. Beale was searching him with thoroughness and rapidity.

"No gun, all right, put your hands down. Now turn out your pockets."

The man said something in a language which the girl could not understand, and Mr. Beale replied in the same tongue. He put the contents, first of one pocket then of the other, upon the table, and the girl watched the proceedings with open eyes.

"Hello, what's this?"

Beale picked up a card. Thereon was scribbled a figure which might have been 6 or 4.

"I see," said Beale, "now the other pocket--you understand English, my friend?"

Stupidly the man obeyed. A leather pocket-case came from an inside pocket and this Beale opened.

Therein was a small packet which resembled the familiar wrapper of a seidlitz powder. Beale spoke sharply in a language which the girl realized was German, and the man shook his head. He said something which sounded like "No good," several times.

"I'm going to leave you here alone for awhile," said Beale, "my friend and I are going downstairs together--I shall not be long."

They went out of the flat together, the little man with the big head protesting, and she heard their footsteps descending the stairs. Presently Beale came up alone and walked into the sitting-room. And then the strange unaccountable fact dawned on her--he was perfectly sober.

His eyes were clear, his lips firm, and the fair hair whose tendencies to bedragglement had emphasized his disgrace was brushed back over his head. He looked at her so earnestly that she grew embarrassed.

"Miss Cresswell," he said quietly. "I am going to ask you to do me a great favour."

"If it is one that I can grant, you may be sure that I will," she smiled, and he nodded.

"I shall not ask you to do anything that is impossible in spite of the humorist's view of women," he said. "I merely want you to tell nobody about what has happened to-night."

"Nobody?" she looked at him in astonishment. "But the doctor----"

"Not even the doctor," he said with a twinkle in his eye. "I ask you this as a special favour--word of honour?"

She thought a moment.

"I promise," she said. "I'm to tell nobody about that horrid man from whom you so kindly saved me----"

He lifted his head.

"Understand this, Miss Cresswell, please," he said: "I don't want you to be under any misapprehension about that 'horrid man'--he was just as scared as you, and he would not have harmed you. I have been waiting for him all the evening."

"Waiting for him?"

He nodded again.

"Where?"

"In the doctor's flat," he said calmly, "you see, the doctor and I are deadly rivals. We are rival scientists, and I was waiting for the hairy man to steal a march on him."

"But, but--how did you get in."

"I had this key," he said holding up a small key, "remember, word of honour! The man whom I have just left came up and wasn't certain whether he had to go in No. 8, that's the doctor's, or No. 6--_and the one key fits both doors!_"

He inserted the key which was in the lock of her door and it turned easily.

"And this is what I was waiting for--it was the best the poor devil could do."

He lifted the paper package and broke the seals. Unfolding the paper carefully he laid it on the table, revealing a teaspoonful of what looked like fine green sawdust.

"What is it?" she whispered fearfully.

Somehow she knew that she was in the presence of a big elementary danger--something gross and terrible in its primitive force.

"That," said Mr. Beale, choosing his words nicely, "that is a passable imitation of the Green Rust, or, as it is to me, the Green Terror."

"The Green Rust? What is the Green Rust--what can it do?" she asked in bewilderment.

"I hope we shall never know," he said, and in his clear eyes was a hint of terror.