He paused, tapped the machine and went on, "This is a polarimeter--a simple polariscope--a step beyond the saccharimeter," he explained, with a nod at Sandoval. "It detects differences of structure in substances not visible in ordinary light.

"Light is polarized in several ways--by reflection, by transmission, but most commonly through what I have here, a prism of calcite, or Iceland spar, commonly called a Nicol prism. Light fully polarized consists of vibrations transverse to the direction of the ray, all in one plane. Ordinary light has transverse vibrations in all planes.

Certain substances, due to their molecular structure, are transparent to vibrations in one plane, but opaque to those at right angles.

"Here we have," he explained, tapping the parts in order, "a source of light, pa.s.sing in through this aperture, here a Nicol polarizer, next a liquid to be examined in a gla.s.s-capped tube; here on this other side an arrangement of quartz plates with rotary power which I will explain in a moment, next an a.n.a.lyzer, and finally the aperture for the eye of an observer."

Kennedy adjusted the gla.s.s tube containing the liquid which bore the substance sc.r.a.ped from the cartridge--he had picked up in the office of Jose. "Look through the eyepiece, Walter," he directed.

The field appeared halved. He made an adjustment and at once the field of vision appeared wholly the same tint. When he removed the tube it was dark.

"If a liquid has not what we call rotary power both halves of the double disk appear of the same tint," he explained. "If it has rotary power, the halves appear of different tints and the degree of rotation is measured by the alteration of thickness of this double quartz plate necessary to counteract it. It is, as I told Mr. Jameson early to-day, a rather abstruse subject, this of polarized light. I shall not bore you with it, but I think you will see in a moment why it is necessary, perhaps why some one who knew thought it would never be used.

"What I am getting at now is that some substances with the same chemical formula rotate polarized light to the right, are dextro-rotary, as, for instance, what is known as dextrose. Others rotate it to the left, are levo-rotary, as the substance called levose.

Both of them are glucose. So there are substances which give the same chemical reactions which can only be distinguished by their being left or right rotary."

Craig took a bit of crystalline powder and dissolved it in ether. Then he added some strong sulphuric acid. The liquid turned yellow, then slowly a bright scarlet. Beside the first he repeated the operation with another similar-looking powder, with the identical result.

"Both of those," he remarked, holding up the vials, "were samples of pure veratrine, but obtained from different sources. You see the brilliant reaction--unmistakable. But it makes all the difference in the world in this case what was the source of the veratrine. It may mean the guilt or innocence of one of you."

He paused, to let the significance of his remark sink in. "Veratrine,"

he resumed, "is a form of h.e.l.lebore, known to gardeners for its fatal effect on insects. There are white and green h.e.l.lebore, Veratrum alba and Veratrum viride. It is the pure alkaloid, or rather one of them, that we have to deal with here--veratrine.

"There are various sources of veratrine. For instance, there is the veratrine that may be derived from the sabadilla seeds which grow in the West Indies and Mexico. It is used, I am informed, by the Germans in their lachrymatory and asphyxiating bombs."

The mention of the West Indies brought, like a flash, to my mind Sandoval and Senorita de Leon.

"Then, too," continued Kennedy, "there is a plant out in our own Western country, of which you may have heard, known as the death camas, very fatal to cattle when they eat it. The active principle in this is also veratrine."

I began to see what Kennedy was driving at. If it were veratrine derived from death camas it would point toward Page.

"Abderhalden, the great German physiological chemist, has discovered that substances that once get into the blood produce specific ferments.

Not long ago, in a case, I showed it by the use of dialyzing membranes.

But Abderhalden has found that the polariscope can show it also. And in this case only the polariscope can show what chemistry cannot show when we reach the point of testing Senor Barrios's blood--if that becomes necessary."

It was plain that Kennedy was confident. "There are other sources of drugs of the nature used in this case to asphyxiate and kill, but the active principle of all is veratrine. The point is, veratrine from what source? The sabadilla is dextro-rotary; the death camas is levo-rotary.

Which is it here?"

As I tried to figure out the ramifications of the case, I could see that it was a cruel situation for one or the other of the girls. Was one of her lovers the murderer of Anitra's brother? Or was her own brother the murderer of Eulalie's lover? I looked at the faces before me, now tensely watching Kennedy, forgetful of the wire-like strings which they held in their hands. I studied Teresa de Leon intently for a while. She was still the enigma which she had been the first time I saw her.

Kennedy paused long enough to look through the eyepiece again as if to rea.s.sure himself finally that he was right. There was a tantalizing suspense as we waited for the verdict of science on this intensely human tragedy. Then he turned to the queer instrument over which the needle-hand was moving.

"Though some scientists would call this merely a sensitive form of galvanometer," he remarked, "it is, to me, more than that. It registers feelings, emotions. It has been registering your own every moment that I have been talking.

"But most of all it registers the grand pa.s.sion. I might even call it a love meter. Love might seem to be a subject which could not be investigated. But even love can be attributed to electrical forces, or, perhaps better, is expressed by the generation of an electric current, as though the attraction between men and women were the giving off of electrons or radiations of one to the other. I have seen this galvanometer stationary during the ordinary meeting of men and women, yet exhibit all sorts of strange vibrations when true lovers meet."

Not used to Kennedy's peculiar methods, they were now on guard, ignorant of the fact that that alone was sufficient to corroborate unescapably any evidence they had already given of their feelings toward each other.

Kennedy pa.s.sed lightly over the torn and bleeding heart of Eulalie.

But, much as he disliked to do so, he could not so quickly pa.s.s Anitra.

In spite of her grief, I could see that she was striving to control herself. A quick blush suffused her face and her breath came and went faster.

"This record," went on Kennedy, lowering his voice, "tells me that two men are in love with Anitra Barrios. I will not say which exhibits the deeper, truer pa.s.sion. You shall see for yourself in a moment. But, more than that, it tells me which of the two she cares for most--a secret her heart would never permit her lips to disclose. Nor will I disclose it.

"One of them, with supreme egotism, was so sure that he would win her heart that he plotted this murder of her brother so that she would have the whole estate to bring to him--a terrible price for a dowry. My love meter tells me, however, that Anitra has something to say about it yet.

She does not love this man.

"As for Teresa de Leon, it was jealousy that impelled her to follow Jose Barrios from Cuba to New York. The murderer, in his scheming, knew it, saw a chance to use her, to encourage her, perhaps throw suspicion on her, if necessary. When I came uncomfortably close to him he even sent an anonymous telegram that might point toward her. It was sent by the same person who stole in Barrios's office and shot him with an asphyxiating pistol which discharged a fatal quant.i.ty of pure veratrine full at him.

"My love meter, in registering hidden emotions, supplements what the polarimeter tells me. It was the levo-rotary veratrine of the fatal death camas which you used, Page," concluded Craig, as again the electric attachment clicked shut the lock on the laboratory door.

VIII

THE VITAL PRINCIPLE

"That's the handwriting of a woman--a jealous woman," remarked Kennedy, handing to me a dainty note on plain paper which had come in the morning mail.

I did not stop to study the writing, for the contents of the letter were more fascinating than even Kennedy's new science of graphology.

You don't know me [the note read], but I know of your work of scientific investigation.

Let me inform you of something that ought to interest you.

In the Forum Apartments you will find that there is some strange disease affecting the Wardlaw family. It is a queer disease of the nerves. One is dead. Others are dying.

Look into it.

A FRIEND.

As I read it I asked myself vainly what it could mean. There was no direct accusation against any one, yet the implication was plain. A woman had been moved by one of the primal pa.s.sions to betray--some one.

I looked up from the note on the table at Craig. He was still studying the handwriting.

"It's that peculiar vertical, angular hand affected by many women," he commented, half to himself. "Even at a glance you can see that it's written hastily, as if under the stress of excitement and sudden resolution. You'll notice how those capitals--" The laboratory door opened, interrupting him.

"h.e.l.lo, Kennedy," greeted Doctor Leslie, our friend, the coroner's physician, who had recently been appointed Health Commissioner of the city.

It was the first time we had seen him since the appointment and we hastened to congratulate him. He thanked us absently, and it was evident that there was something on his mind, some problem which, in his new office, he felt that he must solve if for no other purpose than to justify his reputation. Craig said nothing, preferring to let the commissioner come to the point in his own way.

"Do you know, Kennedy," he said, at length, turning in his chair and facing us, "I believe we have found one of the strangest cases in the history of the department."

The commissioner paused, then went on, quickly, "It looks as if it were nothing less than an epidemic of beriberi--not on a ship coming into port as so often happens, but actually in the heart of the city."

"Beriberi--in New York?" queried Craig, incredulously.