Rhona went back to her old quarters at the little hotel in Surrey Street for that night, and next morning Hetherwick came round to her, with an armful of newspapers. Finding her alone, he laid them on the table at her side with a significant nod of his head at certain big black letters which topped the uppermost columns.  
"Matherfield must have given plenty of informing news to the pressmen last night," he remarked with a grim smile. "It's all in there—his own adventures at Southampton yesterday; mine and Robmore's in Westminster, and all the rest of it. I believe the newspaper people call this sort of thing a story—and a fine story it makes! Winding up, of course, with the dramatic arrest of Baseverie at Waterloo! I'm afraid we're in for publicity for a time, worse luck!"
 
"Shall we—shall I—have to appear at that man's trial?" asked Rhona.
 
"That's unavoidable, I'm afraid, and at other things before that," answered Hetherwick. "There'll be the proceedings before the magistrate, and the adjourned inquest, and so on. Can't be helped; and there'll be some satisfaction in knowing that we're ridding the world of a peculiarly cruel and cold-blooded murderer! That chap Baseverie is certainly as consummate a villain as I ever heard of. A human spider—and clever in his web-spinning. But I wish one had a few more particulars on one point—and yet I don't see how one's to get them."
 
"What point?" asked Rhona.
 
"That sealed packet, containing the details, or formula, or whatever it is, of your grandfather's invention," replied Hetherwick. "Where is it? What, precisely, is it? Did Ambrose get it from him? Has Baseverie got it? So far as I can make out, the whole thing began with that. Whether it was really worth a farthing or a fortune, your grandfather brought to London something which he honestly believed to be of great value, and there's no doubt that he got into the hands of those two men, Ambrose and Baseverie, because of it. There's no doubt, either, that in conversation with them, he told them, perhaps jokingly, what he knew about Madame Listorelle. Nor is there any doubt that these two murdered him. Nor is there any doubt, in my mind, as to how they murdered him! You must remember that both men were trained medical men, and, obviously of a scientific turn of mind into the bargain. Each had doubtless made a deep study of poisons. Such a knowledge is of value to such men as they were—men of criminal tendencies. Probably they knew of a subtle poison easily administered, the effects of which would not be evident for some hours. No doubt they timed their work, so that their victim should die swiftly and suddenly when well away from their laboratory. And, of course, they did the same thing in the case of Granett. Granett paid the penalty of being with your grandfather. But for what did they murder your grandfather? Did they get rid of him so that they could keep his secret about Madame Listorelle to themselves, and blackmail her and her sister, or that they could rob him of his invention and turn it to their own profit? If the latter, then——"
 
He paused, looking inquiringly at Rhona, as if he expected her to see what he was after. But Rhona shook her head.
 
"I don't follow," she said. "What then?"
 
"This," replied Hetherwick. "If their desire to get hold of your grandfather's secret was their motive, then that secret's worth a lot of money! Money which ought to come to you. Don't you see? Where is the secret? Where's the sealed packet? I suppose the police would search Baseverie last night—perhaps they found it on him. We shall hear—but, anyway it's yours."
 
Rhona made a gesture of aversion.
 
"I should hate to touch or have anything to do with it if it had been in that man's possession!" she said. "But I don't think there's any doubt that they murdered my grandfather because of that secret. Only, I think, too, they'd a double motive. The secret about Madame Listorelle was their second string. Probably they believed that Lady Riversreade would be an easy prey. And I think she would have been, if she hadn't had Major Penteney to fall back on. I know she was dreadfully upset after Baseverie's first visit. So I put it this way—always have done: they thought they could sell grandfather's invention for a lot of money, and get another lot out of Lady Riversreade and Madame Listorelle as blackmail."
 
"Black money, indeed, all of it!" exclaimed Hetherwick. "Well——"
 
A woman-servant put her head into the little parlour in which they were sitting, and looked significantly at Rhona.
 
"There's a policeman downstairs, miss, asking for you," she announced. "Leastways, he wants to know if you can tell him if Mr. Hetherwick's here or been here."
 
Hetherwick went to the head of the stair; a policeman standing in the hall below looked up and touched his helmet.
 
"Inspector Matherfield's compliments, sir, and could you step round and bring Miss Hannaford with you?" he asked. "There's new developments, Mr. Hetherwick. Important!"
 
"We'll come at once," assented Hetherwick. "Ten minutes!" He went back and hurried Rhona away. "What now?" he asked as they hastened towards Matherfield's office. "Perhaps they've extracted something out of Baseverie? Or possibly the newspapers have attracted the attention of somebody who can give further news?"
 
The last suggestion strengthened itself when, on entering Matherfield's room, they found him closeted with two strangers whose appearance was that of responsible and well-to-do commercial men. All three were discovered in what looked like a serious and deep conversation, and Hetherwick was quick to notice that the two unknown men looked at Rhona with unusual interest. Matherfield made haste to introduce her as the late ex-Superintendent Hannaford's granddaughter, and Hetherwick as a gentleman who had been much concerned in the recent proceedings.
 
"These gentlemen, Miss Hannaford and Mr. Hetherwick," he proceeded, waving his hand at the others, "are Messrs. Culthwaite and Houseover, manufacturing chemists, of East Ham—incidentally, they've also a big place in Lancashire. And having seen this morning's papers, in which, as you've no doubt noticed, there's a good deal about our affair, they've come straight to me with some news which will prove uncommonly useful when Baseverie's put in the dock before the magistrate this afternoon. The fact is, Mr. Hetherwick, these gentlemen have supplied a missing link!"
 
"What link?" asked Hetherwick eagerly.
 
Matherfield nodded at the elder of the two men, Culthwaite, who produced a pocket-book, and extracted from it a sheet of paper. Silently, he passed it over to Matherfield, who turned to Rhona.
 
"Now, Miss Hannaford," he said, with a note of triumph in his voice, "I dare say you can positively identify your grandfather's handwriting and his way of making figures? Can you swear that this has been written by him?"
 
Rhona gave but one glance at the paper before looking up with a glance of positive assertion.
 
"Oh, yes!" she exclaimed. "That is his writing, without a doubt! Nothing could be more certain!"
 
Matherfield turned to Hetherwick.
 
"That's the formula for the ink!" he sai............