“Dreams now!” murmured Garbould contemptuously; and he leaned back and folded his hands over his stomach.

“I didn’t think much of the dream, of course,” Ruth went on. “I had been puzzling about it all so hard for so long that it was only natural to dream about it. But after breakfast I had a sudden feeling that the secret was in the laboratory if I could only find it. I did not attach any importance to the feeling; but it went on growing stronger; and after lunch I went to the laboratory and began to hunt.

“I looked through all the drawers and could find nothing. Then I went round the room looking at everything and into everything, instruments and retorts and tubes and so on. Then I went into the middle of the floor and looked slowly round the room pretty hard. Against the wall, near the door, lying ready to be taken away, was a gas cylinder rolled it over to see what gas had been in it and it had no label on it.”

She paused to look round the Court as if claiming its best attention; then she went on: “Now that was very queer because every gas cylinder must have a label on it – so many gases are dangerous. I turned on the cylinder and nothing came out of it. It was quite empty. Then I went to the book in which all the things which come in are entered, and found that ten days before Dad died he had had in a cylinder of CO2 and seven pounds of ice. Also he had had seven pounds of ice every day till the day of his death. It was the ice and the CO2 together that gave me the idea. CO2, carbon dioxide, has a very low freezing-point – eighty degrees centigrade – and as it comes out of the cylinder and mixes with the air it turns into very fine snow; and that snow, if you compress it, makes the hardest and toughest ice possible. It flashed on me that Dad could have collected this snow and forced it into a mould and made a weapon that would not only inflict that wound but would disappear instantly!”

She paused again to look round the Court at about as rapt a lot of faces as any narrator could desire. Then she went on: “I knew that that was what he had done. I knew it for certain. Carbon dioxide ice would make a hard, tough dagger, and it would melt quickly in the hottest room of a Turkish bath and leave no smell because it is scentless. So there wouldn’t be any weapon. And it explained the tea-leaf too. Dad had made a carbon dioxide dagger perhaps a week before he used it, perhaps only a day. And he had put it into the thermos flask as soon as he had made it. The thermos flask keeps out the heat as well as the cold, you know. But to make sure that it couldn’t melt at all he kept the flask in ice till he was ready to use the dagger. It’s the only way you can explain that tea-leaf. It came out of the flask sticking to the point of the dagger and was driven into the wound!”

She paused again and one might almost say that the Court heaved a deep sigh of relief.

Then Garbould asked in an unpleasant and incredulous voice: “Why didn’t you take this fantastic theory straight to the police?”

“But that wouldn’t have been any good,” she protested quickly. “It was no use my knowing it myself; I had to make other people believe it; I had to find evidence. I began to hunt for it. I felt in my bones that there was some. What I wanted was the mould. I found it!”

She uttered the words in a tone of triumph and smiled at Willoughton; then she went on: “At least I found bits of it. In the box into which we used to throw odds and ends, scraps of material, damaged instruments, and broken test tubes, I found some pieces of vulcanite; and I saw at once that they were bits of a vulcanite container. I took some wax and rolled it into a rod about the right size and then I pieced the container together on the outside of it – at least most of it – there are some small pieces missing. It took me nearly all night. But I found the most important bit – the pointed end!”

She dipped her hand into her handbag and drew out a black object about nine inches long and three quarters of an inch thick and held it up for every one to see.

Someone, without thinking, began to clap; and there came a storm of applause that drowned the voice of the Clerk calling for order and the bellowing of Garbould.

When the applause died down, Hazeldean, who never misses the right moment, said: “I have no more questions to ask the witness, my lord,” and sat down.

That action seemed to clinch it in my eyes, and I have no doubt, it clinched it in the eyes of the jury.

The purple Garbould leant forward and almost bellowed at Ruth: “Do you expect the jury to believe that a well-known man like your father died in the act of deliberately setting a dastardly trap to hang the prisoner?”

Ruth looked at him, shrugged her shoulders, and said with a calm acceptance of the facts of human nature one would expect to find only in a much older woman: “Oh, well, Daddy was like that. And he certainly believed he had very good reasons for killing Mr. Willoughton.”

There was that in her tone and manner which made it absolutely certain that Kelstern was not only like that but that he had acted according to his nature.

Greatorex did not re-examine Ruth; he conferred with Hazeldean. Then Hazeldean rose to open the case for the defence. He said that he would not waste the time of the Court, and that in view of the fact that Miss Kelstern had solved the problem of her father’s death, he would only call one witness, Professor Mozley.

The grey-headed, grey-bearded, stooping man, who had come to the Court so late, went into the witness-box. Of course his face had been familiar to me; I had seen his portrait in the newspapers a dozen times. He still carried the brown-paper parcel.

In answer to Hazeldean’s questions he stated that it was possible, not even difficult, to make a weapon of carbon dioxide hard enough and tough enough and sharp enough to inflict such a wound as that which had caused Kelstern’s death. The method of making it was to fold a piece of chamois leather into a bag, hold that bag with the left hand, protected by a glove, over the nozzle of a cylinder containing liquid carbon dioxide, and open the valve with the right hand. Carbon dioxide evaporates so quickly that its freezing-point, 80 degrees centigrade, is soon reached; and it solidifies in the chamois-leather bag as a deposit of carbon dioxide snow. Then turn off the gas, spoon that snow into a vulcanite container of the required thickness, and ram it down with a vulcanite plunger into a rod of the required hardness. He added that it was advisable to pack the container in ice while filling it and ramming down the snow, then put the rod into a thermos flask; and keep it till it is needed.

“And you have made such a rod?” said Hazeldean.

“Yes,” said the Professor, cutting the string of the brown-paper parcel. “When Miss Kelstern hauled me out of bed at half-past seven this morning to tell me her discoveries, I perceived at once that she had found the solution of the problem of her father’s death, which had puzzled me considerably. I had breakfast quickly and got to work to make such a weapon myself for the satisfaction of the Court. Here it is.”

He drew a thermos flask from the brown paper, unscrewed the top of it, and inverted it. There dropped into his gloved hand a white rod about eight inches long. He held it out for the jury to see.

“This carbon dioxide ice is the hardest and toughest ice we know of; and I have no doubt that Mr. Kelstern killed himself with a similar rod. The difference between the rod he used and this is that his rod was pointed. I had no pointed vulcanite container; but the container that Miss Kelstern pieced together is pointed. Doubtless Mr. Kelstern had it specially made, probably by Messrs. Hawkins Spender.”

He dropped the rod back into the thermos flask and screwed on the top.

Hazeldean sat down. The juryman who had been reprimanded by Garbould leaned forward and spoke earnestly to the foreman. Greatorex rose.