January of 1920 came, and the river was frozen over. The flowing oxygen was solid enough for men and horses to walk on. There they went in beaten paths, warm water trudging along on cold! Jean had always been amazed by such miracles, but their potency was doubled for her now, and they kept her from breaking her heart.

Twice a week she and the dog tramped up the ice to the post-office, always with the hope of not hearing from Marvin. Any letter from him would mean that he had found a new girl. She ought to be hoping for such a letter, but somehow her wicked heart declined to hope.

In her little laboratory she was perfectly free to risk her neck with any experiment she chose. And considering that she had got the oxygen from potassium chlorate, and that Horatio had barely missed passing out by chlorine before he met his death by other forces, she found herself wishing to see chlorine.

She read that it was usually a green gas, though it could be liquefied and stored in steel tanks. She read that it combines with sodium to make table-salt, though how a green gas could come to look so sparkling white she did not understand. At first she thought she would coax it out of salt, but having no electric battery she had to abandon that plan.

It was like fluorine and bromine, curious things of which she knew nothing. It was also like iodine, with which people paint themselves on the slightest provocation, though she suspected that soap and water will ordinarily do as well. It was furthermore related in some mysterious way to a heavy metal called manganese. She had not the faintest suspicion that manganese conceals twenty-five charges of electricity, or that its two undiscovered relatives conceal forty-three and seventy-five respectively. But her book gave her to understand that chlorine can easily be extracted from manganese chloride.

So she asked Ojeeg, who was now become so respectable that he was carrying the mail, to bring her from the Soo a little manganese dioxide. She wrote the magical words for him on a piece of birch bark, and next time he not only brought the stuff to her in the clearing, but stood by and watched her mix it with hydrochloric acid, heat it, and collect the chlorine in a bottle.

They sniffed it and understood perfectly why it choked soldiers to death. And long after Ojeeg left her, she stood pondering that greenish yellow mist through its glass prison. It seemed incredible that human beings would deliberately roll that death along the ground to fill the nostrils of other human beings. But it had been done, and when the madness again seized on men it would be done again. But there in her silent retreat among life-giving wintry airs she determined that no son of hers should ever be compelled to breathe chlorine.