“No doubt they are working hard to solve the mystery. Unfortunately the best they can do is to guess. We hold the key. That is--I should correct that statement--we hold the knowledge, and they hold the keys.” “The keys?” Harry wanted to know more. The professor pointed to the three empty sockets in the great white stone above their heads. “These three missing stones are the keys. Until they are reset we cannot control the Spot. I had found two of them before I came through. I take it that both of you remember the blue one?” “I think,” agreed Chick, “that neither of us is ever likely to forget it! Eh, Harry?” The professor smiled. He was holding the light up to the snow-stone, at a spot that would have been the point of intersection had lines been drawn from the three missing gems, and the resulting triangle centred. He held his hand up to the substance. It was slightly rough at that point, as though it had been frozen. Then he ran his fingers across the surrounding surface. “Ah!” he exclaimed. “I thought so! That helps considerably. Chick--put your hand up here. What do you feel?” “Rough,” said Chick, feeling the intersection point. “Slightly so, but cold and--and magnetic.” “Now feel here.” “Cool and magnetic, doctor; but smooth. What does it prove?” “Let's see; do you understand the term 'electrolysis'? Good. Well, there should be another clue--not similar, but supplementary, or rather, complementary--on the earth side. Perhaps one of you found it while you lived in that house.” The professor eyed both men anxiously. “Did either of you find a stain, or anything of that sort, on the walls, ceiling, or floor of any room there?” Both shook their heads. “Well, there ought to be,” frowned the doctor. “I am positive that, should we return now, we could locate some such phenomenon. From this side it is very easy to account for; it's simply the disintegrating effect of the current, constantly impinging at the point of contact or the intersection. Having acted on this side, it must have left some mark on the other.” Watson was still running his hand over the snow-stone. Once before, when he had stood barefooted in the contest with the Senestro, he had noted its cold magnetism. “What is this substance, professor?” “That, I have not been able to discover. I would call it neutral element, for want of a more exact term; something that touches both aspects of the spectrum.” “Both aspects of the spectrum?” “Yes; as nearly as the limitations of my vocabulary will permit. If you recall, I showed you a simple experiment the other day in the palace. By means of an inductor I drew out the iron principle from the ether and built up the metal. Only it was not precisely iron but its Thomahlian equivalent. Had you been on the earth side you would have seen nothing at all, not even myself. I was on the wrong aspect of the spectrum.