As soon as they had passed from the sunlight to darkness, the captives knew they were in a sort of cave.  The blackness was intense, but in a short time there was a faint glow observed ahead, caused by a number of the stone lamps burning.

At that point were gathered several more of the Esquimaux, all attired in white furs.  There was some excitement when the prisoners were brought in, and all crowded around to see them.

Then began what seemed a discussion among the natives.  They talked loud and long.  Finally from some other part of the cave two tall men, dressed as the others were, in white, came in.  They seemed to be in authority, for when they had spoken all the others were silent.

While the captives waited in anxiety for what would happen next, the whole cave was illuminated with a wonderful light.  It was rosy red at first, then changed to a golden hue, then to green, yellow, blue and purple.  The captives could not see where the fire came from, but they gazed at the light in mingled fear and admiration.

“It is the northern light; the aurora borealis!” exclaimed the professor.  “It is the most beautiful light in the world.”

“What makes it?” asked Andy.

“Electricity, magnetism, the sun and the intense cold; no one knows exactly what produces it,” replied Amos Henderson.  “It is quite likely that there is some opening to this cave, and the sun shines in it, or the lights may be reflected from outside by reason, of the ice, which acts as a mirror.”

“It’s pretty,” observed Andy, “but all the same I’d rather—­”

He got no further for the two giant-sized natives advanced quickly toward them.  One roughly seized Professor Henderson, and, with the help of his companion, began stripping off his clothes.  Andy started forward to aid the captain, but the other natives held him back.  Washington, too, was restrained by several hands.

In a few minutes the professor was stripped, except for a piece of fur about his middle.  Then a rude litter was brought in.  The two big natives, after pouring some oil over the old man, placed their victim on the stretcher, and then began a march up the cave.  Washington and Andy were forced to walk directly behind the inventor, and were surrounded by natives on every side.  The poor professor soon became half insensible from the cold.

“This is terrible!” groaned Andy.

“Dey is goin’ to slaughter him!” wailed Washington.  “He’ll be sacrificed and burned up!  See, de altar ob de sacrifice am just ahead.  Oh!  I wisht we wuz all dead!”

“We’re likely to be, soon enough,” muttered Andy.  “But keep up your courage!”

At that instant the head of the procession was close to the ice altar.  Behind it the mysterious lights played and flickered in streamers of red, green and gold.  Up the steps went the two gigantic men, carrying the professor.  They were about to sacrifice him in a horrible way!