Professor Reuter's oily voice boomed out. "I regret to say that although this metal responds to the spectroscopic tests for beryllium, it will not do for our purpose. It is, in fact, an isotope of beryllium, a metal which resembles it spectroscopically but not physically. The weight is wrong; the metal is much too heavy and will not do for making armor for space ships." "But--but--" babbled Adam. "I didn't weigh it, of course, but it seemed very light to me." "And to me also," remarked Dr. Perkins. There was a frown of puzzlement if not of suspicion between his brows. "Miss de Vries, you will not make a record of this useless argument," snapped McCausland. "Professor Reuter will explain--" "I will explain that the sample in its original form was full of air particles, like a fine sponge," remarked the professor easily, but with his mouth working. "The Plutonians evidently have some process of lightening in this fashion." "Isn't there any way of treating it?" McCausland turned to Dr. Perkins. "Will you explain to our young but over-enthusiastic friend about that?" Dr. Perkins shook his head. The frown still persisted. "Not if it's a true isotope. That would be the same thing as transmuting elements. But I still confess I do not entirely understand." McCausland took up the word swiftly. "Meanwhile, since it is certain that the metal used by these Plutonians will not do for our purpose, I think it important that we at least look into the composition of their civilization to some extent," he remarked. "I propose to investigate them with Professor Reuter's co-operation. Mister Longworth, you will take charge of the ship guard." Paulette spoke up suddenly. "Captain McCausland, I think it is important that I go with you on making this contact with the Plutonians. Certainly everyone on the three planets will want to know about them." The captain's mouth writhed a moment, and Adam noticed the glance he shot at Professor Reuter, but his voice was smooth. "To be sure, Miss de Vries. I think, then, this is all that comes before the present meeting. Dismiss." * * * * * The space in front of the metal wall that shut off the Plutonian domain had been cleared. As Paulette, accompanied by Professor Reuter, McCausland, and Bjornsen of the engine-room staff, reached the bottom of the dig and stepped over to it, they noted that although the wall still showed part of the jagged break, a door had been fitted to fill most of the gap.