By and by she remarked that they were more alert. Some were making notes, and one or two looked past her with frank curiosity. The door was behind the board, and Agatha had heard nobody come in, but when she looked round she saw a gray-haired gentleman standing near the lady principal. He seemed to be listening to what she said and she thought his eyes twinkled as if he understood the difficulty of rousing her pupils' interest. This was somewhat embarrassing; but the school was famous and visitors were now and then shown round.
She paused, and the stranger turned to the principal. "If you will allow me—"
The principal smiled and he came up to Agatha, holding out his hand for the chalk.
"Suppose we alter the formula this way?" he said and wiped out the letters and figures.
Agatha studied him as he wrote fresh symbols. He was plainly dressed and about sixty years of age. There was nothing else worth noting, but he obviously knew his subject and she liked his face. She saw that the girls could follow his explanation, but while suited to their understanding, it was, in one respect, not quite accurate.
"I don't know if I've made it much plainer," he said deprecatingly when he stopped.
Agatha indicated a group of letters. "It is plainer, thank you! But does the combination of the two elements take place exactly as you have shown? At a normal temperature, the metal's affinity for oxygen—"
"Ah," he said, "you know that? It looks as if you had studied the new Austrian theory. But perhaps one may make a small concession, for the sake of clearness."
"Science is exact," Agatha replied.
"It's a bold claim for us to make," he rejoined, smiling. "Our symbols are guess measures; our elements split up into two or three. But I gather that you refuse to compromise about what, in the meantime, we think is the truth?"