As they stood at their places around the table, Miss Soddern gave a resumé of what they had already learned about dust and its dangers. They talked with a fluency that surprised Brenda about bacteria and yeasts and spores and moulds, and in most cases showed by examples that they knew what they were talking about. "I am glad that all these bacteria are not harmful," said Brenda, "for otherwise I should stand in fear of instant death when caught in one of our east winds," and she looked with interest at the plate that showed a great many little spots irregularly distributed within a circle. Each spot represented a colony of bacteria, and though the showing was rather overwhelming, it was not nearly as bad as another exposure made at a crossing in a certain city where the old-fashioned street-cleaning methods prevailed. An exposure made just after the carts had been collecting heaps of dirt showed an almost incredible number, quite beyond counting. So interesting did Miss Soddern make her lesson that Brenda stayed quite through the hour. "I've gathered one or two new ideas on the subject of trailing skirts," she whispered to Julia in one of the intervals of the lesson. "I always thought it was just a notion, this talk about their being so unclean, but now I shall always think of them as regular bacteria collectors. Also I've learned one or two things about dusting, and I'm going to watch our maid to-morrow, and if she isn't using a moist cloth, I'll frighten her by asking her why she insists on distributing death-dealing germs around the room." Half of the class that day had to report the result of their own observation of bacteria colonies collected on the gelatine plate, and half were to prepare the little glass boxes to take home. Brenda watched the process with great interest,--the preparation of the boxes in a vacuum, so that there would be no air inside them when they should be first exposed in the new locality. "It's something," said Julia, "to get these girls to acquire habits of accuracy." "Oh, it reminds me of the class in physics at Miss Crawdon's," replied Brenda. "I never would take it myself, but some of the girls said that it was splendid; it taught one to be accurate." At that moment Miss Soddern began to address the girls. They had been so absorbed in their work that they had talked very little during the hour. "How many of you have anything to report regarding the boxes that you took home last week." One by one the outside girls gave accounts of their observations, each one vying with the others to describe the most prolific growth of bacteria. "As the boxes were to be exposed simply in their living-rooms, I am surprised at the results," said the teacher in an aside to Julia; "I'm afraid that some one must have been stirring up the dust. What does your family think of these experiments?" she continued, turning to a bright-eyed American girl.