"It is a sort of muslin bag." said Norah. "Quite right. But why does she use this bag?""The muslin is porous. The holes, or pores, in it are only large enough to let the clear liquid run through. The thick coffee-grounds cannot pass through them, and are left in the bag.""Teacher had a bag made of flannel. You know flannel is porous. We can see the pores in it. Well, he poured some muddy water in this bag, and held the bag over a basin. The pores were not large enough to let all the solid mud pass through. That was kept back, and the water, that dripped through into the basin, was nearly clear. Well then, he poured this into another bag made of some kind of cloth with still smaller pores. When the water came through this it was quite clear. All the little grains ofmud, that had passed through the pores in the first bag, were kept back now. The pores in this second bag were too small to let them pass.
"I thought at once of mother"s coffeestrainer. But teacher gave us another name for these porous bags. He calls them filters. He says when we pass a liquid through them, we filter it.