英文科學(xué)讀本 第二冊(cè)·Lesson 04 Filters

Lesson 04 Filters

You know, Norah, when mother makes the coffee for breakfast, she pours it through a strainer. said Fred.

What is the strainer made of?

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 of mud, 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 coffee-strainer. 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.

I am going to show you a very pretty little filter now, made of blotting-paper. Teacher made some for the lesson. He gave me these to try with at home. See, there are three pieces of blottingpaper, cut round, and folded across the center at right angles. When I open them out, I can make a little funnel with them.

Now I will stand the funnel in a tumbler, and pour into it some of this chalk and water. See, the liquid looks thick and white like milk. Watch what happens when I pour it into the filter. Drop by drop, the water trickles through the pores of the blotting-paper. But it is clear water now. All the little pieces of solid chalk, that were floating in the water, are left behind in the blotting-paper. The blotting-paper is a filter. It filters the liquid I put into it.

SUMMARY

Filters are made of porous substances. They let the clear water pass through their pores, but keep back the little pieces of solid matter. Blotting-paper is porous; it makes a good filter.