Page:Popular Science Monthly Volume 6.djvu/354

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NDER the low eaves at the back of the house was a long, deep wooden trough for catching the rain that fell on the roof. This old trough was to me a never-failing source of wonder and delight during my childhood. The inside of it was all lined with a beautiful, green, velvety mould, and, when there had been no rain for some time, the water itself would turn a greenish color. We used to catch our little downy yellow ducks and put them in the trough to see them swim, and sometimes they would break off and eat the green mould with their curious shovel-bills. What this queer, green stuff was, and how it came there, was a great mystery to us children. Charley declared it came down in the rain just as the angle-worms that he used for fish-bait. I had to wait a long time to find some one to explain to me all about these simple things. No doubt I might have learned about them here at home, if I had tried hard enough; but it so happened that I found a great professor in London, who was teaching his students just what I wanted to know, and he explained so well what I had seen in the old water-trough, and many other curious things, that I have thought my young friends might like to hear about them also. I am sure I should have been very glad if I could have found any one to explain them to me when I was a child.

Probably you have no trough in which you can find this green mould, but there is plenty of it on old palings, stone-walls, and the trunks of trees. That which comes on the top of water, and makes it look green, is a little different from that which covers old wood and stones, and we shall speak of this difference by-and-by. In order to see what there is in this green, mouldy matter, and what it is made of, you must look at it through the microscope. The word microscope comes from two words which mean little, and to view, and so this instrument is used to magnify, or make larger, things which are too small to be seen with the naked eye. Under it the dust of the butterfly's wing looks as large as the feathers of a canary-bird. Each of you ought to have a microscope of your own to study the things we are going to talk about, or several of you might club together and buy one, and use it "turn about." I am sure you would never regret the investment.

If you carefully scrape off a little of this mould from the trees or fences and look at it through the microscope, you can see that it is made up of exceedingly small bladders or bags. You will find little sacs