Note 8 — HOME STUDIES IN CHEMISTRY AND PHYSICS.

“Diatoms,” A species of microscopic plants belonging to the Algæ or sea weeds, growing in both salt and fresh water. For a long time they were thought to belong to the animal kingdom, and were classed among the animalcules, as they had the strange power of secreting from the water silicious shells. They grew in some places in such abundance as to form large beds of their shelly remains, the material of which is used for the polishing powders known under the name of tripoli. The species found now are exactly like the fossils. Ehrenberg says: “Species are to be found, in a living state, in situations where they have been propagated from times far anterior to the existence of man.” They may be obtained by allowing the water in which they grow to stand for a time, and then pouring off all but the slimy part at the bottom. The diatoms may often be seen to move a little in the water, from which it was supposed they belonged to the animal kingdom. Professor Bailey describes one very interesting species, as follows: “At one moment the needle-shaped frustules lie side by side, forming a rectangular plate; suddenly one of the frustules slides forward a little way, the next slides a little also, and so on through the whole number. These motions are constantly going on, and with such rapidity that the eye can scarcely follow them. The cause of the motion is wholly unknown, but it is probably mechanical and not vital.” One species in its growth takes on a fanlike appearance and is exceedingly beautiful. Fossil diatoms have been found in all the strata of rock formations. These with some of the minute creatures of the animal kingdom have left greater records of themselves than most of the higher forms of life. Extensive deposits of their silicious shells are found in various parts of the world. The city of Richmond, Va., stands on a layer of them which is eighteen feet in thickness.