302
JOURNAL OF CHEMICAL EDUCATION
FEBRUARY, 1931
periment. If the same experiment is used to demonstrate more than one point is should be performed a different time for each point. 4. The experiment must work quickly and thus avoid the loss of valuable class time. 5. The apparatus must be as simple as possible so that i t will be easy to follow the process, and so the apparatus will not attract the attention of the student from the process demonstrated. 6. The action to he studied must be clearly visible from all parts of the room. If the students on the back row cannot see what is taking place they will get very little benefit from the demonstration, and will often distract the attention of those who could observe the reaction and so spoil the experiment for them. 7. Test the experiment just before the demonstration to be sure that the desired result will be obtained. It is poor teaching, and very embarrassing, to explain why an experiment is a failure. 8. Permit the students to examine the apparatus and samples of the materials and products after the class period. The success of the demonstration depends upon the teacher. There are no definite rules that may be laid down and the results guaranteed if the teacher follows them. One teacher may have fine results and another fail utterly with the same demonstration when using the same apparatus and the same general plan of procedure. Therefore the extent to which demonstrations are used and the details of procedure in the experiments depend upon the individual teacher, and he must work out his own policy. In conclusion, remember that the demonstration is a method of teaching and so the student should be held responsible for the principles shown and for the facts. The demonstration teaches principles and facts while the individual experiment adds technic to the list. The demonstration is a means to an end and not an end in itself.
Humble Water Plant Makes Much Limestone. Limestone, or what will eventually be limestone, is manufactured in thousand-ton lots in shallow lakes in the Middle West. So much is indicated by researches conducted by Prof. H. A. Schuette and Hugo Alder of the University of Wisconsin and the Wisconsin Geological and Natural History Survey. The two chemists analyzed quantities of Chara, a water-weed that grows freely in the ponds and lakes of limestone regions. Its stems and leaves are harsh and rough t o the touch, indicating the presence of considerable quantities of mineral. The analyses showed the sand-free, air-dry plants to contain over 41 per cent of ash, of which much the larger proportion was calcium carbonate, captured out of solution in the lake water. I n the lake where the analyzed samples were collected, about half the mass of the yearly crop of aquatic plants is accounted for by Chara. With this as a basis, Prof. Schuette and Mr. Alder calculated that this one plant yearly returns t o the bottom of this lake something like a thousand tons of calcium carbonate.-Science Service
