19. Porous Cups for Two-fluid Cells. Ordinary unglazed earthen flower-pots make good cups. The hole in the bottom should be closed with a cork, or by fastening a piece of pasteboard over the hole with paraffine. The pasteboard may be fastened to the under side of the bottom more easily than to the upper side.
20. Note. It is a good idea to soak the top edge of porous cups for about ¼ in. in paraffine to keep the solutions from crawling up by capillary attraction. If the solutions constantly evaporate from the soaked tops of the cups, they not only waste but they get the whole thing covered with crystals.
CHAPTER II.
BATTERY FLUIDS AND SOLUTIONS.
21. Sulphuric Acid. This acid must be handled with great care, as it (the concentrated) is very strong, and will burn the hands, eat holes in clothing, carpets, etc.; it will even char wood. Do not let any of it drop anywhere accidentally. If you wish to pour concentrated acid into a bottle, place the bottle to be filled upon a plate, and wipe all drops of acid from the outside of it afterward. The concentrated acid should be kept in tightly-corked bottles, as it absorbs moisture from the air very rapidly. Ordinary corks should be paraffined if they are to be used in acid bottles, or they will be soon eaten up.
22. Mixing. When sulphuric acid and water are mixed, considerable heat is produced. Never pour water into the acid, as the heat would be produced so rapidly that the vessel containing the mixture might break. Always pour the acid into the water, and thoroughly stir the mixture at the same time. Earthen vessels do not break when heated as easily as glass ones. The mixing may be done in ordinary glass fruit-jars, if care be taken to pour the acid slowly into the water. The jars should be set in some larger dish, or in the sink, before adding the acid. If they get too hot, allow them to cool a little before proceeding with the mixing. As the acid is much heavier than water, it will immediately sink to the bottom of the jar, unless constantly stirred.
23. There are different grades of acid upon the market. For battery purposes you do not need the chemically pure (C P) acid. The ordinary "commercial acid" is all right, even though it is a little dark in color. You can get this at any drug-store. Get 5 or 10 cents' worth at a time.