502. Cold cream makes your hands and face soft. 503. Glowworms and fireflies can be seen on the darkest nights. 504. A lake looks gray on a cloudy day and blue on a clear day. 505. Dried fruit will keep much longer than fresh fruit. 506. If you scratch a varnished surface, you can rub the scratch out completely by using a cloth wet with alcohol. 507. Soda is usually dissolved in a little water before it is added to a sour-milk batter. 508. Iron rusts when it gets wet. 509. Peroxide is usually kept in brown bottles. 510. Dry lye may be kept in tin cans, but if the lye is _moistened_ it will eat the can. SECTION 54. _Acids._ Why are lemons sour? How do acids act? Some acids are very powerful. There is one, called _hydrofluoric acid_, that will eat through glass and has to be kept in wax bottles; and all acids tend to eat or corrode metals. You saw what hydrochloric acid did to the zinc shavings when you wanted to make a balloon; or, to be more accurate, you saw what the zinc shavings did to the acid, for the hydrogen gas that bubbled off was driven out of the acid by the zinc. Then the zinc combined with the rest of the acid to form what chemists call a _salt_. If we were to let the soft metal, sodium, act on hydrochloric acid, we should get hydrogen also; but the salt that formed would be regular table salt (NaCl). You cannot do this experiment, however, as the sodium does its work so violently that it is dangerous. EXPERIMENT 105. _To be done by the teacher before the class. If acid spatters on any one's skin or clothes, wash it of immediately with ammonia or a strong soda solution._ Drop a little candle grease on a piece of copper about 3/4 inch wide and 2 or 3 inches long. In the flame of a Bunsen burner, gently heat the end of the copper that has the candle grease (paraffin) on it, so that the paraffin will spread out all over the end. Let it harden. With a nail, draw a design in the paraffin on the copper, scratching through the thin coat of paraffin to the copper below. Pour a couple of drops of concentrated nitric acid on the paraffin-covered end of the piece of copper, and spread the acid with a match so that it can get down into the scratches. Let it stand by an open window for 5 or 10 minutes. Do not inhale the brown fumes that are given off. They are harmless in small amounts, but if breathed directly they are very irritating. Now wash off the acid by holding the copper under the hydrant, and scrape off the paraffin. [Illustration: FIG. 182. Etching copper with acid.]