EXERCISES Levers are of three classes: (1) The fulcrum is between the weight and the power. (2) The weight is between the fulcrum and the power. (3) The power is between the fulcrum and the weight. Pulleys are wheels over which ropes can be pulled. 187 The wheel and axle is a form of the lever used for circular motion. The wedge and the screw are really forms of the inclined plane. Friction prevents us from getting the full amount of work out of a machine; but some is necessary in order that machines may work at all. A sailboat is a device for getting some forward motion out of any wind except one that is "dead ahead." The kite gets upward motion out of a horizontal wind. The aeroplane gets upward motion by the pushing of its obliquelyset planes horizontally against the air. The windmill converts a horizontal wind into circular motion. 212. Exercises. 1. What kind of a machine is used for lifting window awnings? A door transom? A weighted window sash? 2. How could you arrange a system of pulleys so that by pulling downward with a force of 20 lbs. you could raise 40 lbs.? 3. If the crank of a winch (Fig. 179) makes a circle of 36 in. while the rope is wound up 6 in. for each complete revolution of the crank, how much force must I exert on the crank to lift a pail of water weighing 100 lbs.? 4. What kind of a machine is a nail, a coffee-grinder, a carpenter's brace and bit, a pendulum bob, a snow plow, a pin-wheel, a door knob, a spoon, a spade, a chisel? 5. How do you hold your knife and your fork when cutting a tough piece of meat? What kind of machine are these implements then? What machine are the tines of the fork and the blade of the knife? 6. A man who can exert a force of 100 lbs. wishes to raise a 300 lb. barrel into a wagon 3 ft. above the ground. He uses an inclined board; how long must it be? 7. In loading a wheelbarrow, where should you put the load in order to make the force needed for lifting as small as possible? 66 CHAPTER XI ACIDS, ALKALIES, AND CLEANING Dilute Alcohol 213. Acids. We learned in § 5 that no two substances have exactly the same special, or specific, properties. There are, however, many substances that are alike in some particular property or properties. This is true of the substances that make up the class we call acids," as well as of those we call " bases," or " alkalies." The word "acid" means sour; sourness is a characteristic property of most acids. The juices of ripe fruits are sweet because they contain sugar; but when they are exposed to the air, the juices "ferment": their sugar is changed to alcohol and carbon dioxide. This particular fermentation is due to yeast (cf. § 129). The change does not, however, stop here. When the fermented juices, which now contain alcohol, stand longer in the air, they be Air Air FIG. 192. Air Shavings Air Vinegar Quick Vinegar Process. come sour, and we have cider vinegar, currant vinegar, wine vinegar, etc. (Fig. 192). The cause of this second fermentation is another plant, mould," or "mother of vinegar." is due to acetic acid (cf. § 124). colorless, sharp-smelling liquid, known as the "vinegar The sourness of vinegar In a pure form this is a which freezes at 16° C., CLASSES OF ACIDS 189 and blisters the skin. Vinegar usually has only 3% or 4% of it. The materials present in plants undergo other fermentations besides the one that gives acetic acid. Thus, "Dill" pickles are small cucumbers fermented so as to give lactic acid; " sauer kraut" contains the same acid. Sweet milk becomes sour because the milk sugar in it is changed to lactic acid. Pure lactic acid is a thick liquid of very sour taste. 214. Classes of Acids. It is not only by fermentation of fruit juices that acids are formed. The fruits themselves, as well as other parts of plants, contain acids. Tomatoes, cherries, rhubarb, etc., are strongly acid. Lemons and oranges contain much citric acid; grape juice, much tartaric acid; apples contain malic acid. All of the acids already named in this chapter are compounds of carbon with hydrogen and oxygen (cf. § 123). These acids are called organic acids (cf. § 3). There are many acids that contain no carbon at all, but have some other element in place of the carbon. These belong to the class of inorganic, or mineral, acids. Thus, nitric acid contains nitrogen, combined with hydrogen and oxygen; sulphuric acid consists of sulphur, hydrogen, and oxygen; phosphoric acid contains phosphorus, hydrogen, and oxygen. These three acids are all thick, colorless liquids. They are commonly "diluted" with much water. All acids contain hydrogen, but not all have oxygen. Thus, hydrochloric acid (cf. § 111) contains only hydrogen and chlorine. This is the acid of the gastric juice (cf. § 364). The gastric juice of man contains about 0.22 of 1% of it; the adult dog has several times as much. 215. Acids and Coloring Matter.- Besides having the property of sourness, acids have a definite action on certain coloring matters, such as purple cabbage solution and litmus. Acids often change the color of the dyes in our clothing. Thus, "navy blue" fabrics are colored red by the common acids of the laboratory. Litmus is the coloring matter we generally use in testing for acids. It is a blue substance obtained from certain plants called lichens (cf. § 324). Either the solution, or filter paper that has been soaked in the solution, may be used. The prepared paper is called litmus paper. Blue litmus is changed to red litmus by sour plant juices and by other acids. A substance which is able to change blue litmus to red is said to have an "acid reaction." 66 216. Action of Acids with Metals. A third important property of acids is that they corrode, or eat," metals. We have learned that hydrogen is made by the action of some of the acids upon zinc, iron, etc. (cf. § 103). When the metals are so used, they are "eaten up," and disappear. Some metals, such as copper, do not act with dilute acids to give hydrogen (cf. § 150); but if the metal and the dilute acid are kept in contact with air, the metal is gradually corroded. Here the oxygen of the air acts with the metal to give the oxide of the metal (cf. § 48); the oxide then reacts with the acid. Copper and lead cannot be used for cooking utensils, because they act with the air and the acids of fruits and other food, producing poisonous compounds. Copper is "eaten" very readily by dilute nitric acid. If a design is ACTION OF ACIDS WITH CARBONATES 191 painted upon copper with asphalt paint (Fig. 193), and the copper is put into nitric acid, the part not covered by the asphalt is "etched" by the acid. When the asphalt is removed, the design stands out "in relief." 217. Action of Acids with Carbonates.— The action of acids with carbonates may also be used as a test for acids. Marble (cf. § 132) and hydrochloric acid react with much effervescence, because carbon dioxide escapes as a gas (cf. § 126). The FIG. 193. This design may be etched on copper by dilute nitric acid. other product is calcium chloride. It may be obtained as a white solid by the evaporation of its solution. The limestone of bones, oyster and clam shells, and of coral is rapidly eaten out by acids, and only the animal material is left. As a result, bones that are treated with acids lose their stiffening. The large amount of acid in a dog's stomach enables him to digest bone. Other carbonates react with acids as marble and limestone do. Washing soda is sodium carbonate, which is made up of sodium, carbon, and oxygen. Baking soda is sodium hydrogen carbonate, or sodium bicarbonate (cf. § 130). Both washing soda and baking soda effervesce rapidly when an acid is added. The acid used may be tomato juice, sour milk, or lemon juice, as well as a mineral acid (cf. § 214). When either of the two "sodas " is treated with hydrochloric acid, there is formed, besides water and carbon dioxide, sodium chloride, or common salt (cf. § 108). Soda was formerly rare and expensive; but it is now made, on an enormous scale, from common salt. Wood ashes contain potassium carbonate, or " potash." The potash is obtained from the ashes by the use of water. Potash reacts with hydrochloric acid as soda does; but it gives potassium chloride instead of sodium chloride. |