Soap Bubbles, Chapter 6

          "Father," said Jimmy the following Monday at the breakfast table, "I know a lot more about soap than I did once, but I haven't yet found out why it is such a dirt chaser. Will you tell me tonight?"
          "Why not give Miss Dean the first chance to do that, son?" father replied.
          "All right, dad," said Jimmy. "Only, I'm like the elephant child in the Just-So Stories. I have a 'satiable curtiosity.'"
          School time came. By and by Miss Dean said:
          "Well, I suppose everyone used soap this morning?"
          Everybody had.
          "All right, but does anybody know what that soap really did and how it did it?"
          Nobody knew.
          "Then come around the big table and let's find out.
          "First, to be sure that soap will do what water alone cannot, we will have a washing contest. The girls are good washers, I know. They may take these pieces of soiled muslin and wash them in clear water. Boys, you may wash these similar pieces in soapy water.
          "All ready? Begin, and do your best."
          Everybody worked with a will. The boys were not going to let the girls beat them, and so on one side of the table the soapsuds foamed and flew, and on the other side the girls worked manfully.
          "Stop!" came the signal from Miss Dean. "Squeeze out. Rinse. Lay your pieces in line along the table. Is there any difference in the two rows as to appearance?"
          There was! And there was a row of proud boys, and another of not so proud girls.
          John said: "I'd be sorry to have mother use water alone for some of my clothes."
          Then said Miss Dean:
          "Since you all seem sure that it takes soap to remove dirt, let's make some tests and find out why it does.
          "First smear a thin film of oil on one side of these strips of glass. Now place a drop of water and one of this soap solution near each other on the oil. Is there any difference in the shape of the drops?"
          "Why, yes," said Gertrude, "that's funny. The drop of water is round and high, and the soap solution spreads out flat."
          "Try on the other side of the glass, without the oil."
          "It is about the same each time," said Gertrude. "The soap spreads farther than the water."
          "Here is another way to show how soap solutions spread," said Miss Dean. "I have smeared oil around the inside of this glass tumbler. I fill it with water and then shake out the water. Look at it."
          "There are drops of water clinging to the glass in places," said George, "and they don't run together."
          "This time I am filling the glass with a soap solution and shaking that out. Now what do you see?"
          "There are no drops now. The soap has spread over the whole glass," they all said.
          "Then if our hands or dishes or clothes are greasy, which will spread more quickly and evenly over them, soapy water or clear water? Which will get farther and more evenly into the fibers of our clothing?"
          "But why will soapy water spread out more than plain water?" asked Tom.
          "Which stretches farther when you blow it up, a balloon with a thick skin or one with a thin skin?" asked Miss Dean.
          "the one with a thin skin."
          "This will help us to understand the difference between soapy water and plain water in the way they spread out. In the balloon with a thin skin the particles of rubber can spread out easily, while in the balloon which does not stretch so well, the particles are held closer together. In some way, the particles in the surface of a soap solution are able to spread out very far indeed, and we say that soapy water has a low surface tension. On the other hand, the particles in the surface of plain water draw together and make a round drop, as we have seen, so we describe this by saying that water has a high surface tension.
          "Because soap solutions have a very low surface tension, they can spread easily through our clothing. "Shall we write this on the board as one of the reasons why soap cleanses?" asked Miss Dean.
          "Now let us find out just how the soap removed the grease and dirt in the muslin the boys washed," said Miss Dean.
          "Here are two bottles. Jimmy, put the same amount of this dirty-looking oil in each of them. Fill this bottle half full of water, and this other one half full of soap solution. Notice how the oil rises to the top in each. Now, Tom, take the bottle with the water in it. Cork your bottles, and shake hard, until I tell you to stop.
          "Stop!" said Miss Dean, after a few seconds.
          The two boys placed their bottles side by side for the class to see.
          "In Tom's bottle the oil is still on top of the water," said George.
          "But it isn't in Jimmy's," said Gertrude. "That looks like whipped cream all through. What has happened to the oil?"
          "Let's see what has happened to the oil," said Miss Dean, and she put a drop of the mixture in Jimmy's bottle under a microscope.
          Everybody crowded near to take his or her turn looking in.
          "Can you decide what has happened to the oil?" asked Miss Dean.
          "I think the oil is broken up into those tiny drops that we can just see in the microscope," said Mary.
          "That is right," said Miss Dean. "What then is all around those tiny drops and keeping them apart?"
          "The soap solution," said several children.
          "What did you have to do in order to make the soap solution surround the oil?"
          "We had to shake the two together."
          "So that must have broken up the oil into little drops and let the soap stretch around them," said Jimmy.
          "There is a big word for what the soap has done to the oil," said Miss Dean; "we say the soap has emulsified the oil, and the mixture is an emulsion."
          "I know," said Mary. "I've had to take emulsified fats as a medicine."
          "And I've taken an emulsion of cod-liver oil," said several.
          "And you've all eaten mayonnaise, which is another emulsion," said Miss Dean.
          "Made with soap?" asked Tome, with a grin.
          "Some other things, such as egg yolk, make an emulsion with oil, but nothing makes a better emulsion than soap does," said Miss Dean.
          "But to come back to the way soap cleanses - which would be easier to remove, a big mass of solid grease or these little tiny droplets all surrounded with soap?" asked Miss Dean. "So what shall we write on the board as another way in which soap helps to cleanse?"
          "Remember, though," said Miss Dean, "that you did some shaking to get that emulsion. How did you boys get those pieces of muslin clean a while ago? By sitting down and looking at them?"
          "I'll say we didn't!" said the boys in chorus. "We did some work on those pieces!"
          "All right," said Miss Dean. "Work is necessary to break up the grease and dirt in small particles, and to help the soap to surround these particles and pry them loose. You work when you wash your hands; washing machines work when they keep the clothes in motion and force the soap solution through, back and forth."
          "Shall we add another sentence to our last one on the board," asked Miss Dean, "as to the need for work in helping to form an emulsion and cleanse with soap?"
          "But Tom worked when he shook the oil and water together," said George. "Why didn't he get an emulsion?"
          "Marvelous question!" laughed Tom. "Didn't you just learn that water won't stretch? How could it get around the oil?"
          "I suppose the more soapsuds we make, the more the soap is stretching," said Jimmy. "I never knew before why we couldn't blow bubbles with clear water."
          "That's a good thought, Jimmy," said Miss Dean. "A good, thick soapsuds shows that the soap is stretching and doing work, and we know that when, for instance, our hands are dirty, we like to rub up a good suds, or lather, all over them."
          "And I guess," said Jimmy, "that 'keep moving' is as good a rule for anyone who is washing as it is for a crowd."
          "And now," said Miss Dean, "I will show you how soap really likes to pick up dirt. Here is a funnel in which I am fitting filter paper. The next thing will be to smear the filter paper with lampblack, like this, and then let water drain through it into the glass below.
          "You see that the water brings a little of the lampblack through. If we pour this water through a second time, how does it look now?"
          "It is clear now," said John. "No lampblack came through."
          "Now watch carefully," said Miss Dean, and she poured a soap solution through the lampblack on the filter paper, and collected what came through in a clean glass.
          "Why," said everybody, "the soap is bringing all the lampblack through with it. Isn't that queer!"
          "It shows," said Miss Dean, "that the soap solution has power to make the dirt or lampblack stick to it, more than the filter paper has to hold it. If we let the filter paper represent our clothing, and let the lampblack stand for the dirt which collects on it, we can see how the dirt leaves the clothing and sticks to the soap. Is there another word for 'stick' which we could use?"
          "'Adhere,'" suggested Grace.
          "Good. So here is another reason why soap cleanses more than water does. What shall we add to our list on the board?"
          "We must say one more thing about the cleansing power of soap," went on Miss Dean. "All soaps, when they dissolve in water, change a little, and with the water some free lye forms. Let us use a word that chemists use, and call lye alkali. Some soaps allow more of this alkali to form than others do. Alkali helps to cleanse by making new soaps or emulsions with grease, but sometimes it harms clothing, as we shall see later. So for some clothing we shall learn to choose soaps that are not strong, or harsh, with alkali when they dissolve in water, but are mild instead.
          "And now we have our last reason why soap chases dirt," said Miss Dean. "Let us complete our list of reasons on the board."