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29. What difficulties lay in the way of one who should try to help Burns with money? What could have been done for him that was not done?

30. Where does the chief blame for Burns's failure lie?

31. How might he have divided his time?

32. What points of likeness between Burns and Byron?

33. What do you think of the affirmation: "he has no Religion"? 34. Quote the final sentence.

HELPS, HINTS AND SUGGESTIONS.

USES OF A SOAP FILM.

By J. A. Culler.

Every one has blown soap-bubbles and has admired the beauty of

their gorgeous hues. The blowing of bubbles, however, is not always a mere diversion but may be called to our aid in illustrating some valuable principles of science.

To begin with, a good solution is indispensable and should be pre

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pared with care. Many directions for this are found in books. following is good and easily prepared, and with it the writer has been able to blow a bubble eighteen inches in diameter. To one quart of rain water that has been boiled and allowed to cool add about one ounce of the best white castile soap scraped into fine shavings. Shake from time to time till well dissolved. Then set aside for fortyeight hours, when all undissolved soap should have settled to the bottom leaving the top liquid perfectly clear. Then very carefully pour off the clear liquid into another bottle and to every two parts of this soap solution add one of good glycerine. Shake these well together. This solution should now be clear and sparkling and is ready for use. I have found the conventional clay pipe the most serviceable instrument in blowing these bubbles.

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Now, if a class is reciting a lesson in Physiology on respiration and the pupils are thinking of a page in the book rather than of themselves, it may bring the subject home and add much interest if two scholars be called before the class, one the largest boy-to take a deep breath and blow all he can into the bubble while the other with ruler measures its diameter. Then the volume of this sphere is calculated on the board by those who know some mensuration and the "vital capacity, 230 cubic inches" as stated in the book will begin to mean something.

While our fleet was lying before Santiago it was not certainly known for a long time whether or not Cervera was in the harbor nor how much of his fleet was there.

So Americans raised balloons from their ships to get a view of the harbor. A reporter for one of our papers described these as filled with oxygen. Such a reporter would be

dismissed from a first-class newspaper, and he would deserve it. The buoyancy of air can be nicely shown by the bubble with some simple contrivance to generate hydrogen with which good balloons are filled.

Prepare apparatus as in Fig. 1, a stout bottle. a cork through which passes a short piece of glass tubing, a rubber tube one foot long, and a pipe. Now pour water into the bottle to a depth of about two inches, and add some scraps of zinc, old sheet zinc will do or, if zinc cannot be procured, carpet tacks or small nails will do. Now pour upon these sulphuric acid about one-twelfth as much as there is water. Gas will be given off which will now blow the bubble. At first hold the mouth of the pipe down until the bubble is the size of a hen's egg, then suddenly invert it and allow to blow until it shows signs of breaking away, when a quick motion to one side. I will liberate the bubble which ascends to the ceiling. The pipe can again be dipped into the soap solution and the operation continued at pleasure. This gas is hydrogen for Zn+H,2SO, ZnSO4+2H It is used in balloons because it is the lightest gas and the balloon rises for the same reason that a cork will rise to the surface of water. Air is 14 times as heavy as hydrogen, while oxygen is 16 times as heavy as hydrogen, hence if a bubble is blown with oxygen it

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will rapidly fall to the floor. It is heavier than air and sinks for the same reason that a stone will sink in water. This same apparatus may be used to generate (CO2) carbon dioxide, the only difference being that marble must be substituted for zinc. CaCO3+H2SO4= CaSO4+H2O+CO2, hence the gas is CO2 and a bubble blown with this will sink very rapidly to the floor, and if allowed to fall upon a candle flame it will extinguish it, while an oxygen bubble falling upon the flame will cause it to burn brilliantly. This will illustrate the difference between these two gases in an important particular.

If, while the hydrogen bubble, above described, is ascending a lighted match be plunged into it there will be a sudden, though quiet, flash of light and all will disappear. This nicely illustrates the union of the hydrogen in the bubble and the oxygen in the air. This subject of combustion is poorly understood even by some educated people. They say, for example, that wood and coal will burn as though any one thing could be a combustible. Combustion is always a chemical union of at least two elements. We forget the atmosphere of diluted oxygen in which we are constantly bathed. Now, that burning hydrogen bubble, raised to the proper temperature by the lighted match, simply united with the oxygen of the air and water was the product: thus, 2H+

O=H2O, and if the room had been filled with hydrogen and the bubble filled with oxygen exactly the same phenomena would have been observed, except that the bubble would descend instead of ascend. Blow a small bubble with air from the lungs, then connect the pipe with the hydrogen generator and swell the bubble to about three times that size. Release the bubble and light it, when a loud report will be heard because of the sudden union of the oxygen and hydrogen which were intimately mixed. This fact has suggested a way of producing rain in dry seasons. It was noticed, during the civil war that almost invariably after a battle in which there was heavy cannonading there would be a heavy shower of rain, so that there seemed to be some connection between these powder explosions and the condensation of the water vapor in the air; so it seemed reasonable that if huge balloons filled. with one part oxygen and two parts hydrogen be allowed to ascend and have attached to them a time fuse to cause the union of these great volumes of gas, there would be such agitation of air in this region that condensation and precipitation would follow. After all methods have been tried, however, the rainmakers do not yet enjoy the full confidence of the public.

The soap bubble is spherical because the film is stretched, or under tension. der tension. This can be easily

shown by blowing a bubble five or six inches in diameter and then remove the pipe from the mouth, when the bubble will rapidly contract driving all the air it contains out through the pipe stem. As compared with other shapes a sphere has the least surface for the same volume of substances, and so, as the film is trying to make its area as small as possible it puts its contents in the form of a sphere. This is well shown in falling drops of rain, which are round because of the surface tension of water, and when this surface tension bears a certain ratio to the mass of water within, one drop will rebound on striking another and it cannot be easily broken up. So that, under ordinary conditions, we are neither deluged by great masses of water coming down from the clouds altogether, nor does the rain fall as fine mist. Even the shot-maker takes advantage of this principle. If his melted lead be separated into small globules, they will be spherical while freely falling and if they fall far enough will harden before they strike the water at the bottom.

The surface tension of water is much greater than that of the soap solution, but the soap solution has another property called surface viscosity by virtue of which it can be enormously stretched as in blowing a large bubble. Water has this property only in a slight degree.

Let a drop of petroleum fall up

on a basin of water and instantly it will spread over the whole surface. Now, the surface of water acts very much as if a thin elastic membrane were tightly stretched over the water and fastened all around to the sides of the basin and so when the oil broke this, it dragged the oil out with it in all directions.

The old experiment of floating a needle on water is a clear demonstration of this strong surface tension or water-skin.

In the soap solution it is the surface tension which keeps the film stretched on both sides from point to point of its support. This can be nicely shown by preparing a wire as shown in Fig. 2 and ty

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the figure, but if the part of the film within the loop be now pricked with a pencil point and broken the

thread will immediately be pulled out in all directions forming a circle as illustrated in Fig. 3.

The most striking exhibit of the film is seen in Fig. 4. Dip a tumbler into the solution and, taking away a film, place the tumbler as in

the figure. Place the eye so as to receive the rays of light reflected from the film and soon will appear a beautiful display of colored bands which no drawing can accurately represent. These colors are caused by the interference of light waves by which some of the components of white light are destroyed while others come on to the eye. Knowing the length of the waves of light. it is possible to calculate the thickness of the film and hence to approach the size of the molecule.

When we speak of the film as being only a molecule in thickness we are speaking of exceedingly small quantities, but the physicist and chemist knows something about molecules and atoms as certainly as the mechanic knows about

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The number of waves in red light is 395X1012 in one second and for

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