578 THE POPULAR SCIENCE MONTHLY. 



the film upon one side of the thread, and leave it uninjured upon the 

 other. The surface-tension, destroyed upon one side, will remain in 

 action in the unruptured film, and therefore we should expect the 

 thread to be pulled toward the uninjured side. We can easily put 

 the matter to the test of experiment. I break one side by touching it 

 with a hot Avire, and what we foresaw occurs, the thread is instantly 

 pulled toward the side which the wire did not tear. This experi- 

 ment proves that the surface of a liquid is in a state of tension. 

 I have, however, another experiment to show you upon the same 

 point, which will add to our knowledge upon the subject, for it will 

 not only show that the tension exists on the surface, but that in dif- 

 ferent liquids it exists in different degrees of intensity. You now see 

 upon the screen the image of a few drops of colored water, which are 

 placed upon a glass plate. I dip a glass rod into some pure water, 

 and touch the colored film with the drop which adheres to the end. 

 As you see, nothing very particular happens. There is only a slight 

 diminution of the blueness in the centre, owing to the fact that the 

 colored water has been mixed with the pure water. Now, I will dip 

 the rod into alcohol, and you will see a different result. As soon as 

 I touch the blue water with the alcohol a motion occurs, and it moves 

 rapidly away from the point at which the contact took place. Now, 

 let us consider shortly what the explanation of this phenomenon is. 

 The surface of the water and the surface of the alcohol are alike in a 

 state of tension ; but the tension of the surface of the water is greater 

 than that at the surface of the alcohol. At the moment I put the drop 

 of alcohol upon the water we had a small drop of alcohol surrounded 

 by a large quantity of water, and between the two there was a line of 

 demarkation, which we may, for simplicity's sake, liken to the thread 

 you saw just now. When I destroyed the force of the tension in the 

 first experiment on the one side, the force which remained on the other 

 side pulled the thread toward it. In this case the force was acting 

 on both sides ; but the force at the surface of the water pulling away 

 from the centre of the alcohol-drop was greater than the force at the 

 surface of the alcohol-drop pulling toward its centre. The conse- 

 quence was, that we obtained a motion in the direction in which the 

 greater force was acting that is, in the direction in which the water 

 was pulling away from the centre ; and this continued the water and 

 alcohol moving farther and farther away, until the two became entire- 

 ly mixed together. An experiment similar to this may be performed 

 after dinner in the evening: When we pour some wine into a glass 

 we generally in doing so wet the sides, and the resxdt is, a thin film of 

 liquid adheres to them above that portion of the glass which is filled 

 with wine. This film soon contracts into drops, and each of these 

 drops consists, as all wine does, of a mixture of water, alcohol, and 

 certain other substances, the presence of which we may for the mo- 

 ment neglect. Alcohol, as you know, is an extremely volatile fluid, 



