SOAP-BUBBLES. 583 



thus become smaller. I will blow a bubble at one end of a glass 

 tube, and leave the tube open at the other end ; we shall thus have a 

 small hole formed in one side of the bubble, which, if our theory is 

 correct, will gradually contract and disappear. You now see on the 

 screen images of the ends of two tubes. I have the power of cutting 

 off one tube entirely from access to the other ; and I do so now, so 

 that you will, if you please, consider for the purposes of this experi- 

 ment that tube only as existing at the extremity of which I shall 

 blow the bubble. You now see the image of the soap-bubble, which 

 as long as the tube is closed remains unaltered in size ; I open it, and 

 it now at once contracts and disappears. This, then, conclusively 

 proves that the air in the bubble was compressed. I will now go a 

 step farther, and show that the amount of this compression depends on 

 the size of the bubble. If it be large, the air is not so much compressed 

 as if it btt small. Let us consider what would happen if I formed bub- 

 bles at the two ends of a tube. If they were of the same size, evidently 

 the one pressing the air in one direction, and the other pressing it in 

 the other with equal force no effect would follow. If, however, one 

 bubble were smaller than the other, and what I have said be true, the 

 small one would compress the air within it, and drive it from left to 

 right (say) with greater force than the other would tend to drive it 

 from right to left : hence the air would flow from the small bubble to 

 the large one ; the large one would increase, and the small one dimin- 

 ish. The smaller the bubble, the more the air would be compressed ; 

 and thus the current would become greater and greater, until at last 

 we should see the small bubble entirely disappear, the large one hav- 

 ing absorbed all the air which it previously contained. I will try to 

 show you this on the screen ; first disconnecting the two tubes, I blow 

 at their ends bubbles of unequal size. I will now place them in com- 

 munication, so that the air can pass from the one to the other. You 

 see, the small bubble contracts and the large one expands, and we 

 thus learn that the pressure of the smaller or more curved bubble 

 upon the air is greater than that of the less curved one. 



I now come to the third property of liquids of which I wish to 

 speak ; and that is, that the surface of a liquid is generally either more 

 or less viscous than the interior. With reference to the word viscous, 

 you will find a familiar example of two liquids which differ very much 

 in this property of viscosity in treacle and water. Take a vessel of 

 treacle and a vessel of water, pour the liquids out, and note the dif- 

 ferent way in which they behave ; the water flows out smoothly, one 

 part slipping over another, whereas the treacle comes out in a great 

 rolling mass, which seems to stick to the sides of the vessel. Again, 

 put a spoon into a vessel of water, and move it through the liquid, 

 you will find little resistance to its motion, the water seems to flow 

 away to make room for it and closes in again immediately behind. 

 Try the same experiment with the treacle and you will find the resist- 



