254 SCIENTIFIC AMUSEMENTS. 



coloured spot left when dry does not extend quite up to the pencil 

 mark ; and by dipping the paper in some chemical fluid that will 

 react with the substance absorbed by the paper (e.g., ferrocyanide 

 of potassium with perchloride of iron or sulphate of copper, sulphur- 

 etted hydrogen solution with the latter, and so on), it can be 

 rendered still more obvious that the salt dissolved in the fluid 

 originally dropped on to the paper has been more or less removed 

 from solution and retained in the central parts, whilst a weaker 

 solution, and finally little but water has passed on to the extreme 

 edge of the circle. 



CHAPTER XVIII. 

 SOAP BUBBLES AND FILMS. 



One of the most familiar observations is that whereas ordinary 

 pure spring or distilled water will not develop a permanent froth, 

 or permit of bubbles being blown therewith, water in which a little 

 soap is dissolved possesses the property of adhering together in 

 such fashion that it can be obtained in the form of thin flat films 

 (as when a ring of wire is dipped in soap water and taken out 

 again), or as nearly spherical shells with very thin walls, or 

 " bubbles," as when a tobacco pipe is dipped in soap water and 

 the flat film formed at the opening of the bowl enlarged by care- 

 fully blowing into the stem and finally dislodging the bubble by 

 a dexterous jerk. Films and bubbles can also be obtained with 

 many other kinds of liquids besides soap water, more especially 

 with " collodion " solutions (guncotton dissolved in ether) such as 

 are employed to form a coating on glass plates for photographic 

 purposes. 



Expt. 299. Analogy between a Soap Bubble and an India- 

 rubber Balloon. Blow a good-sized bubble by means of a tobacco 

 pipe, but do not dislodge it by shaking from the pipe ; remove 

 the stem of the pipe from the mouth and hold it near a lighted 

 candle ; you will now see that the candle flame is blown on one 

 side ; a current of air issues from the pipe stem, and at the same 

 time if you watch the bubble you will see that it gets smaller or 

 "collapses:" in short, the film of soap water behaves exactly as 

 if it were a thin india-rubber ball, capable of expansion when 

 blown into, but collapsing again when the air is allowed to escape. 



