516 ILLUSTRATIVE EXPERIMENTS 



water-liair surface is increased, e.g. by removing the brush from the water, the 

 hairs form a compact pencil. 



9. Boy's Leather Sucker (p. 392). 



To show that surface tension is the causative factor suspend a microscope 

 slide horizontally in the receiver of an air pump. By means of a drop of 

 water between, cause a second slide to adhere to the lower surface of the first 

 slide in such a way that the second slide may be loaded. Load with the 

 maximum weight and exhaust the receiver. Repeat the experiment under 

 various conditions, e.g. trace of oil ester, bile salts, etc. 



10. Work Done by Altering Surface Tension. 



In performing the experiment detailed on p. 175 (Fig. 42), it is convenient 

 to have a clean copper frame, easily prepared by bending a piece of copper 

 wire (No. 16 gauge) into a circle about 2 inches in diameter, with a handle 

 about 2 inches long. The frame is freed from grease by washing in dilute 

 soda and then thoroughly rinsed. A piece of silk thread is attached to one 

 portion of the frame and a loop n::ado on its loose end. Some of the soap 

 solution is placed in a large Petri dish. The frame is dipped into this and 

 removed horizontally with a fine film of soap covering the circle. Satisfy 

 yourself that the loop moves freely over the film by gently rotating the 

 frame. Now break the film inside the loop, using for this purpose a bluntly 

 pointed piece of blotting pai)er. 



11. Effect of Soap Formation on Surface Tension. 



Take four simihir watch-glasses. In the first two put a few c.c. of water, 

 and in the other two about the same quantity of a 1 per cent, solution of 

 sodium carbonate. With a fine pipette place one drop of rancid olive oil 

 on the surface of the lic[uid m glasses 1 and 3. Place drops of liquid 

 paraffin of similar size on the fluid in glasses 2 and 4. Note {a) whether the 

 rancid oil and the paraffin differ in spreading power on a water surface, and {h) 

 the movements occurring in glass 3. Can you explain these movments in the 

 light of what you have learned from experiments 9 and 10 ? 



12. Camphor "Water-beetle." 



Prepare a rectangular piece of camphor. To one short side aflnx a short 

 piece of stick and place the whole thing on the surface of water in a large 

 dish. How do you explain the direction of the movements ? Remove the 

 stick and replace the camphor in the water. 



13 Camphor-Benzene " Amoeba. " 



(Brailsford Robertson.) The amoeba is made of a saturated solution of 

 camphor in benzene to which a dye {e.g. carmine) has been added to make 

 the solution easily visible when placed in water. Place a drop of the solution 

 on the surface of clean water in a dean Petri dish. The movements may be 

 slowed down by the addition of the faintest trace of oil. Generally the 

 first ■■ amoeba " disintegrates rapidly. Do not throw out the fluid in the 

 dish, but add another drop of the camphor solution. What eft'ect has increase 

 or decrease in temperature on the movements ? What happens when a solid 

 particle is suspended in the water near the " amoeba " ? What is the effect 

 of putting two separate drops on the surface of the water at the same time. 

 [a) when the drops are equal in size, {h) when they are unequal in size ? What is 

 the effect of the addition of a trace of fat, obtained, for example, by touching 

 the surface of the water with a glass rod which has been rubbed on the side of 

 the nose ? 



14. Mercury "Amoeba." 



Place a small globule of mercurv in a large Petri dish and cover it with 



