168 INTRODUCTION TO GENERAL PHYSIOLOGY 



as long as the film is present inside as well as outside the loop. If, 

 now, the film inside the loop is broken by touching it with a pointed 

 bit of filter paper, the tension outside pulls the loop into a circular 

 shape. 



This film on a ring may also be used to show that a needle can 

 be dropped through without breaking the film. The needle should 

 first be wetted with the soap solution. 



To see the spherical form taken by drops of liquid requires some 

 trouble and skill. The method recommended by Boys is to pour 

 into a bottle a tablespoonful of olive oil together with a mixture of 

 nine parts by volume of rectified spirit (or methylated spirit con- 

 taining no petroleum oil) and seven parts of water. Shake up and 

 leave until the oil has separated again. It may be either above or 

 below the alcohol, according to the specific gravity of the latter. 

 Fill a beaker with a similar mixture of alcohol and water. If the 

 oil had risen to the top in the bottle, add a little water to the liquid 

 in the beaker by a pipette dipping about half-way down. Take 

 some of the oil in a pipette and empty into the middle of th2 beaker. 

 If it sinks, a little more water is wanted in the lower half cf the 

 beaker ; if it floats, a little more alcohol is wanted in the upper half. 

 To see the shape truly, it must be looked at from above the liquid, 

 since the curvature of the sides of the beaker distorts the image. 

 When the right specific gravity of the alcohol has been obtained, 

 more oil may be added to the drop, best from a tap-funnel 

 slowly. 



Adsorption. A bubble is blown as above, but using a dilute 

 solution of saponin in distilled water (about 0.5 to I per cent). 

 Allowing the bubble to contract, or by sucking air out of it, it 

 becomes more or less rigid and goes into folds (Ramsden). Solid 

 particles of saponin will be seen in the film, and especially in the 

 drop at the lower end of the bubble. 



Add charcoal in powder to a dilute solution of crystal violet 

 until the colour is removed. Filter off the charcoal and wash it on 

 the paper with acetone. After the water is replaced by acetone 

 the drops falling through will be stained violet. It is better to 

 make the acetone slightly acid with a drop of hydrochloric acid, on 

 account of the fact that it is the free base of the dye that is adsorbed. 



Cell Membrane and Permeability. Having found an amoeba 

 or leucocyte, which is known to be living by its movements, add a 

 dilute solution of aniline blue by placing a drop at one end of the 

 cover-slip and a fragment of filter paper at the opposite end. The 

 organism does not stain as long as it is alive, although any bits of 

 dead material become blue. 



Slices of red beet are washed under the tap in order to remove 

 the contents of the cells injured by the knife. Left in tap water, the 



