404 ILLUSTRATIVE EXPERIMENTS 



rapidly. Do not throw out the fluid in the dish, but add another drop of 

 the camphor solution. What effect 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, (6) 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 ? 



15. Mercury " Amoeba." Place a small globule of mercury in a large 

 Petri dish and cover it with potassium bichromate (sat. solution) to which 

 has been added some nitric acid (bench reagent). How do you explain 

 the movements ? 



16. Electrical Alteration of Surface Tension. Carry out the experiment 

 detailed on p. 48. Place a small globule of mercury on an iron or enamelled 

 plate and cover with water. Note shape of globule. Add sulphuric acid 

 (bench reagent) drop by drop (to make about a 10 per cent, solution). 

 What happens to the globule ? Explain. Connect the plate and globule 

 to a single cell of a battery through a commutator. First pass the current 

 through the globule to the plate and note alterations in shape, then reverse 

 the commutator. 



17. Ostwald's Physical " Heart." (Verworn's Physiologisches PraJc- 

 tikum.) The method of carrying out this demonstration of electrical 

 alterations in surface tension is indicated in the diagram (Fig. 76). A 

 globule of mercury about an inch in diameter is placed in a clock glass 

 almost rilled with 10-15 per cent, sulphuric acid. Potassium bichromate 



.CORK. 



SEWING NEEDLE 



.-WATCH GLASS 



GLOBULE Of I 



FIG. 76. Mercury " Heart." 



solution (say N/10), is added drop by drop till the fluid becomes a light 

 yellow in colour. A clean sewing needle thrust through a cork is placed 

 in a diagonal position so that the point of the needle just touches the margin 

 of the mercury globule. At the moment of contact the globule becomes 

 more spherical. This breaks its contact with the needle and it loses its 

 semispherical form and so again makes contact. These rhythmic pulsa- 

 tions may go on for hours. When the action has stopped remove the 

 needle and note the odour of acetylene. How do you account for this ? 

 What is the reason for adding bichromate ? 



18. The Capillary Electrometer. This instrument for measuring differ- 

 ences of electric potential depends for its action upon the alteration of surface 

 tension between mercury and sulphuric acid with alterations of the potential 

 difference of the interface (see p. 48, and Expts. 16 and 17). For class use 

 the simplest satisfactory form is that made by the Harvard Apparatus Co. 

 It consists of a capillary tube containing mercury which is continuous with 

 a reservoir in the form of a plunger pump. The position of the mercury 

 in the capillary may be altered by adjusting the plunger by means of a 

 micro-screw. The glass capillary dips into a small test tube containing 

 dilute sulphuric acid and a drop of mercury to make good contact with the 



