MEASrnKMENT OF SURFACE TENSIOX 517 



potassium bichromate (sat. solution) to uhidi has been added some nitric acid 

 (bench rea<j,ent). TTow do you ex])laiii the movements ? 



15. Electrical Alteration of Surface Tension. 



Carry out the exporinu-nt detailed on ]). 49. 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 ? ILxplain. 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 cotnnuitator. 



16. Ostwald's Physical "Heart." (N'erworn's Physiuloijiachen 

 Pmktikuui.) 



The method of carrving out this demonstration of electrical alterations in 

 surface tension is indicated in the diagram (Fig. 11, p. 49). A globule of 

 mercury about an inch in diameter is placed in a clock glass almost filled 

 with 10-15 per cent, sulphuric acid. Potassium bichromate solution (say 

 iV/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 pulsations may go on for hours. When 

 the action has stopped remove the needle and note the odour of acety- 

 lene. How do you account for this ? What is the reason for adding 

 bichromate ? 



17. Measurement of Surface Tension. 



Simple Stcdugmometer. A simple stalagmonieter which is sufhciently 

 accurate for any of the experiments detailed here may be made from com- 

 mercial capillary glass tubing. Cut a piece of this tubing into sections of 7 

 to 10 cm. long and grind the ends flat on fine emery cloth moistened with 

 paraffin oil. By means of a short length of pressure tubing fix the capillary 

 tube (glass to glass) to the lower end of a .3 to 5 c.c. pipette graduated to 

 1/10 c.c. and clamp rigidly in a vertical position over a narrow graduated 

 cylinder or other vessel to catch the drops. 



Note. — The stalagmometer should be dissociated into its component parts 

 except when actually in use and the glass parts kept in cleaning mixture. 

 Before use they should be thoroughly rinsed with warm distilled water and 

 dried by attachment to a sucking or blowing apparatus. 



To Calibrate the Stalagmometer. It is convenient to calibrate the apparatus 

 at room temperature. Fill it to above the top graduation mark with dis- 

 tilled water which lias attained room temperature. Start counting and 

 collecting the drops when the meniscus just passes the top mark. Note 

 the graduation mark when the sixtieth drop falls. Repeat the experiment 

 to confirm this point and then by a deep file mark or other means intensify 

 this jnark. 



To compare the surface tensions of distilled water and 0"i per cent, soap 

 solutions. 



(i.) Stalagmometer Method. 



[a] Confirm the calibration of tlie stalagmometer by counting (i.) the 

 number of drops, (ii.) the volume of the fluid dropped, and (iii.) the time 

 taken in dropping between the two marks. 



(ft) Wash out the apparatus with the soap solution and then fill to above 

 the top mark with the soap solution and proceed as in (a). 



