50 Mr. W. B. Hardv on th 



fall in surface tension occasioned by its presence, by the 

 sweeping away of motes of dust on the surface as it spreads, 

 and by the fact that it can be scraped oft' the surface, the 

 surface tension being thereby raised to the full value of clean 

 water. The movement of motes during the process of 

 scraping shows that the invisible him confers on the surface 

 the property of tangential elasticity or contractibilitv. This 

 is an exceeding delicate test for its presence. The film is at 

 an electrical potential different from that of the water ; there 

 is a contact difference of potential between it and the water*. 



In the paper already referred to I described how drops of 

 certain fluids of low chemical reactivity, and ivhose vapour- 

 pressure was negligible, such as a paraffin of high boiling-point, 

 could be placed on a clean surface of water without either 

 displacing motes on the surface or lowering its tension, or 

 making the surface " contractile.*' 



Some fluids therefore will not spread on water at all, and 

 that despite the fact that they are capable of lowering the 

 tension of water. Saturated paraffins of low boiling-point 

 will form a film, but that only by condensation of the vapour, 

 and not by direct spreading of the drop, for reasons given in 

 the earlier paper. 



The point of interest and, so far as I know, of novelty, is 

 that all these relations hold quite simply for a clean solid face. 

 From a drop of certain fluids primary spreading occurs, if 

 the fluid in question is not rigorously pure secondary 

 spreading follows, and certain fluids will not spread at all. 



A drop of acetic acid which has been purified by re- 

 crystallization when placed upon a surface of glass which has 

 become contaminated by exposure to the atmosphere does 

 not spread. On clean glass the drop flattens out rapidly, 

 dust particles or drops of water being swept away. It was 

 found, however, that the flattening was due to the water 

 vapour present in the air. The remarkable character of the 

 action as an instance of the operation of small quantities of 

 matter upon surface energy cannot be exaggerated. 



Attention was drawn to it by the following observation. 

 A clean plate of glass was placed in a chamber through which 

 a rapid current of dry air was flowing. After the plate had 

 been in the chamber for 30 minutes, the cover was lifted 

 slightly for a moment to allow of a drop of acid being placed 

 on the plate from a fine capillary tube. The drop at once 

 spread to an even layer, thin but distinctly visible. After 

 being for some minutes in the dry air the layer altered. 



* Proc. Eoy. Soc. B. lxxxiv. p. 220 (1911) ; A. lxxxvi. p. 608 (1912). 



