4 
NOTE ON THE lEON-TURNINGS CELL. 
It is inconceivable that a perfectly smooth projecting solid 
point should be the seat of such an entrapped bubble ; but it is 
to be observed that what appears to the naked eye to be such a 
smooth point, may in most cases be seen with a microscope to be 
an exceptionally rough portion of the surface ; indeed, a point 
cannot project from a portion of the surface that has been well 
polished : it is the sign of a place that has escaped smoothing ; 
thus the microscope shows the point to be the roughest place in 
a needle. 
Again, it is well known that a piece of porous earthenware 
{e, [/., a bit of clay tobacco pipe) is one of the best means of 
securing a regular delivery of bubbles in a boiling liquid, and the 
author has observed that bubbles which were detached in great 
abundance from the apparently smooth surface of a sewing- 
needle immersed in a glass of soda water had their origin in a 
scratch which it required the microscope to reveal. 
But the most conclusive argument of all against the forma- 
tion of bubbles at true points is to be found in the consideration 
of the necessarily constant value of the contact angle between 
the liquid surface bounding the bubble and the solid against 
which it abuts. In the case of a 
liquid which wets the solid, the 
liquid angle between the solid-liquid 
surface and the liquid-gas surface is 
acute, and a bubble at a vertical 
point must have some such a form 
as that shown in fig. 1, and must 
necessarily slide up the point and 
entirely come away. Nor may it be 
urged that air bubbles really do 
escape in this way from points, and 
that their formation and delivery is 
rendered possible by the slowness of 
F,g. 1. 
