166 Dr. Shaw and Mr. Garrett on 
sundered by forces which are small, but measurable. The 
best proof that coherence has taken place is, not that the 
P.D. between the surfaces has fallen (for this always occurs 
when the pressure between the surfaces is great), but that 
definite force is necessary to sunder them, always provided 
pseudo-coherence is avoided *. 
In this paper coherence is tested and measured solely by 
the sundering forces. Ina former paperf one of us showed 
how, by taking the simplest case of two surfaces only, it is 
possible to measure the forces necessary to sunder them when 
cohered, It was also shown that the surfaces recohere, 7. e. 
show a power of clinging when brought together after co- 
herence has been broken, and that this clinging power is 
comparable with that occurring in the original coherence. 
In the present paper the experimental investigation is 
carried further than before, and an attempt is made at ex- 
planation. 
Fig. 1. 
m 
Apparatus. 
Two forms are used. 
I, The Boom Apparatus (fig. 1).—Similar to an apparatus 
* See Shaw, Phil. Mag. March 1901, p. 271. 
tT Shaw, Phil. Mag. March 1901. 
