PRESIDENTS ADDRESS—SECTION A. 53 
application of pressure. Professor Liversidge, to whom I men- 
tioned this matter, has informed me that dry alum and dry lead 
acetate become liquid on rubbing together in a mortar ; hydrate of 
chloral and camphor do the same. It is a question as to the 
combination produced by strongly pressing together powdered 
metals, or corresponding salts. I have made some experiments 
on solid paraffine, and believe, though I desire to repeat the 
experiments before being absolutely sure, that paraftine melting 
normally at about 53° C, may be caused to begin to melt some 
two degrees lower by having undergone intense pressure. The 
melting point also seems to be less definite. Again, the electric 
charges produced by compressing crystals are well known, and 
since on this theory such charges are the sign of forced chemical 
change, since the amount of charging occurring in any crystal is 
associated with the relation of the compression to the crystallo- 
graphic axes, it is suggested that the state of chemical combination 
(of the higher order—generally called physical) is different along 
different axes. Again, Strouhal and Barus have given good 
reason for thinking that the molecular grouping of steel alters 
under stress ; and, in fact, explain viscosity—after Maxwell—in 
this manner. ‘Taking these and other facts into account, my 
position is that if chemical change (including the reconstruction of 
so-called ‘ physical compounds ”) is accompanied by the freeing of 
atoms, even if they be afterwards combined in another way, only 
this freeing, if requiring the expenditure of energy, will be accom- 
panied by the phenomena of charge. Applying this reasoning, in 
connection with Helmholtz’s important views as to “ boundary 
layers” to the case of an ordinary frictional machine, I regard the 
first electrification as due to actual chemical change occurring in 
the boundary layers between the rubber and the glass plates. 
This change need not be large. Thus I have calculated roughly 
that if a large Holtz machine, which charged about ten jars to 
such a potential in about a hundred seconds as would allow them 
to givea spark two or three inches long, had been replaced by an 
equivalent plate machine, with a layer of water vapour on the 
plate surface, then an amount of decomposition corresponding to 
about a millionth of a gramme per second would account for the 
effects observed. I had no reason for taking a layer of water on 
the glass surface, except to enable me to make a rough calculation. 
Nothing can be better than a colloid like glass itself. A very 
small surface change would account for all the results, especially 
as it might undo itself while the machine was not working, and 
particularly if it got reversed ; and would, I think, be so small as 
to escape detection by a balance, or rather discrimination from 
the effects of abrasion. That the glass surface gets more or less 
changed is well known to everyone. I have ventured to put 
forward these views—not because they are free from objections, 
but because I think that the time has come for a beginning to be 
