298 Proceedings of the Royal Society 
Thomson, I am indebted for various hints, usually in the especially 
valuable form of criticisms and reasons for doubt), has lately called 
my attention to a paper by Listing, of date 1847, part of which is 
devoted to the subject of knots. I have this morning obtained it 
from the Cambridge University Library, but have not yet thoroughly 
read it. As was to be expected, I find that the author has antici- 
pated some of the contents of my papers ; and he mentions at least 
one very curious fact, which I had thought possible, but had not 
observed, though it is very directly connected with one of the results 
of the present note. He virtually shows, by giving a particular case, 
that the method of deformation which I employ does not always 
give all possible forms of a completely knotted wire. I believe 
that this depends on the fact that a part of the scheme is amphi- 
cheiral. I propose to give the Society an account of Listing’s 
method and results on the earliest opportunity. 
3. Note on the Effect of Heat on Infusible Impalpable 
Powders. By Professor Tait. 
Several years ago Professor Dewar gave me a specimen of silica 
in a state of exceedingly minute division, which had been produced 
in Dr Playfair’s laboratory in the preparation of fluosilicic acid. I 
noticed at the time how much its great mobility is increased by 
heating — so that it behaves almost like a liquid. And I fancied 
that I observed close to the surface a thin stratum of what might 
by the same analogy be called a vapour ; consisting of particles 
thrown up and falling back again, like the little drops thrown up at 
the surface of soda-water. I was inclined to ascribe these pheno- 
mena to heat directly — supposing that the particles were fine 
enough to behave, though in a very imperfect way, as the kinetic 
theory assumes the particles of a gas to behave. However this may 
be, the extreme mobility of such powders when heated on a plati- 
num dish ; and the fact, noticed by chemists, that a bath of calcined 
magnesia is capable of propagating waves when heated ; seem to 
show that valuable results might be obtained by seeking for evi- 
dence of inter-diffusion as the result of experiments made by very 
long-continued heating of vessels containing fine silica and mag- 
