1888-89.] Sir W. Thomson on Constitution of Matter. 719 
an oblate with equatorial diameter twice as long as the polar axis. 
Hence I have been led to think it probable that the molecular struc- 
ture of Iceland spar is not that of Hypothesis (1), but is as follows. 
§ 59. Hypothesis (2). — Take a primitive cube § 53 of the assemblage 
and distort it by shrinkage along any one of its four line-diagonals, 
with (for simplicity) no change of length in directions perpen- 
dicular to it. This reduces each globe to an oblate ellipsoid of 
revolution, and the cube to a rhomb, which is the rhomb of Iceland 
spar if the shrinkage-ratio is J2 to 1. 
§ 60. Let FG (fig. 7) be one edge and G one of the two obtuse 
trihedral angles of a rhomb of Iceland spar ; and F', G' the corners 
Fig. 7. 
opposite to F, G (so that G r G is the optic axis). Let HKK'H' be a 
diagonal plane parallel to FG and F'G'; HK, H'K' being parallel to 
FG, F'G'. How consider rows of oblates parallel to KK' and HH\ 
The oblates are in contact at the ends of equatorial diameters in 
the lines of these rows. Turn the oblates of each row round the 
line of the row, in the half of the assemblage above HKK'H' (suppos- 
ing this plane horizontal and G to the right) all through equal angles 
against the motion of the hands of a watch till their equators become 
horizontal. The assemblage of centres shears to the right (with rota- 
tion in the direction of the hands of a watch), FG moving rightwards, 
till the angle between FG and the end face through F becomes a 
right angle. Simultaneously with this shearing motion there is a 
shrinkage of the assemblage in the direction perpendicular to the 
plane HKK'H', entailed by the fact of the equatorial planes of the 
oblates turning from their primary inclined positions, with equatorial 
planes perpendicular to the optic axis, to their present horizontal posi- 
tions. This is most readily seen by confining attention to the single 
row of oblates which initially had their centres and points of mutual 
contact in the short diagonal GF' of the right-hand end face. The 
