1887.] Sir W. Thomson on Doctrine of Extraneous Pressure, 33 
tXuires the fulfilment of (38), as did the other supposition : but 
instead of (37) we now have [in order to make (36) constant] 
A = B = C (59), 
and therefore each, in virtue of (14), zero; and therefore by (43), 
a = /? = y=l; 
so that we are driven to complete isotropy. Hence our present form 
(§ 7) of the stress theory of double refraction cannot be fitted to give 
line of vibration in the plane of polarisation. We have seen (§21) 
that it does give line of vibration perjpendieidar to the plane of 
polarisation with exactly FresneVs form of wave-surface, when 
fitted for the purpose by the simple assumption that the potential 
energy of the strained solid is expressed by (52) with A: constant ! 
It is important to remark that k is the rigidity-modulus of the un- 
strained isotropic solid. 
23. From (58) we see that the velocities of the waves correspond- 
ing to the three cases, Z=l, m = l, 1, respectively, are ^(A:/a), 
J{klj3), slikjy). Hence the velocity of any wave whose vibrations 
are in the direction parallel to any one of the three principal elonga- 
tions, multiplied by this elongation, is equal to the velocity of a 
wave in the unstrained isotropic solid. 
6. Exhibition of Models. 
The President exhibited Models of the Minimal Tetrakaide- 
kahedron. His paper on the subject is printed in the London^ 
Edinburgh, and Dublin Philosophical Magazine, vol. xxiv. 5th 
series, p. 503, December 1887. 
7. Researches on Micro-Organisms, including ideas of a 
new Method for their destruction in certain cases of 
Contagious Diseases. Part II. By Dr A. B. Griffiths, 
F.R.S. (Edin.), E.C.S. (Bond, and Paris), Principal and 
Lecturer on Chemistry and Biology, School of Science, Lincoln; 
Science Master in the Lincoln Grammar School, ^’c. 
In the Proceedings of the Royal Society of Edinburgh, vol. xiv. 
[No. 123], pp. 97-106, there is a paper of mine under the above 
title. I wish in the present memoir to communicate to your dis- 
voL. XV. 7/6/88 c 
