38 
In the instance of the instrument I use the stem of the 
thermometer is 18 inches long and the eighth of an inch or 
thereabouts in bore. 
Mercury in consideration of its weight is the fluid I em- 
ploy, and in conjunction with it some sulphuric acid is 
enclosed, because of the mobility which is thereby gained. 
I am aware that in these circumstances mercuric sulphate 
is very slowly formed, but after two years’ lapse of time no 
inconvenience has been caused thereby and the mobility of 
the mercury remains. 
The bulbs of the thermometer are two inches in diameter 
or thereabouts, and that they may be more rapidly affected 
the glass thereof is thin. Both are blacked, and the one 
intended to receive the radiance of the sun projects above 
the box in which the apparatus is contained into a dome of 
glass. 
“ On some Notices in Classical Authors of the Action of 
Sunlight on Purple Bye,” by James Bottomley, B.Sc., F.C.S. 
At a meeting of the Society on October 21st, Br. Schunck 
exhibited to us some of his specimens of purple extracted 
from shell-fish, and called attention to the remarkable action 
of light in developing the colour. At the time I remem- 
bered having seen some passage relative to a supposed action 
of light upon the colour, showing that the ancients had 
some obscure notions of the matter. This seems to me 
additional evidence of the identity of the colour obtained 
by Br. Schunck with the famous purple of the ancients. 
The passage which I had in my mind is a note by Paley on 
the following verses of the Helena of Euripides : 
Kvavoeidlg afi^ vSwp 
^ > V A / 
erv)(ov EAiKa r ava ^Aoav 
(j)OLVLKag aXtfj) ireirXovg 
avyoLcnv iv ^pvaiaig 
aju^i^aXrrova £v re SovaKog epvecnv. 
