4.8 M E M O IR S of tbe 
taken, becaufehe fomewhere mentions heaps oFthofe fhells, where 
the Officlnce 'Purpurce anciently were, and alio from the purple 
Sanies the fifa yields of itleJf, he mentions one or two more 
Ipecies of turbinate Ihails, to be found in the Mediterranean, 
which yield a purple juice. Upon the whole, it is indifferent, 
what Ibrt of Aiell we ule in the ftiops, if it be to be calcined, 
providing it be a fea-fhell5 and neither does 2) iofcor ides or JEtius 
diftinguilh betwixt the Ofirea ^Purpuray or Succimm calcined, 
but they afcribe to them all the lame cauftic virtue; pofTibly 
Ibme one fpecies may have it in a higher degree; as the various 
forts of lime-ftone, if calcined, differ in ftrength. Tho' the Ipecies 
of fhell or Purpura be fcarcely known to our iliops at this day, 
vet the ufe of the purple juice hath been by tradition at leaft 
handed down to our times, and kept as a lecret, till it W2S difco- 
vered and publifhed by Mr. Cokj and from apaflage of Veda's 
ccclefiaftical hiftory, /. i. c. i. it appears that the purple trade of 
dying was ufed in England, and very much valued. 
'The E%par,fion and Contra5iion of Fluids Ipy Heat and Cold, in 
order to afcertain the Divifions of the Thermometer; by 
Mr, Edm. Halley. Phil Tranf. ISI° 197. p. (J50. 
(r\ Ualities, as heat and cold, moifture and drynels, ^c. are 
Vz_ not otherwile to be eflimated, but by their effe^is, as by 
encreafing or leffening the dimenfions of any body they aft on, or 
elle by the motions they produce; both which fubjecl them to 
menluration ; but it is Itill a queftion how to afcertain the propor- 
tional heat or cold, ^c. that is between any twp climates or 
feafons, fo as to conclude the one, for inftance, twice as hot, or 
twice as cold as the other, tho' the inflruments now in ufe abun- 
dantly fuffice to lliew, when the temper of the air is the fame, 
and when it is warmer or colder ; the reafon is, that we know not 
the caufes of the expanfion of fluids by heat, or of their contraftion 
by cold, as arifmg from the nature of their conflituent parts, 
which are fo far fl-om being objeftsof our ienfes, that they even 
furpafsour moll refined reafoning; for the fame degree of heat 
does not proportionally expand all fluids; fome Iwelling with a 
gentle warmth, and others not till they are confidcrably hot ; 
Ibme boiling with a mc ^derate heat, and others not at all; lome 
capable of a great expanfion, others of very little; lo that it may 
well be concluded, that not one of them cncreales or diminifl-ies 
in the fame ratio with the hrat, and confequently, that the 
thermometers graduated by equal parts of the expanfion of any 
fluid, are not iufficient ftandards of heat or cold : This will be 
more 
