T H E 
T H E 
I Mercury seems to expand more equably 
than any other fluid. Yet its increments of 
bulk are not quite proportional to the incre- 
ments of heat. With other fluids the irregu- 
larity of expansion is very considerable. 
I One cubic inch of mercury, or one measure 
whatever of it, at 32° of temperature, when 
heated to the temperature of boiling water, 
iviz. at 212°, will be found increased in bulk 
by the quantity 0.01835. This fluid metal 
boils and becomes a vapour at 600° of Fah- 
renheit's thermometer, and it becomes a solid 
at — 40° ; viz. 72° below melting ice. Be- 
low that point, viz. — 40°, it contracts ir- 
regularly. 
, Spirit of wine boils at about 180° and the 
purest probably never freezes. When bran- 
dy, or a mixture of water and spirit, freezes, 
it is the water that becomes solid, but the spirit 
will be found .collected together in one or 
more bubbles, in some part of the ice. 
From all that has been said with respect to 
(the expansion of fluids, it appears that, on 
account of the great irregularity of the rate 
of expansion, mercury and spirit of wine are 
the only two fluids which can be used for 
thermometers ; observing that some com- 
pensation must be made in the scale of the 
spirit thermometer, in order to make it cor- 
respond with the scale of the mercurial ther- 
mometer. But the mercurial thermometer 
cannot indicate a temperature higher than 
500. Hence various ingenious persons have 
endeavoured to contrive instruments capable 
pf indicating the higher degrees of heat, 
Which would be ©f great use in philosophy, 
chemistry, and various arts ; but the only 
useful contrivance of this. sort was made by 
the late Mr. Wedgewood. This ingenious 
gentleman applied to the measuring of high 
degrees of heat, a singular property of argil- 
laceous bodies, a property which obtains 
Lore or less in every kind of them, as far as 
has been examined. This property is, that 
fln argillaceous substance, when exposed to 
tire, is diminished in bulk by it, nor does the 
bulk increase again after cooling ; and this 
jdiminution of bulk is proportionate to the 
degree of heat to which the substance has 
been exposed. 
I This property may seem to be a deviation 
from the general rule, viz. that heat expands 
all sorts of bodies, and that a diminution of 
heat enables them to contract their dimen- 
sions ; but in this case it must be considered 
#hat the clav'pieces contract and remain con- 
tracted, because some substance, viz. water 
and an aeriform fluid, is separated from them 
by the action of the fire. 
Mr. Wedgewood’s thermometer, or appa- 
ratus for measuring the high degrees of heat, 
consists of small pieces of clay of a determin- 
ed length, which are to be placed in the fur- 
»ace, crucible, &c. whose degree of heat is 
to he ascertained ; and of a gauge to measure 
the contracted dimensions of the clay pieces, 
after they have been exposed to the lire. 
Fig. 241 represents the gauge, which is 
either of brass or of porcelain, b ig. 242 re- 
I presents a section of the same ; and the let- 
ters refer to the like parts in both figures. 
! EFGH is a smooth flat plate ; AC, BD, are 
two rulers or flat pieces, a quarter of an inch 
thick, and fixed last upon the plate, so as to 
form a converging canal ABCD, whose width 
at CD is three-fifths of the width at AB. 
| The whole length of the canal from AB to 
CD, is divided into 240 equal parts, and the 
divisions are numbered from the wider end. 
It is evident that if a body, so adjusted as to 
fit exactly the wider end of this canal, is 
afterwards diminished in its bulk by the action 
of lire (as the thermometrical pieces which 
will be described in the next paragraph), it 
may then be passed further in the canal, and 
more so according as the diminution is 
greater. 
'i'he thermometrical pieces are small cy- 
linders of clay, a little flattened on one side. 
They are nearly as much in diameter as they 
are In length. * When one of these pieces is 
to be used, it is proper to measure it first by 
placing it in the gauge at AB ; for sometimes 
t lie pieces are a few degrees larger or small- 
er than the distance AB, which excess or 
defect being ascertained, must afterwards be 
allowed for. P represents one of these pieces 
set in the gauge for measurement. 
The piece is then placed in the furnace, 
or crucible ; and if it is taken out either at 
the end of the operation, or at any period, 
and, when grown cold, is measured by sliding 
it as far as it will go, into the canal of the 
gauge, the number of divisions against the 
p'ace where it stops will shew the contracted 
dimensions of the piece, and of course the 
degree of heat to which it has been exposed. 
It will be found that these pieces will go very 
little beyond 0 in the canal, if they have been 
exposed* to a visible red heat ; will go to 27° 
if they have been exposed to the heat in which 
copper melts ; to about 90° if exposed to the 
welding-heat of iron ; about 1G0° if exposed 
to the greatest heat that can be produced 
with charred pit-coal in a well constructed 
common air-furnace, See. 
The same thermometrical piece which has 
been used before, may be used again for 
higher degrees of heat, but not for lower de- 
grees. 
It is now necessary to shew the correspond- 
ence between the scale of this, and the scale 
of Fahrenheit’s mercurial thermometer. 
As the mercurial thermometer cannot shew 
a temperature higher than 600°, and Wedge- 
wood’s thermometer cannot shew a tempe- 
rature lower than red heat, which is by seve- 
ral degrees higher than 600°, therefore it was 
necessary to contrive a measure for the inter- 
mediate degrees, and which might reach 
some degrees below 600°, and some degrees 
above the temperature of a red heat. Mr. 
Wedgewood chose a piece of silver, the ex- 
pansion of which measured in a gauge made 
for the purpose, similar to the gauge fig. 
241, might indicate the degrees of tempe- 
rature between the two thermometers ; with 
this instrument he first found the correspond- 
ence between the degrees of Fahrenheit’s 
scale and the last-mentioned gauge, by pla- 
cing them alternately in water of the tempe- 
rature of 50°, and in boiling water. Then he 
found the correspondence between the de- 
grees of the gauge of the silver piece, and 
that of the earthen thermometrical pieces, by- 
placing them both at the same time in dif- 
ferent and higher degrees of heat; lastly, 
by computation from those results, lie deter- 
mined the correspondence between the de- 
grees of Fahrenheit’s scale and those of his 
own thermometrical gauge. 
It was found that one degree of Wedge- 
wood’s thermometer is equal to 130° of Fah- 
renheit’s; and that the 0 of Wedgewood’s 
5 112 
T H 1 795 
coincides with the 1 077, 1 ’5 of Fahrenheit’s ; 
from which data a comparison ot the two 
thermometers may bo. made, or rather ol the 
imaginary extensions of their two scales ; 
for, in fact, Fahrenheit’s thermometer can- 
not shew higher than 500', and Wedgewoccl’s 
cannot reach near so low. It is likewise to 
be observed that the degrees of Wedgewood’s 
scale are supposed to shew equal increments 
of heat, whereas in truth we do not know 
whether the clay thermometrical pieces con- 
tract in proportion to the increments ot heat ; 
which shews that, though this is the best 
known thermometer for measuring the higher 
degrees of heat, yet an improvement of the 
same, or some other manageable and more 
accurate, contrivance, is highly desirable. 
Upon the whole it appears, that the spirit 
thermometer enables us to measure the de- 
grees of heat as low as has ever been experi- 
enced, either naturally, or by artificial cool- 
ing : that the mercurial thermometer enables 
us to measure the heat from — 40° to 600 c ; 
and that Wedgewood’s thermometer enables 
us to measure from a red heat up to the far- 
ther extent of that scale, viz. to its 240th 
degree, which is reckoned equivalent to 
32277 J of Fahrenheit’s scale. 
riiESIUAt; Base fluellin>, a genus of 
plants of the class pentandria, and order mo- 
nogynia. The calyx is monophyllous, with 
the stamina inserted into it ; there is only 
one seed, which is inferior. There are nine- 
teen species ; one of which is a British plant, 
the linophylhtm or bastard toad-flax. It has 
a foliaceous panicle with linear leaves, and 
flowers in June and July. 
TI1LASPI, BASTARD-CRESS, OP MITHRI- 
date mustard, a genus of plants of the 
class telradynamia, and order siliculosa ; and 
in the natural system ranging under the 39th 
order, siliquosa. T he pod is emarginated, 
obcordate, and polyspermous ; the valves 
are boat-shaped, and marginated and cari- 
naled. There are 14 species; of which six 
only are natives of Britain. 1. The arVense, 
treacle-mustard, or penny-cress. It smells 
like garlic, and has a white flower. 2. The 
hirtum, or perennial mithridale mustard. 
3. The campestre, or mithridate mustard. 
4. The montanum, or mountain mithridate 
mustard. 5. The perfoliatum, or perfoliate 
treacle-mustaid. 6. The bursa pastoris, or 
shepherd’s purse. The seeds of some of 
these species ha\e an acrid biting taste, ap- 
proaching to that of the common mustard ; 
with which they agree nearly in their phar- 
maceutic prope.ties. 
THIRD, in music, an interval so called 
because it contains three diatonic sounds. 
The Greeks not admitting the third as a 
consonance, it obtained no general name 
amongst them ; but took that of the lesser or 
greater interval from which it was formed. 
There arc four species of thirds ; two con- 
sonant, and two dissonant. The consonants 
are ; first, the major third, called by the an- 
tients ditone, composed of two tones; se- 
condly, the minor third, called h emit one, 
consisting of a tone and a half. The dis- 
sonant thirds are, first, the diminished third, 
composed of two major semitones ; se- 
condly, the siiperfluous^third, composed of 
two tones and a half. This last interval, 
not having place in the same mode, or key, 
is never used either in harmony, or in me- 
lody. The Italians sometimes introduce the 
