PYR , 
the length to which it can be slid into the 
converging groove or gage. 
The utility of this instrument, it was ob- 
vious, would be much increased by con- 
necting it with the mercurial thermometer, 
and by ascertaining the proportion between 
the degrees of each ; and this was done by 
Mr. Wedgwood. The scale of his pyrome- 
ter commences at red-heat fully visible in 
day-light. The mercurial thermometer 
cannot easily measure any temperature 
above 500“ or 550° ; and hence, between 
the termination of the scale in the one, and 
its commencement in the other, there is 
a range -of temperature requiring to be 
measured. This Mr. Wedgwood did, by 
the expansions of a square piece of silver, 
measured in a gage ^ earthen-ware, con- 
structed in the same way as his pyrometer; 
and by the same method, he found out 
the proportion between each degree of his 
scale, and that of any of the usual tlier- 
mometrical scales. Each degree of his py- 
rometer he found to be equal to 130° of 
Fahrenheit. The sommencemeiit of his 
scale, or the point marked 0, corresponds 
with 1077i° of Fahrenheit’s scale. From 
these data, it is easy to reduce either to 
the other, through their whole range. The 
scale of Wedgwood includes an extent of 
temperature equal to about 32,000° of Fah- 
renheit, or 54 times as much as that be- 
tween the freezing and boiling points of 
mercury. Its commencement, as has been 
stated, is at 10771° of Fahrenheit, or red 
heat fully visible in day light ; its extremity 
is 240° ; but the highest heat that he mea- 
sured with it is 160", or 21,877° of Fah- 
renheit; being the temperature of a small 
air-furu;sce, and 30° of his scale above the 
point at which cast-iron melts. 
Guyton has proposed a pyrometer for 
measuring high temperatures, in which pla- 
tina, a metal not fusible even at very in- 
tense he^ts, is employed as the measure 
of expansion. A rod or plate of this metal 
is placed horizontally in a groove framed 
in a mass of hardened white clay ; one ex- 
tremity of the rod is supported on the mass 
which terminates the groove ; the otlier 
PYR 
presses against a bended levet of plating, 
the longest arm of which forms an index 
to a graduated arc. The expansion. Which 
the rod of metal suffers from exposure to 
heat, is indicated by the change of posi- 
tion in this index. The mass of clay, being 
highly baked, will not introduce any im- 
portant error from its contraction ; and the 
expansion, which it may suffer during the 
exposure to heat, will affect only the small 
distance between the axis of motion of the 
index, and the point of contact of the 
plate, so as rather to diminish the effect 
than to increase it. Platina, having the 
important advantage of not melting by any 
heat we have to measure, and of not suP. 
fering any chemical change from it, is well 
adapted to the construction of a pyro- 
meter. 
Besides these, various metallic pyrome- 
ters have been invented capable of mea- 
suring low temperatures, by the expansion 
being multiplied by the aid of wheels, 
levers, or other mechanical contrivances, 
or being magnified by microscopes. Such 
are the pyrometers of Muschenbroeck ; 
that described by Ferguson ; one invented 
by Mr. Ellicot, with which he measured 
the expansions of various metals ; one by 
Mr. Smeaton, and applied to the same pur- 
pose ; Mr. Ramsden’s, superior to the pre- 
ceding ones in delicacy and accuracy ; Mr. 
Crichton’s, in which advantage is taken of 
the difference of expansion between a rod 
of zinc and a rod of iron, to give a curva- 
ture to the bar composed of the united 
rods, proportioned to the temperature to 
which they are raised ; by which bending 
motion is given to an index that, at its 
other extremity, w'here the scale is marked, 
describes a considerable space ; and, lastly, 
one by Regnier, on a principle somewhat 
similar, of which a report is presented to 
the French National Institute. The strict 
accuracy of these instruments may, from 
the nature of their construction, be regarded 
as doubtful. It has been found, by Eilicot’s 
pyi'ometer, that the expansion of bars of 
different metals, by the same degree of 
heat, is as follows ; 
Gold. Silver. Brass. Copper, Iron. Steel. Lead. 
73 103 95 B9 60 56 149 
PYROMUCUS add. When sugar and 
other sweet tasted substances are distilled, 
among other products there is always a 
notable quantity of an acid liquid. This acid, 
when rectified, obtained the names of sy- 
rupous acid, aad afterwards pyromucous 
acid. It is now known from the recent e.x- 
periments of Fourcroy and Vanquelin, that 
this acid is nothing else than the acetic, 
holding in soiution a portion of empyreuma- 
tic oil. 
PYROPE, in mineralogy, a species of the 
