‘FPUNGYT. 
ylai glongitindinal groove: These; in’all the species of the 
F are of a black colour. Some grow on wood, ov 
fe Wark of trees, while others: rocks or walls, 
They are arranged with the li by different au- 
thors. 
_ Verrucaria sanguinularia, E. Bot. 155. Aenean 
ash-coloured, with a tinge of green, granular, wrin 
thin, and irregular, » The receptacles are scattered, he- 
mispherical, black on the outisde, bright red within. 
When -magnified, a small pore may be observed at the 
summit of the receptacles. 
Genus IX. Perrusaria. The plants of this genus 
sist of a plain n 
or cavity of which is defended from the exterior air by 
some infusible substance, the least capable of conduct- 
ing heat. That portion of the cavity which is occupied 
by the fuel, may be called the body of the furnace. 
This is separated from the lower portion, called the 
ash-pit, by a grate for the admission of air. The —_ 
portion above the body is called the chimney. hen 
the fire is kindled, all the air above the grate becomes 
specifically lighter than the outer air. This induces a 
current of air through the furnace, the velocity of which 
will be as the difference of temperature between the 
inner and outer air, and as the square root of height, 
reckoning from the grate to the summit of the column. 
In a furnace so constructed, the air would meet with 
the least resistance from not being required to change | 
its direction after it enters the grate, and its practical 
effect would come the nearest to the law above laid 
down. But for the sake of convenience, the body of 
the farnace is detached from the chimney, by a con- 
necting cavity called the throat of the furnace. This 
will interrupt the free of the air, in proportion 
to the deviation of its direction from the ndicular 
line, and inversely as the area of its section. The in- 
tensity of the heat of every furnace, will therefore be as 
the quantity of oxygen consumed in a given time, and 
‘inversely as the space in which the combustion is pro- 
‘duced. This will therefore be as the velocity of the 
current and the density of the air, all other things be- 
ing equal. ; ; 
61 
lant. They differ from the preceding genus in aspect, 
ft the number‘of capsules, and in thee recuptatlen being 
of the same colour with the crust. 
_Pertusaria communis. Lichen pertusus of En. Bot. 
677. The crust is smooth, of a greenish ash colour 
when moist, cinereous when dry, and covered with nu- 
merous, crowded, smooth, angular warts, of the same 
colour as the crust. Upon the head of, each wart are 
small black impressed punctures, from one to ten in 
number, as if made with the point of a pin, under 
each of which punctures, if the wart be cut transverse- 
ly; will be found a hollow, spherical cell. In the old 
have an indistinct crust supporting their receptacles, specimens, the punctures open of themselves, and dis- 
_ which are-pierced by many pores, corresponding to as cover the cell. It is found on the trunks of trees, and 
many internal capsules; the pores sometimes unite, also upon rocks. 
and form an irregular cup during the old age of the 
——————e} 
FUR FUR 
FUNGUS. See Surcery. Let D- =the density of the outer air, 
FUR. See Darrur. d =the density of the air in the chimney, 
FUR Trave. See-Petrry. ; hk = the height of the chimney, 
FURNACE is the name given to an apparatus, which g =the velocity produced by gravity in a se- 
consists of a certain cavity containing combustible mat- cond of time, 
ter, with various means of supplying it with air to ef- i =the velocity ofthe ascending current ; 
fect its combustion. nee then the velocity which the height h would give will be 
Furnaces may be divided into two general classes, — ,/9} p. and the velocity of the t 
sis Win aie Ramanan iad Mankaedin inthe Te ee ey isha Tae! 
first of these, the air is induced to pass through the fire gy 
by the draught of a funnel or chimney communicating D 
with it. Inthe second, the air is supplied by the ac- — The practical effect, however, will fall short of this 
tion of bellows, or other pneumatic apparatus. The formula, from the interference of several causes, the 
term furnace, however, is more platioulatly applied to principal of which is the interruption of the current, 
such as are used in the manufactures for the fusion and partly by the change of the direction of its motion, and 
-calcination of substances, and in the laboratory of the partly by the roughness of the sides of the chimney. 
chemist. ’ The first of these may be in some measure obviated, 
The most simple and effective air furnace, would con- by making the throat sufficiently wide, and as little out 
atic or cylindric column, the interi- of the dicular as circumstances will admit. The 
friction upon the sides may be decreased considerably, 
by rubbing those sides of the bricks which have-to form 
the interior surface, in order to make it as even as pos- 
sible. It will also be advantageous that the walls of 
the chimney should conduct away the least possible 
quantity of heat. This will be effected, by first build- 
ing an inner wall of one course of bricks, and then sur- 
rounding it with an outer wall, leaving a cavity be- 
tween the two walls equal to the thickness of a brick, 
which cavity is to be closed firmly at the top. This 
will serve to keep the temperature of the air in the 
chimney hotter than it otherwise would be, and by that 
means increase the velocity of the current. The inte- 
rior of the body of the furnace, the throat, and a small 
portion of the chimney, must be of fire brick. 
It is difficult to give a maximum’ for the height of 
chimnies ; although from the’ above theorem, the 
power of the furnace would increase as the square 
root of the height, to any extent. Yet in practice, it will 
be found to be limited by two causes. The one, the 
friction of the sides of the chimney ; the other depends 
upon the gradual diminution of the temperature of 
the inner air as it ascends till it becomes equal to 
that of the external. This first will vary with the faci- 
lity with which the heat is conducted away through the 
sides of the chimney.» It will be obvious, however, un- 
der the most favourable circumstances, that the changes 
of power by altering the height will not be very con- 
spicuous, from its being-as the square root of the height, 
Hypoxyla. 
—— 
Furnace. 
—— 
