STEAM 
ENGINE. 
547 
before, and the vessel will be quite transparent. The 
visibility therefore of the matter which constitutes the steam 
is an accidental or extraneous circumstance, and requires 
admixture with cold air. 
The opaque and cloudy appearance of steam, is explained 
by saying that the vapour is condensed by coming into 
contact with the cooler air. But there is something in the 
form of this cloud which is very inexplicable. The particles 
of it are sometimes very distinguishable by the eye; but 
they have not the smart star-like brilliancy of very small 
drops of water, but give the fainter reflection of a very thin 
film or vesicle like a soap bubble. If we attend also to their 
motion, we see them descending very slowly in comparison 
with the descent of a solid drop ; and this vesicular con¬ 
stitution is established beyond a doubt by looking at a 
candle through a cloud of steam. It is seen surrounded by 
a faint halo with prismatical colours, precisely such as we 
can demonstrate by optical laws to belong to a collection of 
vesicles, but totally different from the halo which would be 
produced by a collection of solid drops. It is very difficult 
to conceive how these vesicles can be formed of watery 
particles, each of which was surrounded with many particles 
of fire, now communicated to the air, and how each of these 
vesicles shall include within it a ball of air; but we cannot 
refuse the fact. We know, that if, while linseed oil is 
boiling or nearly boiling, the surface be obliquely struck 
with the ladle, it will be dashed into a prodigious number 
of exceedingly small vesicles, which will float about in the 
air for a long while. Mr. Saussure was the first who dis¬ 
tinctly observed this vesicular form of mists and clouds; 
and lie makes considerable use of it in explaining several 
phenomena of the atmosphere. 
STEAM ENGINE, s. An engine so called, because its 
first movement takes place in consequence of the generation 
and condensation of steam. 
The steam engine is the first of a series of machines which, 
no doubt, will hereafter become numerous; differing from 
all those which preceded it in the circumstance that it is at 
once a chemical and a mechanical instrument. The minute 
changes in the fabric of matter that are produced by the 
agency of fire, form the prime movers of an engine which, 
in other respects, differs not from the machines moved by 
wind, water, or animal strength. 
In the article Mechanics, we have already so far antici¬ 
pated the subject under consideration, that the reader is in¬ 
formed that by means of heat applied to a boiler, water is 
evaporated into a cylinder furnished with a valve; that the 
steam thus sent into the cylinder drives the air through the 
valves, and then that the steam is suddenly condensed by 
cold, so that instead of filling the whole cylinder, it scarcely 
occupied To r 0g of its previous space: that, consequently, 
a vacuum is formed which operates so as to pull down 
forcibly a piston inserted into the cylinder, and that thus 
the prime movement of the engine is effected. This prin¬ 
ciple, belongs to all engines, except the high-pressure 
—so that, in fact, the former are properly pneumatic ma¬ 
chines, and it is only to the last that the term steam 
engine ought properly to be applied. But whatever kind of 
engine is used, the elasticity of the steam on the one hand, 
and its condensation on the other, are the only agents em¬ 
ployed. The first points to be known, then, are the laws of 
this elasticity and of this condensation. 
It is a well-known law of heat, that all bodies, when they 
press from a solid to a fluid state, or from fluidity to the 
aeriform or gaseous state, imbibe, during such change, a 
considerable quantity of heat, which is not rendered appa¬ 
rent by the thermometer, or perceptible to our sense. As, for 
example:—a sheet of ice, and any fluid of the same tempe¬ 
ratures being placed in an exalted temperature, the fluid will 
become warmer immediately, but the ice will not; and the 
ice will require many times more heat to melt and exalt it to 
the heat of the fluid. The reverse, also, holds good. Water 
returning to the state of ice, indicates the loss of many times 
more caloric than the said water does if it be made to return 
to the temperature of ice, without being allowed to freeze 
(a restriction which we may easily command). So, also, 
water heated to 212° of Fahrenheit, requires 800 degrees of 
heat to convert its whole mass into steam;—yet this steam 
only makes the thermometer rise to 212°. And thus again, 
the steam, when condensed, gives out many times more 
caloric' than the difference of its apparent temparatures in the 
states of vapour and water. 
A vast number of experiments have been made with the 
view of discovering the precise quantity of the heat thus 
latent in steam. They disagree considerably, but perhaps 
we shall not err much in stating, that under the ordinary pres¬ 
sure of the atmosphere, steam, exhibiting 212 degrees of 
heat, actually contains about 1200°. Steam, therefore, when 
mixed with 6 times its weight of water at 32°, will raise the 
temperature of the latter to 212 degrees. 
The preceding principle is of considerable use in its appli¬ 
cation to practical purposes. Wherever large quantities of 
boiling water are wanted, as in dyeing-houses, breweries, 
and the like, it is of great advantage to employ steam for 
heating the water; It is also used extensively in manufacto¬ 
ries, for heating rooms; which it does very effectually, 
and without any of the unpleasant effects resulting from 
common stoves. 
Now temperature being the cause of the evaporation 
of fluids, and atmospherical pressure the force that opposes 
that change, it follows that the phenomenon itself varies 
either with the temperature or the pressure: and elasticity being 
a property counteracting this pressure, and regulated solely 
by it, this depends on both temperature and pressure. Thus it 
happens, that when we diminish atmospherical pressure by 
an air-pump, the water contained in the vacuum boils and 
evaporates at a lower temperature than 212 °; and thus it 
happens, also, that under an increased pressure, water may be 
forced to imbibe more than 212 degrees of heat ere it boils. 
With regard to the law that governs the elasticity of steam, 
as many experiments have been made to discover it as 
have been performed with the view of finding out the law of 
heat. The former experiments have not had better success 
than the latter; for, though an approximation has been suffi¬ 
ciently near for most practical purposes, yet the results 
arrived at by the first experimentalists offer discrepancies 
that are not accounted for. Perhaps the fairest mode that 
we can adopt, is to lay before our readers some brief descrip¬ 
tions of the manner in which different scientific experimen¬ 
talists have proceeded in their investigations, and the con¬ 
clusions they have respectively arrived at. Omitting the 
earlier experimenters, such as Mr. Kairne, Lord Charles Ca¬ 
vendish, and others, we shall commence with the descrip¬ 
tion Professor Robison has left us of his own mode of 
endeavouring to ascertain the relation between elasticity and 
temperature. He used an instrument of the kind engraved 
in PI. I. fig. 1. 
ABCD (fig. 1.) is the section of a small digester made of 
copper. Its lid, which is fastened to the body with screws, 
is pierced with three holes, each of which had a small pipe 
soldered into it. The first hole was furnished with a brass 
safety-valve V, nicely fitted to it by grinding. The area of 
this valve was exactly jth of an inch. There rested on the 
stalk at top of this valve, the arm of a steelyard carrying a 
sliding weight. This arm had a scale of equal parts, so ad¬ 
justed to the weight that the number on the scale corre¬ 
sponded to the inches of mercury, whose pressure on the 
under surface of the valve is equal lo that of the steelyard on 
its top; so that when tne weight was at the division 10 , the 
pressure of the steelyard on the valve was just equal to that 
of a column of mercury 10 inches high, and 5 th of an inch 
base. The middle hole contained a thermometer T, firmly 
fixed into it, so that no vapour could escape by its sides. 
The ball of this thermometer was but a little way below the 
lid. The third hole received occasionally the end of a glass 
pipe SGF, whose descending leg was about 36 inches long. 
When this syphon was not used, the hole was accurately 
shut with a plug, 
Tli? 
