ted

ed of

Salts Ting-pu the attr

resist the

al press

under the ower fe

been w

of ste

ed a t

cles of water is equal to a pressure of about three atmospheres, and to this strong cohesive force he attributes the irregular jumping motion observed in ebullition, and also some of those explosions of steam-boilers which heretofore have perplexed engineers. It is well known that cases have occurred in which an open pan of boiling water has exploded, producing fatal results, and such explosions cannot be explained on the usual hypothesis. M. Donny says that liquids by boiling lose the greater part of the air which they hold in solution, and therefore the molecular attraction begins to manifest itself in a sensible manner. The liquid consequently attains a temperature considerably above its normal boiling-point, which determines the appearance of new air-bubbles, when the liquid separates abruptly, a quantity of vapour forms, and the equilibrium is for the moment restored. The phenomenon then recurs again with increased violence, and an explosion may eventually ensue.

Spheroidal Condition of Liquids on Hot Surfaces.-If a drop of water or other liquid be thrown upon a hot metal plate or other highly heated surface, it does not moisten the surface or diffuse itself over it, but forms a flattened ellipsoidal mass; and if the drop be sufficiently small, it forms a minute spheroid, which revolves rapidly round a shifting axis, and evaporates very slowly without entering the state of ebullition. From Church's experiments it appears that it is necessary for the liquid to emit vapour before it can assume the spheroidal state. Molten lead dropped upon a very hot platinum plate did not assume the spheroidal state, whereas mercury dropped upon this plate assumed the spheroidal state at once. The most remarkable experiments, however, which have been made in illustration of the phenomena of the spheroidal state are those of M. Boutigny, and to him engineers are mainly indebted for calling their attention to the subject. One of the most singular results obtained by M. Boutigny is the power of making ice in a red hot crucible. A small crucible or capsule of platinum being made white hot, some anhydrous sulphurous acid in the liquid state is poured into it. The boiling-point of this liquid is as low as 14° Fahrenheit; but as it immediately on being projected into the

capsule assumes the spheroidal state, it remains upon the white hot metal without touching it; and if a few drops of water be now let fall upon the liquid acid, the water will be immediately frozen, and a piece of ice may be turned out of the crucible. M. Boutigny has also shown that if acids and alkalies in solution be poured into a clean red hot platinum crucible they will not unite, but both will assume the spheroidal state and roll about the bottom of the crucible without entering into combination. Not merely the gravitation of the liquid, therefore, but also its chemical affinity, appears to be superseded by the causes which make it assume the spheroidal state.

When a liquid assumes the spheroidal state it does not wet the surface, but appears to avoid touching it, like water sprinkled upon grease. Instead of entering into violent ebullition when it reaches the hot surface, its temperature will rise very little, and the drops of liquid will either remain at rest or will acquire a gyratory motion. When the surface is cooled down to 400° to 500°, depending on the nature of the surface and also on the nature of the liquid, the liquid will begin to diffuse itself, and will be suddenly scattered in all directions. The requisite temperature of a platinum plate to make water at the boiling-point assume the spheroidal state is 120° Centigrade, or 248° Fahrenheit; but if glass be used instead of platinum, the temperature must be raised to 180° Centigrade, or 324° Fahrenheit. For water at 0° Centigrade, the temperatures required are 400° and 800° respectively.

When water assumes the spheroidal state, it is possible by placing the eye on the level of the hot surface to see between the surface and the liquid. The electric circuit, moreover, is interrupted, showing that there is no actual contact between the liquid and the plate. The repulsion existing between the liquid and the plate is usually imputed to the existence of an atmos phere of vapour upon which, as upon a cushion, the spheroids are supposed to rest. There is no reason to conclude, however, because vapour is raised from a liquid, that therefore its gravity must be suspended, and the cause is rather to be sought for in the motion of the spheroid, or of its internal particles, whereby the motion to which gravity is due is partially counteracted.

n the white

of water le
mmediate
he crucill

in soluti

ey will

roll abs)

ombinati

Dut also

uses whi

es not

Spri

ition

Spheroidal State of the Water in Boilers.-There can be no doubt that the water of boilers is sometimes repelled from the metal in the same manner as would be done if it were in the spheroidal state, and explosions have, no doubt, frequently had their origin in this phenomenon. Land boilers, whether of the cylindrical or waggon form, frequently bend down in the bottom where the strongest heat of the furnace impinges, and lead rivets, inserted in them for purposes of safety, are sometimes melted out. The water appears to be repelled from the iron in those parts of the boiler bottom where the heat is greatest, and the iron becomes red hot, and is bagged or bent out by the pressure of the steam. In some boilers the bottom can at any time be made red hot by very heavy firing, and in most factory boilers the bottom will be more or less injured if the stoker urges the fire very much. If gauge cocks be inserted at different levels, in a small upright cylindrical boiler, so that one cock is near the top, another near the bottom, and the rest in intermediate positions, it will follow, that if sufficient water be introitself duced into the boiler to show at the lowest gauge cock, it will continue to show there so long as a moderate heat is maintained. So soon, however, as the fire is made to burn fiercely, so as to impart a strong heat to the bottom, the water will disappear from the bottom cock and show in the top cock, thus proving that the water has been repelled by the heat until it occupies the top part of the boiler instead of the bottom part.

ling

n to

COMMUNICATION OF HEAT.

Heat may be communicated from a hot body to a cold one in
three ways-by Radiation, by Conduction, and by Circulation.
The rapidity with which heat radiates varies, other things
being equal, as the square of the temperature of the hot body in
excess of the temperature of the cold one; so that a body if made
twice as hot will lose a degree of temperature in one-fourth of
the time;
if made three times as hot, it will lose a degree of
temperature in one-ninth of the time; and so on, in all other
proportions. This explains how it comes that a very small pro-
portion of surface in a boiler of which the furnace is maintained

at a high temperature is equivalent to a much larger proportion of surface when the temperature is somewhat lower. Radiant heat may be concentrated into a focus by a reflector, in the same manner as light, and, like light, it may likewise be made to undergo refraction and polarisation.

The conduction of heat through different substances varies very nearly in the same proportion as their conducting powers for electricity. Taking the conducting power of silver as 100, the following are the conducting powers of metals according to the best authorities:

CONDUCTING POWERS OF METALS.

The conducting power of marble is about the same as the conducting power of bismuth; and the conducting powers of porcelain and bricks are each about half that of marble. The conducting power of water is very low, and hence heat is transmitted downwards through water only very slowly. But upwards it is transmitted rapidly by virtue of the circulation which then takes place.

The efficiency of the heating surface of a boiler will depend very much upon the efficiency of the arrangements which are in force for maintaining or promoting a rapid circulation of the water. In like manner, the rapidity of the circulation which is maintained in the water used for refrigeration in surface con

and Fr

r proport er. Rafir ector, in th vise be mad

tances rare

eting pove

ilver &

www.dr

Condai

To Es

densers will mainly determine the weight of steam condensed in the hour by each square foot of refrigerating surface. Peclet found by a number of experiments that water, when used as the refrigerating fluid, was about ten times more effectual than air; and he further found that when water was used for refrigeration, each square foot of copper surface was able to condenso about 21 lbs. of steam in the hour. Mr. Joule, however, found that a square foot of copper surface might, by maintaining a rapid circulation of the cooling water, be made to condense 100 lbs. of steam in the hour-the cooling water being contained in a pipe concentric with that containing the steam, and, flowing in the opposite direction. With this rapidity of refrigeration, the cooling surface of a condenser need only be about one sixteenth of the heating surface of the boiler which supplies the engine with steam. In ordinary land boilers 10 square feet of heating surface will boil off a cubic foot, or 624 lbs. of water in the hour; and one square foot of heating surface will therefore boil off one-tenth of this, or 6.25 lbs. of water in the hour. To boil off 100 lbs. in the hour would at this rate require 16 square feet of heating surface. But the 100 lbs. of steam thus boiled off will, according to Mr. Joule, be condensed by one square foot of cooling surface; so that, if this authority be accepted, the surface of a well-constructed condenser need only be about one-sixteenth of the heating surface of the boiler, the steam of which it condenses. The importance of maintaining a rapid circulation in the water of boilers has not yet been sufficiently recognised. It is desirable that solid water and not steam should be in contact with the heating surface, else the metal plating will be liable to become overheated, and any given area of heating surface will be much less effective. The species of boiler invented by Mr. David Napier, called the haystack boiler, and in which the water is contained in vertical tubes, is about the best species of boiler for keeping up a rapid circulation of the water. But it necessary to apply large return pipes or a wide water space all round the exterior of the boiler, with a diaphragm to permit ascending and descending currents, in order that the water carried upward by the steam may be immediately returned.