all the power or energy of cohesion appears destroyed between the particles; yet we know it remains in existence, and reasserts its power and presence in dragging the steam back into the form of water or ice. We find, therefore, affecting our very existence, some close mysterious relation between the powers of heat and cold and the power of the attraction of cohesion ; and also that we are, in fact, surrounded in our daily life and work by strange mysterious influences of nature, of deep interest to everyone personally, and well worthy of thoughtful consideration. Apply heat to lead, and witness its effect in the diminution of the attraction of cohesion ; mould this lead into a bar, allow the heat to disperse, and witness that cohesion is again established ; use force and break this bar in twain, the cohesion then appears to be destroyed at the points of fracture. Scrape the fractured ends and make them smooth and bright, and by great pressure together they may be caused to cohere, so that the upper piece will lift the lower ; and that this effect is not due to adhesion alone may be seen by observing, when the two pieces of lead are again separated by force, a limited number of small lead pin-points where cohesion occurred. Cohesion, which in metals is known as the property of tenacity, is not to be confounded with adhesion, which refers to the clinging together of bodies of different natures, as illustrated ELEMENTARY SCIENCE FOR PLUMBERS. 41 by the use of glue and paste in uniting surfaces. In the two pieces of lead mentioned the attraction of cohesion is seen to be very imperfect ; it cannot completely re-establish its power until sufficient pressure or force, applied by heat or otherwise, brings all the particles of lead into normal contact Relative tenacity in metals is determined by ascertaining the exact weights required to break wires formed of the metals in equal lengths and gauge. Lead possesses a low degree of tenacity, while iron possesses tenacity in a high degree. <Callout type="tip" title="Tip:">Understanding cohesion helps predict how materials will behave under stress.</Callout> Capillarity, or capillary attraction, is a natural force with which plumbers are obliged to reckon. Instances of this capillary action occur, and may be observed, if a glass tube of very small bore, having both ends open, is dipped into a vessel containing water. The water will rise in the tube to a higher level than the water in the vessel, and so remain in equilibrium, apparently in opposition to the law by which all liquids are stated to seek one level If the same tube be dipped in liquid mercury, we may observe that the mercury will not rise in the tube to the level of the mercury in the vessel, but will sink and remain in equilibrium at a lower level. The surface of the water in the tube will be concave, the outer edges curved upwards, clinging to the walls of the tube as if attempting to climb ; but, strangely enough, the surface of the mercury will be convex, the outer edges curved downwards, as if seeking to descend in the tube. The result is the same in air or in a vacuum. When laying sheet lead in gutters and on flat roofs it WATER MERCURT Fio. 8 —Capillary Action. 42 DOMESTIC SANITARY DRAINAGE AND PLUMBING. has been found, when two sheets of lead are allowed to overlap and lie one upon another closely, that unless precautions are taken to prevent it, water will rise to a considerable height between the two sheets, and cause serious injury, by soaking the underlying plaster and woodwork, which the lead is intended to protect. In water-traps, placed under troughs and sinks to protect houses from the entrance of drain air, it has been found that a piece of rag, a few strands of fibre or filament, caught and hanging over the outer weir of the trap, will have the dangerous effect of emptying the trap ; for, by the action of capillarity, the threads or filament allow the water to rise through them out of the hollow of the trap, and to escape by rapid evaporation, or by trickling slowly away down the outer waste pipe, leaving the house exposed to the danger of free entrance of foul air. From damp subsoils the water will rise to a considerable height through basement walls by the same cause, and the marks may be seen in all damp houses. This capillary action is explained by the theory of the existence of a surface tension, instanced by the experiment of placing carefully a fine steel needle on the surface of water; it will sometimes float, apparently supported by a thin membrane in tension, which bends under the weight of the needle. Experiment has led to the formulating of two laws relating thereto: — 1. At the bounding surface separating air from any liquid, or between two liquids which do not mix, there is a surface tension similar to that of a membrane, which is the same at every point and in every direction. 2. At the line of junction of the bounding surface of a gas and a liquid with a solid body, or of the bounding surface of two liquids with a solid body, the surface is ELEMENTARY s5!ENCE FOR PLUMBERS. 43 inclined to the surface of the solid body at a definite angle depending on the nature of the solid and the liquids. Of all common liquids water possesses the greatest degree of surface tension. When a liquid and a solid surface meet, the particles on both margin surfaces are not attracted equally on all sides, while the particles in the interior of the liquid and solid are subject to equal attraction on all sides, being quite surrounded by the same media. A surface tension thus arises in the surface, separating the liquid and the solid, caused by the interaction of the molecular forces at the free surfaces. When a liquid wets a solid the effects produced are caused by an attraction between the solid and the liquid. When a liquid does not wet a solid the effects are caused by a repulsion between them. For instance, if in- a capillary glass tube the surface of glass be greased so as not to be wet by the water in which it is plunged, the result will resemble those which occur when it is plunged in mercury. Mercury does not wet or cling to the surface of glass. When capillary tubes are plunged in liquid the liquid is raised or depressed, according to as it wets or does not wet the surface. The elevation or depression varies inversely as the diameter of the tube ; it also varies with the temperature and the nature of the liquid, but is independent of the thickness of the tube. In Steam Heating apparatus it is generally supposed that the water condensed from the steam flows back to the boiler along the invert of the pipe, whereas the fact is it clings by capillary attraction all round the perimeter of the pipe, and is strongly affected by the flowing current of the steam, sometimes causing the noise heard in steam-heating pipes. This is a practical point worth knowing. Hydrostatics, hydraulics, and aerometry are branches of science which ought to be studied by all plumbers. 44 DOMESTIC SANITARY DRAINAGE AND PLUMBING. Hydrostatics teaches concerning the equilibrium and pressure of liquid and gaseous fluids. Hydraulics teaches concerning the laws and phenomena of incompressible fluids in motion, especially of water. Aerometry teaches concerning the laws and phenomena of compressible or elastic fluids, especially of air. Many of the works undertaken by plumbers depend for proper construction and proportion upon calculations founded on the laws which regulate the pressure, weight, and motion of fluids — in such works, for instance, as water supply, storage, and distribution, sewen^e, drainage, heating, light- ing, ventilation, water and steam power. The universe is composed, so far as we know, of minute atoms of matter, which have been classified or separated into three great divisions or states — solid, liquid, and gaseous. The plumber has to do with each in turn, and is personally concerned in the laws which govern them. When minute particles of matter are joined in a condition that requires considerable force to separate or disturb them the matter is then solid, such as steel, lead, wood — steel being the most perfect solid of the three named. When minute particles of matter are joined in a condition so that the smallest force causes them to move upon, under, and among each other, and to change their positions with perfect facility, the matter is then liquid^ such as alcohol, water, oil, mercury — alcohol being the most perfect liquid of the four named, water being of the most concern to plumbers. When minute particles of matter are joined in a condition so that they expand without limit if freed from restraint, and contract without limit if pressure is applied; when heat powerfully tends to expand, and cold to contract them, the matter is then gaseous^ such as air, steam, oxygen — air being the principal gaseous fluid of those named. Solids in minute division, such as sand, sawdust, borings. ELEMENTARY SCIENCE FOR PLUMBERS. 45 appear to possess some of the mobile qualities of liquids, but each grain of the mass is yet a perfect solid, formed of many very minute particles or molecules. Liquids are called incompressible fluids. The effect of the greatest possible pressure upon them is practically inappreciable — only to be detected by minutely accurate scientific measurement. Gases are called compressible or elastic fluids, for their volume expands and contracts without limit. It is even understood that some marvellously attenuated gas or ether fills the vast spaces to the distant stars, capable of bearing or of causing the minute vibrations which produce the sensation of light, enabling us to see the light of these stars so many millions of millions of miles away, that no man can ever count the measure or conceive the distance. Liquids may be changed into solids and into gases. Water may be frozen into solid ice or boiled into steam. Lead may be melted by heat or evaporated to vapour. In hydrostatics the fundamental principle which concerns us as hydraulic engineers and plumbers is the equal transmission of pressure in liquids. The results of this action are met in practice every day. The discovery of the principle is due to Pascal, who was bom at Auvergne, in France, in 1623. If a closed vessel, any size or form, be filled with water, and the water be subjected to any pressure at any given point, that identical amount of pressure will be transmitted, equally, undivided, and at right angles, to every point of the liquid and of the internal surface of the vessel. For instance, if a closed hot-water cylinder, say 2 feet diameter x 3 ft. 6 in. long, containing about 4,000 square inches of internal superficial area, be filled with water, and a small pipe of one square inch area be connected to it at any point, and a pressure of 21 lbs. on that square inch be applied. 46 DOMESTIC SANITARY DRAINAGE AND PLUMBING. either by pumping or by a vertical column or head of water 50 feet high, a pressure of 21 lbs. will be transmitted, at right angles, to each and every separate square inch of the 4,000 square inches of the internal surface of the cylinder, pressing up against the top, down against the bottom, and radiating as from the centre against the sides ; so that, as there are 4,000 square inches of inner surface, the total pressure exerted against them will be 84,000 lbs. or nearly 38 tons. If we were ignorant of this law we might naturally suppose that, if a pressure of 21 lbs. was applied to the pipe, that pressure of 21 lbs. would be divided equally against each square inch of internal surface, and that the total pressure on the cylinder would be 21 lbs., instead of 21 lbs. x 4,000 = 84,000 lbs. ! We may see the difference here between the action of liquids and solids, for conceive the column of one square Pressure 21 lbs. aq, in. ^^^^ ^^ water ss frozen solid, or imagine «M that a solid rod of wood, one inch square, was pressed in, instead of the water, of course allowing an equal bulk of the water in the vessel to escape, and that either the ice column or the wood column was pressed by a force of 21 lbs. ; giving the same pressure as the liquid column gave, the only effect produced is that these solid columns will transmit 21 lbs. of pressure to the one square inch of surface on which they rest and press, and they do not influence or transmit pressure to any other part of the cylinder. In this illustration we omit altogether, for convenience and clearness, the differences of pressure on the top and bottom and sides due to the difference of the — WATER =^ :CYLrNDER^ Square Incheg-^ Equals -. 84000 lbs. Fio. 4.— Water Cylin- der under pressure. ELEMENTARY SCIENCE FOR PLUMBERS 47 height of the column of water acting on each different level of the cylinder. If this iron cylinder had been made only just strong enough to contain the water it holds it should burst long before the pressure of 38 tons extra was applied. This 50-feet head is not an uncommon pressure for plumbers to find necessary to apply to hot-water cylinders and boilers. Pascal discovered the fact that, by filling a barrel with water and inserting a high tube of very small diameter into the bung-hole of the barrel, a very small additional quantity of water poured down the tube bursts the barrel. The sides of the barrel have to bear, on each square inch, the same pressure as though they had been surmounted by a column of water of the same diameter as the whole internal superficial area of the barrel itself. The hydrostatic paradox that a quantity of water, however small, may be made to support a weight, however large, is due to this principle discovered by Pascal in his experiment. The square inch, as the standard unit area in England, is chosen for its convenience only ; as there are 144 square inches in a square foot, we find the total pressure of a column of water on a square foot by multiplying the pressure on one square inch by 144. If the bottom of a cistern 3 feetx 12 feet has to support 7 feet depth of water the pressure on each square inch will be 3 lbs. ; how many pounds pressure has the whole bottom to bear, and how many pounds per square foot ? 3 ft. X 12 ft. = 36 sq. ft. x 144 = 5,184 sq. in. x 3 lbs. = 15,552 lbs. pressure on the whole surface of bottom. 3 lbs. X 144 sq. in. = 432 lbs. pressure per square foot. If the bottom of a hot- water cylinder, 24 inches diameter, has to bear a pressure of 18 lbs. per square inch, due to 42 feet column of water, how much has the whole bottom to bear, and how much per square foot ? 24 X 24 = 576 x 7854 = 452-39 sq. in. x 18 lbs. = 8,143 lbs. pressure on the whole bottom. 18 lbs. X 144 sq. in. = 2,592 lbs. pressure per square foot. Another important hydrostatic law for plumbers to bear in mind is that the pressure on each square inch of surface at different depths in any liquid, is equal to the weight of a column of the liquid, whose base is the square inch ; that pressure is always transmitted equally in every direction round every point in the liquid at the given depth. For instance, let us take a 20-gallon vessel full of water, 3 ft. 6 in. deep, and immerse in it an open-ended tube of one square inch area, and 8 ft. 6 in. long, standing on and in water-tight contact with the bottom. The pressure of water on the particular square inch of cistern at the bottom of the tube will be IJ lbs., or the pressure due to the 3 ft. 6 in. column of water above it. Remove the tube and the pressure will remain precisely the same on that particular square inch, although now the whole body of water in the cistern is in direct contact with and above it. Mark the tube across into three equal divisions, and the pressure at the bottom, a, will be IJ lbs. per square inch in every direction inside and outside of the tube, due to the column of water above that point. At the first mark, J, it will be 1 lb. in every direction inside and outside of the tube, due to the column of water above the point 6; a gauge screwed into the side of the cistern at the same level will indicate 1 lb. pressure. At the second mark, c, the pressure will be reduced to J lb. per square inch in every direction inside and outside of the tube, due to the reduced column of water above the point c, and of course a gauge screwed into the side of the cistern at the same level will also indicate ^ lb. pressure. At the third mark, rf, the pressure will be 0, due to there being no column of water above. There is, of course, the atmospheric pressure of about 15 lbs. per square inch above all these water pressures and acting along with each, but in practice it is <Callout type="warning" title="Warning:">Ignoring hydrostatic principles can lead to structural failures in plumbing systems.</Callout> <Callout type="important" title="Important:">Understanding cohesion and capillarity helps prevent leaks and ensure proper water flow in plumbing systems.</Callout>
Key Takeaways
- Cohesion is the force that holds particles together, affecting how materials behave under stress.
- Capillarity can cause water to rise in narrow spaces, leading to leaks if not accounted for.
- Hydrostatic principles are crucial for understanding pressure and flow in plumbing systems.
Practical Tips
- Understand the principles of cohesion and capillarity to prevent leaks and ensure proper water flow in your plumbing system.
- Regularly inspect joints and seals to ensure they remain watertight, especially where materials overlap or come into contact with each other.
- Use appropriate pressure gauges to monitor water pressure in your plumbing system to avoid overloading pipes.
Warnings & Risks
Warning:
Failing to account for capillary action can lead to unexpected water leaks and damage to surrounding materials.
Incorrectly applied hydrostatic principles can result in structural failures, leading to costly repairs or even safety hazards. Ignoring the effects of surface tension can lead to improper sealing and leaks in plumbing systems.
Modern Application
While the specific terminology and experimental methods described may be outdated, the fundamental principles of cohesion, capillarity, and hydrostatics remain crucial for modern plumbers. Understanding these concepts helps prevent leaks, ensure proper water flow, and maintain structural integrity in plumbing systems.
Frequently Asked Questions
Q: What is capillary action, and why is it important in plumbing?
Capillary action refers to the ability of a liquid to flow in narrow spaces without the assistance of, or even in opposition to, external forces like gravity. In plumbing, understanding capillary action helps prevent water from rising between overlapping surfaces, which can lead to leaks and damage to surrounding materials.
Q: How does cohesion affect the behavior of metals in plumbing systems?
Cohesion is the force that holds particles together, affecting how materials behave under stress. In plumbing, understanding cohesion helps predict how metals like lead will behave when subjected to heat or mechanical stress, ensuring proper material selection and installation.
Q: What are some practical applications of hydrostatic principles in modern plumbing?
Hydrostatic principles are used to calculate the pressure exerted by water columns, which is essential for designing and maintaining hot-water cylinders, boilers, and other pressurized systems. Understanding these principles helps ensure proper water flow and prevents structural failures.