It will

in the Bunsen flame so that it will be uniformly heated. then soften, the opening will slowly become smaller, and finally it will make a smooth, spherical end. Why?

9. A thin plate of glass, a lantern slide plate for example, will float when tossed on the surface of water. Why?

10. A piece of cloth, the corner of which dips in a dish of water, will become thoroughly wet in a short time. Explain.

11. Water will not run out of the upper end of a capillary tube, the lower end of which dips below the surface, but sap runs from the end of a maple branch that has been cut off. Explain.

12. Could a lump of sugar be used to take up a blot of ink?

13. A glass rod is sometimes placed against the edge of a cup in pouring a liquid into another vessel. Explain its action.

14. What effect does keeping the surface of soil well cultivated have upon the evaporation of water from the surface?

Problems

What

1. Water rises 25 mm. in a tube of a certain diameter. must be the diameter of a second tube in which it will rise 50 mm.? 2. If a liquid will rise 27 cm. in a tube .12 mm. in diameter, what must be the diameter of the tube for it to rise 81 cm.?

3. How high must a stone column be in order to make the pressure at the base equal to that produced by the osmotic pressure of a sugar solution, i.e., 465 lb. per square inch, if the stone weighs 160 lb. per cubic foot?

II. THE MECHANICS OF LIQUIDS

134. Transmission of Pressure by Liquids. Whenever pressure is brought to bear upon a solid, the molecules, being unable to move freely over one another, will transmit the pressure, undiminished, in one direction only. In the case of liquids, however, the free movement of the molecules over one another secures the transmission of pressure, without change, in all directions.

Demonstration.

-To a thin glass bottle fit a straight cork, of such a size as to go into the neck snugly. Fill the bottle with water.

FIG. 112

Insert the cork and bring pressure to bear upon it by a lever, as in Fig. 112. The shattering of the bottle shows that the pressure was transmitted in all directions.

The action of the molecules of a liquid in transmitting pressure may be

illustrated by filling a bottle with peas and pressing upon the top layers. Since each pea does not lie directly upon another, but in a depression left between those in the next lower layer, a vertical force acting upon any pea will be resolved into other forces in the direction of the points of contact between it and the peas which it touches.

135. Pascal's Law. - Since liquids are perfectly elastic (ie., have no elastic limit) under compression, and since their molecules move freely over one another, pressure brought to bear upon any part of a liquid is readily carried to any other part. After a study of the phenomena, Pascal, a French scientist, stated this law:

Pressure exerted upon any part of an inclosed liquid is transmitted undiminished in all directions. This pressure acts with equal force upon all equal surfaces, and at right angles to them.

136. The Hydraulic Press. An important application of the principle stated in Pascal's Law is made in the hydrostatic or hydraulic press. Figure 113 shows a simple form in section. Two pistons or plungers A and B pass through

water-tight collars into cylinders C and D. The piston A is moved by the lever F by applying the power at P. The body K to be compressed is placed between the platform G and a stationary

framework H above

it. The action is as

follows: Both cylinders and the connecting tube E being full of water, the piston A is forced down, the pressure on the water C closing the valve d, and forcing the valve v open. The water displaced by A is forced through E, and passes into D, where

it pushes B up, and compresses K. When the piston A is raised, the back pressure of B upon the water closes v; d opens, and water from some source of supply passes through it, keeping C full. The next stroke simply repeats the action. By Pascal's Law, if base of cylinder B is 20 times as large as base of cylinder A, the pressure on it will be 20 times as great. This may be stated as follows: The pressure applied by small piston: The pressure delivered to the large piston The area of the small piston: The area of the large piston,

the

=

d and D being the respective diameters of the two pistons. Since Pascal's Law applies to all liquids and since oil prevents

the rusting of the machine, oil is frequently used instead of

water.

The fact that liquids are almost incompressible and perfectly elastic is of very great importance in connection with this machine. A pressure of 100 lb. to the square inch compresses water only .00033 of its original volume, and on the removal of the pressure, that volume is immediately restored.

The hydraulic press is largely used in such work as the compression of bales of cotton and other bulky materials for transportation on shipboard, where the space taken by a package of freight is a determining factor in the expense of carrying it.

137. Pressure Due to Gravity. The principle stated in Pascal's Law holds whether the force employed is due to the pressure of weights placed on a piston resting upon the surface of the liquid, or to the pressure of an added layer of water. When a liquid is at a uniform temperature throughout, the entire mass is in a state of equilibrium, and there are no internal currents, as can be seen by mixing some heavy sawdust through the water. This means that the pressure exerted by the weight of a liquid at any point in the liquid is equal in all directions.

138. Relation of Pressure to Depth. Since every horizontal layer of a liquid has to support the weight of the liquid above, we may write as a result the following laws:

slowly into a vessel of water. Any changes in the level of the mercury will indicate changes in the pressure. Does the pressure increase with the depth? Hold the three tubes in one hand in such a way that the three openings are at the same level, and lower the tubes together into the water. Is the change in the mer

cury level in each tube the same? Why?

139. Vertical Downward Pressure. In the case of a liquid contained in a vessel with vertical sides, the weight of each layer is transmitted undiminished to the layers below. Hence each layer bears the weight of the liquid above it, and the total pressure on the bottom will be the weight of the liquid in the vessel.

der for the vase, the pointer will read the same as before when the water surface reaches the fixed index.

The above demonstration proves that the pressure on the bottom of a vessel containing a liquid is entirely independent of the shape of the vessel; with a given liquid it depends solely upon the depth of the liquid and the area of the base.