composed of red, flattened globules, floating in a colorless liquid called serum. In man the diameter of one of these globules is less than 300 inch. The microscope has revealed to us the existence of insects smaller than these particles of blood. If these insects devour still smaller ones in whose veins blood flows, how infinitesimal must be the magnitude of the component globules!

Experiment fails to determine whether there is a limit to the divisibility of matter. Many facts in chemistry, such as the invariability in the relative weights of the elements which combine with each other, would lead us to believe that such a limit does exist. It is on this account that bodies are conceived to be composed of extremely minute and indivisible parts called atoms.

The remaining properties of matter will now be considered, each under its own heading.

What is porosity?

It is the quality by virtue of which interstices or pores exist between the molecules of a body.

What two kinds of pores are there?

Physical, whose interstices are so small that the surrounding molecules remain within the sphere of each other's attracting or repelling forces; and sensible or actual cavities, across which molecular forces cannot act. Contraction and expansion, resulting from variations of temperature, are due to the existence of physical pores; while the sensible pores are the seat of exhalation and absorption.

Sensible pores are apparent, viz., sponges, pumice-stone, etc. Physical pores are not apparent; but since all matter may be diminished in volume, we conclude that all matter possesses physical pores.

What is compressibility?

It is the property by virtue of which the volume of a body may be diminished by pressure. This property is a consequence and proof of porosity.

Bodies differ with respect to compressibility. The most compressible are gases: they may be compressed in some cases to a few hundredths of the space they occupy at ordinary temperatures. There is generally a limit beyond which pressure reduces them to liquids.

Solids possess less compressibility: cloth, paper, cork, woods, are among the most compressible. Metals are also compressible to some extent, as is shown when a coin receives the impression from the die. There is a limit beyond which, when pressure is increased, bodies are fractured or reduced to powder.

Are liquids compressible?

Experiment has shown them to be very slightly compressible, the degree, however, being so small that it is not easily detected.

What is elasticity?

It is the property by virtue of which bodies resume their original form or volume when the force which altered that form or volume ceases to act.

How may elasticity in solid bodies be developed?

By pressure, traction or pulling, flexion or bending, and by torsion or twisting.

Gases and liquids are perfectly elastic; in other words, after undergoing a change in volume they regain exactly their original volume when the pressure is removed.

There is a limit to the elasticity of solids, beyond which they either break or are incapable of regaining their original form. This is called the limit of elasticity. Within this limit all substances are perfectly elastic.

In gases and liquids, on the contrary, no such limits can be reached. They always regain their original volume when the original pressure is restored.

What is mobility?

It is the property by virtue of which the position of a body in space may be changed. Motion and rest may be relative or absolute: by relative motion or rest of a body we mean its change or permanence of position with respect to surrounding bodies; by its absolute motion or rest we mean the change or permanence of its position with respect to fixed points in space. Thus a passenger in a railway train may be in a state of relative rest to the train in which he travels; but he is in a state of relative motion with respect to the objects, such as trees, houses, etc., past which the train rushes. These houses, again, enjoy merely a state of relative rest; for the earth itself which bears them is in a state of incessant relative motion with respect to the celestial bodies. In short, absolute motion and rest are unknown to us.

What is inertia ?

It is a purely negative though universal property of matter. It is the property that matter cannot of itself change its own state of motion or of rest. If a body is at rest, it remains so until some force acts upon it. If it is in motion, this motion can only be changed by the application of some force. A body when suspended in mid-air does not fall to the earth by virtue of any inherent property, but because it is acted upon by the force of gravity. A billiard-ball gently pushed does not move more and more slowly and finally stop because it has any preference for a state of rest, but because its motion is impeded by the friction on the cloth on which it rolls and by the resistance of the air. If all impeding causes were withdrawn, a body once in motion would continue to move forever in a straight line with unchanging velocity.

What is specific gravity?

It is the ratio of the weight of a body to the weight of an equal volume of some other body taken as a unit.

For solids and liquids, the unit generally taken is water at 60° F. For gases, hydrogen is now taken. Formerly air was employed. Air being 14.43 times as heavy as hydrogen, it is easy to transform from one scale to another.

What do the following symbols represent?

Ꭰ

D= density or specific gravity.

V

=

weight of equal volume of unit. W= weight of substance.

From the definition of specific gravity we derive the following formulæ :

:

W

V

(2) V=

W
D

(3) W = DV.

What instruments are used for determining the specific gravity of solids and liquids?

The specific gravity of solids and liquids is determined by—

1st. Hydrostatic balance for solids;

}

2d. Specific gravity flask (called pyknometer) for liquids.
3d. Hydrometer

What is the law of Archimedes? State different methods of determining specific gravity of solids.

In determining the specific gravity of solids the law of Archimedes is applied, i. e., that when a body is immersed in a liquid its weight is lessened by the amount which the displaced liquid weighs.

By weighing a substance first in air and then in water, the loss in weight will equal the weight of the displaced water. As this water represents exactly the volume of the substance placed in it, we thus have a ready means of ascertaining the specific gravity of solids. Example:

Where substances are lighter than water, a weight is generally used as a sinker. The first weighing is of substance alone, next of substance and sinker together in air, then weight of substance and sinker immersed in water. The difference is the weight of the amount of water displaced

by both. The substance is then detached, and the weight of sinker is found in air and in water. By making use of these data the specific

gravity of substance can be found.

Example:

In case the substance is soluble in water, it is only necessary to employ some liquid having no solvent action and of known specific gravity. Å simple calculation gives the weight of a corresponding volume of water, from which can be calculated the specific gravity of the substance.

How is the specific gravity of liquids determined?

(a) By pyknometer;

By hydrometer.

To determine the specific gravity generally, weigh equal bulks of liquid and water at standard temperature and divide the weight of liquid by the weight of water. The quotient will be greater or less than unity as the liquid experimented with is heavier or lighter than water.

The glass vessel used to measure the volume of a liquid is called a pyknometer. It is of thin glass, having a ground stopper with a flat top, through which is bored a small vertical opening about 1 mm. in diameter. The object of this construction is to facilitate exact measurement. When the vessel is filled with liquid, the stopper is placed in position and the excess of liquid which is forced through the tubular opening is removed by absorption with paper or cloth.

Describe the use of the hydrometer.

If a solid body be immersed in a liquid specifically heavier than itself, it sinks down until it displaces a quantity of liquid equal to its own weight, at which point it floats. A floating body of known specific gravity may thus be used to determine the gravity of solutions by ob

serving the amount that is submerged when placed in the solution. The hydrometer in general use consists of a floating vessel of thin metal or glass, having a weight beneath to maintain it in an upright position and a stem above bearing a divided scale. The liquid to be tested is placed in a small, narrow jar and the instrument floated in it. It is obvious that the denser the liquid the higher will the hydrometer float, because a smaller displacement of liquid will counterbalance its weight. The mark on the stem of the hydrometer at the fluid level is then read off. Instruments of this kind have very extensive use in the arts. They bear different names, according to the kind of liquor for which they are intended. The graduation is often arbitrary, but the principle is the same in all.

Various hydrometers are made: Baumé, Cartier, Sykes, etc.

Other hydrometers are :—

Alcoholometer,

Acidometer,

These are used to show whether the different liquids under examination are more or less concentrated, and not to determine exact specific gravity.

What are the two methods in general use for determining specific gravity of gases?

(1) Dumas's method;

(2) Gay Lussac's method.

1. Dumas: Finds weight of given volume of gas at known pressure and temperature, and makes comparison with the weight of air or hydrogen direct.

2. Gay Lussac: Finds volume occupied by given weight of substance when converted into vapor at a known temperature and pressure, and makes comparison with air or hydrogen under like circumstances.

What are the specific gravities of some of the gases, taking hydrogen as 1?

Specific gravities of a few substances, taking hydrogen as unit :—

It is readily seen that in the case of most elements in a gaseous state their density is the same as one-half the molecular weight. This is due to the law formulated by Avogadro in 1811, and applies to gases only: Equal volumes of gases under equal temperature and pressure contain equal number of molecules. Hence the density of gases will be directly as their molecular weights; and as their weights are referred to hydrogen as unity (i. e., the atom H, and not molecule, which molecule con