pathological work, for the detection of glucose in fluids, such as the urine, for when polarized light falls upon glucose it is rotated to the right and the degree of rotation will depend upon the amount of glucose present and thus it is an easy and accurate way of determining the quantity of glucose in a solution. Levulose, on the contrary, turns polarized light to the left. Also, there are a number of other substances that turn it to either the right or left in varying degrees and thus it is used in many varieties of scientific work, especially as a means of identifying substances which rotate it.

Phosphorescence and Fluorescence

Phosphorescence.-Certain substances, after being exposed to a strong light, will emit light in the dark. This property is termed phosphorescence and matter which possesses this power is said to be phosphorescent. Paints which are luminous in the dark contain some phosphorescent material, usually barium chlorid. A certain class of bacteria, called photogenic, produce this quality in matter, especially fish, in which they produce decay.

Fluorescence.-Certain substances, such as the mineral fluorite, fluorescin, platino-cyanid of barium, uranium glass, and solutions of quinin sulphate, have the power to change the length of ether waves, and thus of changing heat waves to light waves and vice versa to a marked degree, and of reflecting color waves different from those thrown upon them. Substances which do this are said to be fluorescent. name was given, because the mineral fluorite was the first substance in which the property of fluorescence was noted.

This

CHAPTER VII

ELECTRICITY

Theories Regarding the Nature of Electricity and Electrification -Conductors and Non-Conductors-Different Methods of Generating Electric Currents-Nature and Action of Chemical Cells and Batteries for Generating CurrentsElectrolysis-Electroplating.

Derivation of name.-The term electricity was derived from the Greek work electron=amber, because the first record of electrization was in connection with this substance. This record, which was made by the Greeks, some 500 years before Christ, was to the effect that if amber was rubbed with a dry cloth it would attract light objects to itself.

Experiment II. Object: To study electrification. Procedure: (a) Rub a glass rod for a few minutes with a dry woolen cloth or a piece of silk and then drag it through some small pieces of paper scattered on the table.

If the rod is rubbed long enough, under suitable conditions, see page 107, it will attract the picces of paper towards itself.

(b) Hang this rod by a piece of silk thread to a wire bar or stirrup, electrify another glass rod in the same manner as the first rod and hold it against the

free end of the suspended rod, the latter will swing away from it; i.e., it will be repelled.

(c) Rub a piece of sealing wax with silk and hold it near the suspended glass rod, the latter will swing toward it; i. e., it will be attracted.

(N. B. Be careful while doing this experiment not to let the part of the rod that is to be used come in contact with the hand or other object. The reason for this will be seen later in the section on conductors.) In the same section it will be also explained why this experiment is seldom successful in wet weather.

There are two things to be learned from this experiment: (1) Friction will produce electrification. (2) Substances of similar material electrified in the same way repel each other, but electrified objects of different material or of the same material differently electrified may attract each other. This is because there are two kinds of electrification-i. e., of electric charges and one of the laws of electrical action is: Like charges repel each other and unlike charges attract. The two kinds of charges have been arbitrarily named positive and negative, and a positively electrified body is often defined as: An electrified body which acts toward other electrified bodies as does a glass rod that has been rubbed with silk. Likewise a negatively electrified body is said to be: One which acts toward other electrified bodies as does a stick of sealing wax which has been rubbed with flannel.

But one kind of electricity.-Though the expressions negative electricity, positive electricity, static electricity, current electricity, etc., are constantly used, there is but one kind of electricity. For this reason, it has been suggested that the terms negative and positive electrification or negative and positive

electric charges be used instead of negative and positive electricity. By static electricity is meant, that which is not flowing, as, for example, when it is passing over a wire. In the latter case it is known as current electricity, or, more correctly, an electric current. The difference, however, lies, not in the electricity, but in the manner by which it is made manifest.

Some theories regarding the nature of electricity.Though a sufficient degree of knowledge, of the ways in which electric charges and currents can be produced and of the results of their action, has been obtained to allow of electricity being applied in a great many ways to the service of man, its nature is still a matter of conjecture. Some theories that have been advanced are as follows:

The two-fluid theory.-When the ease with which electricity moved over certain substances was first realized, the idea was conceived that it was a fluid. Or, rather, it was thought at first that there were two fluids, one giving rise to positive and the other to negative charges.

The one-fluid theory.-About the year 1750, Benjamin Franklin advanced what is called the onefluid theory, which holds that there is but one kind of fluid and that positive charges are the result of an excess of electricity and negative charges of a lesser amount. Because of this supposition, Franklin used the plus sign (+) as a symbol for positive charges, and the minus sign (-) as a symbol for negative charges. These signs are still used, though, according to the electron theory, which many scientists think is probably nearer the truth, a positively charged body has lost, instead of gained, electrons.

The electron theory.-The electron theory is based

on the assumption that the atoms of all elements are composed of electrons. It holds that each atom of an element consists of inconceivably small positive corpuscles that are surrounded by thousands of still smaller bodies called negative electrons. These negative electrons are, it is thought, constantly revolving with exceedingly great rapidity around the positive corpuscles and were it not for the attraction of the positive corpuscles, they would, it is thought, fall away.

Ordinarily, the majority of substances are in a state of clectrical equilibrium-i.e., neither positive nor negative-but, it is assumed, some of the more easily detached revolving electrons may be transferred from one atom to another without dissociating the atom; the only change being that the atoms of the substance by losing some of their negative electrons have their positive charge strengthened and the substance is then said to have an electro-positive charge. On the other hand, the substance that gained the negative electrons has its negative charge strengthened and is said to be electro-negative. This is what it is thought happens in Experiment II. The friction of the silk on the glass caused the atoms of the latter to part with some of their negative electrons to the silk and thus the glass became positively and the silk negatively charged. On the contrary, the silk gave some of its electrons to the sealing wax and thus it became positively and the wax negatively charged.

The reason why the silk and other material gain electrons in one case and lose them in another is thought to be due to some difference in the number and arrangement of the electrons by which certain substances are able to take electrons more readily from some materials than from others.