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(3) vacuum-plenum system. Ventilation by any of these methods is usually effected with the aid of fans or pumps placed in chimneys or ventilating shafts. In the vacuum system, air is withdrawn from the building as the result of the vacuum created in the shafts and the pipes leading to the rooms, by means of the pumps, etc., and air from outside is, consequently, forced, by atmospheric pressure, into the pipes and ventilators provided for the purpose and from thence into the rooms. In the plenum system it is in the shafts in communication with the outer air that a vacuum is created and the air, which is forced into these by the weight of the atmosphere, is propelled, by fans or other apparatus, into the rooms. The entrance of this additional air forces some of that already in the rooms to leave through the apertures provided for the purpose. The vacuum-plenum system, as the name implies, is a combination of the other two methods.

One means of discovering if artificial ventilation is proceeding as it should is to hold a handkerchief in front of the ventilators; if everything is in order, the handkerchief will be sucked toward a ventilator through which air should leave the room and it will be blown into the room by the incoming air at a ventilator intended for such purpose.

CHAPTER VI

THE ETHER, HEAT, AND LIGHT

The Ether-Absorption, Radiation, Reflection, Refraction, and Polarization of Heat and Light-Color of Light and of Objects-Finsen Light-Phosphorescence and Fluorescence.

Radiation

Nature of radiation.-The process of radiation. was so named because heat and light transmitted in this way pass from their source in straight lines or rcdii. One proof that this is true of heat is demonstrated by holding a screen between the individual and the source of heat, since the direct heat then ccases to be felt. That any heat is then perceived is due to convection currents in the air. Likewise, if an object through which light rays cannot pass is held between the eyes and the light, the latter can no longer be seen and that there is any lighting of a y screened from the direct rays of a light, is due, as will be seen later, to reflection.

The Ether

In order to understand radiation it is necessary to know of the existence of the ether. For various reasons, among others, the transmission of heat and

light through the millions of miles between the sun and the upper strata of air and the nature of radiation, scientists concluded that there must be something besides air permeating all space, not only that above the air but also that existing in all matter, and to this invisible, intangible something they gave the name of ether.

Nature of ether waves.-All matter, but more especially that in which molecular motion is pronounced enough to cause sensible heat, will, by means of its molecular motion, start waves in the ether that, though not precisely similar, are likened to the series of circular waves that arise when a stone is cast upon the quiet surface of a pond. These waves spread out in all directions and, the hotter the body starting the vibrations, the more rapidly will the waves be formed and the greater will be the number of short waves (i. e., those in which there is only a short distance between successive waves).

Length of ether waves.-Ether waves started by very hot bodies, such as the sun, vary greatly in length. Some, for example those made use of in wireless telegraphy, are miles long and others are only about 250,000 part of an inch. As the longer waves produce electric effects, they are termed electric waves. Waves which measure only a few thousandth part of an inch are capable of increasing molecular motion in matter and consequently of raising their temperature; they, therefore, are called heat waves. Waves that are as short as part of an inch affect the endings of the optic nerve in the eye and are therefore called light waves. Waves that are less than part of an inch do not affect the sight, but they do affect the chemicals upon a photographic

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plate and they cause many chemical changes in nature, therefore, they are called chemical or actinic waves or rays.

Origin of color.-Light waves have different lengths and difference in length results in different colors.

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Only the molecules of very hot bodies vibrate with sufficient rapidity to produce waves of a length to affect the optic nerve, i.e., light rays. The temperature of a body must be raised to 525° C. before waves short enough to be visible as light appear. When this temperature is reached, a red glow is seen and as the temperature is increased, though long waves are being generated, shorter and shorter ones (those that give rise to the different colors) are produced. until, between a temperature of 860° C. and 1200° C., waves of all colors are being generated and consequently a so-called white heat or light is reached for, when all colors of the spectrum2 are present in a combined state, a white light is produced. This is the case with sunlight. The sun, being the hottest of all bodies, produces waves in the ether of all possible lengths; thus, in the sunlight, as already stated, there are waves that will give rise to electric effects, heat,

A millimeter (mm.) equals 0.3937 of an inch.

2 The colors seen when light waves are refracted by passing through a prism, etc.

light of all colors, and chemical reactions. That the sunlight contains all the colors of the spectrum is seen when it undergoes refraction, as in the rainbow and in passing through prisms, see page 100.

In one sense, all ether waves are heat waves, since waves of all lengths will increase molecular motion in bodies and thus raise their temperature.

Retention of heat.-Light waves can penetrate many substances easily that the longer heat waves pass through only with difficulty and this is of great value to man. For instance, light waves pass through the glass of hot-beds and conservatories and through snow, paper, etc., covering plants, they are partly absorbed by the earth and plants the molecular motion of which is thereby increased and heat produced which, as it does not readily pass through the plant covers, remains and furthers the growth of the plants. Also, the light waves pass readily through damp air, smoke, and the like, but the heat waves only do so slowly and with difficulty; for this reason, clouds and smoke at night serve to maintain the warmth that the earth acquired during the daytime. Thus a clear night is likely to be cooler than a cloudy one. The fruit growers of California burn smudge fires in their orchards when there is danger of a frost, because heat and also a heavy lowlying smoke that prevents the escape of heat from the ground results from these fires.

How the air is heated.-Heat being the result of molecular motion, it cannot, it is believed, exist apart from matter and consequently ether waves are not themselves hot, though, by increasing molecular motion in all matter which they strike, they are the cause of heat.

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