Gas made from anthracite has a higher proportion of these minor constituents about 20 per cent instead of 10.

Consisting largely of carbon monoxide, water gas is extremely poisonous much more so than coal gas. The smallest leakage of water gas from pipes or cocks is therefore a serious matter. Water gas has from 40 to 60 per cent of the

fuel value of the coal or coke from which it is made. It yields about 350 B. T. U. per cubic foot, but the fuel value is frequently increased by enrichment. (See below.)

Water gas burns with a blue flame, which has very low illuminating power. In order to make it into an illuminating gas for use with old-style burners, it is common to mix with it a gas made by heating petroleum oils to a high temperature. (See "Oil gas," p. 77.) This process is called enriching the gas. Enriched water gas may have a fuel value as high as 700 B. T. U. per cubic foot. In many cities a mixture of coal gas and water gas is used.

Gasoline gas ("air" gas) is made chiefly in private plants for the supply of rural homes or of institutions situated at a distance from a city supply. It is a mixture of gasoline vapor and air. Gasoline consists of the more volatile hydrocarbons of petroleum. The gas is made from it by exposing the liquid on folds of canvas to a current of air. The gasoline evaporates, the vapors mixing with the air, the supply of which is so regulated that the hydrocarbons will not become liquid again in the pipes. The gasoline gas burns with a luminous (and therefore sooty) flame, but a blue flame is obtained by admitting additional air at the burner, which must be of the Bunsen type.

Compressed and Liquefied Gas. When the gas mixtures used for fuel purposes, such as coal gas or oil gas, are subjected to great pressure, some of the constituents (hydrocarbons containing a large proportion of carbon) liquefy. This liquefied portion of the gas may be separated from the portion which remains in the gaseous condition, and the latter

may be stored in cylinders in its compressed state and shipped to houses or institutions which are not supplied with gas through pipes.

A German chemist, named Hermann Blau, has patented a process in which some of the constituents of oil gas are liquefied and removed, then the remaining gases are compressed to a liquid condition. Blau gas is used more for lighting than for cooking purposes.

Surface Combustion

Gas burners have recently been designed which render it possible to mix the gas with exactly sufficient air for its complete combustion and to cause the mixture to burn flamelessly in a pile of granular incombustible material, such as pieces of silica, SiO2, or alumina, Al2O3. Combustion of gas so conducted is termed surface combustion. Surface combustion is very economical because (1) it avoids the heating of air not used in the combustion, and (2) heat radiated from the incandescent pile of refractory material is more penetrative than heat from a gas flame. It is claimed that surface combustion gas stoves, doing the same work as stoves of the ordinary Bunsen type, will use 35 to 45 per cent less gas than the Bunsens.

Further information on liquid and gaseous fuels, including numerous references to the literature of the subject, will be found in Chapter VI of Benson's "Industrial Chemistry" (New York, 1913). For a practical method of comparing the values of the common household fuels, the reader is referred to Lynde's "Physics of the Household" (New York, 1914), pages 152-153.

CHAPTER XIII

LIGHT AND ILLUMINANTS

Experiment 36.*

Materials:

Platinum wire, 2 or 3 inches.

Iron wire, same gauge and length.

Magnesium ribbon, inch.

Quicklime, small lump, say, 1⁄2-inch cube.

Crucible tongs or forceps.

Blowpipe.

(a) With the tongs hold in a Bunsen flame side by side a piece of iron wire and a piece of platinum wire. Note the gradual changes of color as the temperature of the wires rises.

(b) Hold a piece of magnesium wire in the tongs, and ignite it with the Bunsen flame. Note the white light emitted as the magnesium burns. (This is the light used in making flashlight photographs.)

The ash, which remains after the combustion of the magnesium and which retains something of the form of the original ribbon, is magnesium oxide (sometimes called magnesia). Bring this ash again into the flame and note the change of color it undergoes as it is heated. Blow into the flame with the blowpipe, directing a gentle current of the flowing gases towards the magnesia. Does it become brighter? Is its color altered? (c) Hold a piece of lime in the flame a little above the inner cone. Note its color. Blow the flame upon it with the blowpipe. Does the color change? If an oxyhydrogen blowpipe is available, direct its flame against the lime. Is more light now emitted? What is its color?

When substances are heated to a sufficiently high temperature, they give out light. They are then said to be in an incandescent condition. Like heat, light is a form of energy, and incandescence is due to a transformation of some of the heat into light.

Gases can be heated to incandescence, and the colors of the light they then give out are characteristic of the various substances. Each gaseous compound yields its own color, unless when heated to incandescence it is decomposed, in which event it gives the colors of its decomposition products.

EIMER EAMENT

FIG. 35.1

A form of tube in which gases are electrically heated to incandescence.

To obtain the characteristic colors of gases, a glass tube provided with metal electrodes (see Fig. 35) is first filled with the gas. Then most of the gas is pumped out and the tube is sealed by melting the glass. On passing the electric discharge (spark) from an induction coil through the gas from

EIMER & AMEND

FIG. 36. Bunsen and Kirchhoff's Spectroscope. An instrument
used in the analysis of light.

electrode to electrode the characteristic light appears. The color of the light may be analyzed by means of an instrument called a spectroscope.

It was by the color of its light that the element helium was

discovered in the sun's atmosphere many years before it was found upon our planet; and it was from observations of light color that astronomers inferred that the tail of Halley's comet (visible in 1910) contained cyanogen gas (C2N2). Incandescent sodium vapor emits yellow light; potassium, violet; calcium, red; barium and copper, green, etc.; and most compounds of these metals when heated in a Bunsen flame impart to it these characteristic colors.

Experiment 37.*

Materials:

Small portions of the chlorides of sodium, potassium, calcium, strontium, and barium.

Platinum wires sealed in glass handles and carefully cleaned by alternate heating in Bunsen flame and dipping in pure concentrated hydrochloric acid until they do not color the flame.

Heat a clean platinum wire and dip it while hot into one of the salts, then bring again into the Bunsen flame and note the color imparted to it. Repeat with the other salts, using a thoroughly clean wire in each instance.

In practical illumination, however, the incandescence of solids is of much more importance than that of gases. The color of the light emitted by an incandescent solid depends not only on what substance the solid is, but also on the temperature to which it is heated. As the temperature of any solid is gradually raised, red light is first emitted, then the other colors of the rainbow are successively combined with the red until finally white light (which is composed of all the rainbow colors) is given out. The terms commonly used to distinguish high temperatures are said to correspond roughly to the following points on the Centigrade and Fahrenheit scales: