on this basis 1000 watt-hours are equivalent to 3410 calories. The expense of electric heating must in every case be very great, unless electricity can be supplied at an exceedingly low price.

Heating by Means of Gas.-In many towns throughout the natural gas region gas is used for heating as well as for illuminating purposes. In these localities gas is the cheapest mode of heating. It is employed in both the direct and indirect systems of heating. When suitable arrangements are made for carrying off the products of combustion, and there is a proper supply of fresh air for purposes of ventilation, this is a very satisfactory method of heating. In cold weather, where the daily fluctuations. in the temperature are not very great, the gas heater can be lighted and adjusted, and requires practically no attention for weeks or even months. It is therefore a great saving in time and annoyance, and there is no coal to shovel nor ashes to remove.

Heating by Means of Petroleum.-Within recent years petroleum has been brought into common use for heating purposes. The advantages of oil heaters are that they are portable and may be carried from one room to another, and the amount of heat can be readily controlled. These petroleum stoves are objectionable, however, from the fact that the combustion of the petroleum utilizes large quantities of the oxygen of the air of the room, giving off corresponding amounts of carbon dioxid. Babuke' has found that in a room of 12 cubic meters capacity, during the first hour the temperature of the room was raised only 4 degrees C., and rose but slowly afterward. The proportion of carbon dioxid in the air of the room exceeded 1 part per 1000, and reached in the vicinity of the floor 3-10 parts per 1000, and in the upper part of the room 6-12 parts per 1000, amounts which would be detrimental to health when inhaled constantly. The amount of petroleum consumed was about a liter in eight hours.

1 Zeitschrift f. Hygiene, Bd. xxxii., S. 33.

CHAPTER IV.

WATER AND WATER-SUPPLY.

Physical Properties of Water.-Pure water is a colorless, odorless, and tasteless liquid, of neutral reaction, and is taken as the type of all liquids, as air is the type of all gases.

Chemical Composition.-Pure water consists of 2 parts by weight of hydrogen and 16 parts by weight of oxygen, having a molecular weight of 18. Two volumes of hydrogen combine with I volume of oxygen to form 2 volumes of water gas, having a density of 9. The chemical formula for water is H2O. The percentage composition of water is hydrogen, 11.11; oxygen, 88.89.

Chemically pure water does not exist in nature, but is made in the laboratory by mixing the required amounts of hydrogen and oxygen gas and then passing an electric current through the mixture. The gases unite and form water. From the hygienic standpoint, water as found in nature is either pure or impure. Hygienically pure water is one which does not contain any foreign matter which is injurious to health. Impure water is one that is unfit for domestic use. Water as it exists in nature contains a great variety of substances derived from the air through which it has fallen as rain or snow, and from the soil over and through which it has passed. The nature and quantity of the mineral salts dissolved out of the soil by water are dependent upon the chemical composition of the soil. The nature and amount of organic matter contained in the water are dependent largely upon the nature of the soil-covering over which the water has passed.

Water as it occurs in nature may be divided into rainwater, spring-, river-, lake-, and sea-water. Each of these natural waters varies somewhat according to the locality from which it is derived, though in a general way all of these natural waters possess characteristics which are common to all the waters of that particular class.

Rain-water.-If rain-water were collected in a chemically clean vessel at the moment when it was condensed, it would presumably be chemically pure, but in falling through the atmosphere it takes up some of the impurities in the air. Rain-water is one of the purest of the natural waters, but it varies in purity with the nature of the atmosphere through which it has fallen. It is always purer at the end than at the beginning of a shower. It contains dissolved gases derived from the atmosphere— on an average 25 cubic centimeters per liter, of which about 64 per cent. is nitrogen, 34 per cent. oxygen, and 2 per cent. carbon dioxid. The relatively large amount of carbon dioxid, in comparison with the proportion contained in atmospheric air, is due to its large absorption coefficient. Ammonia is also commonly present. The average amount of solid matter in rain-water is 39.5 parts per 1,000,000. Sodium chlorid is the most abundant salt, while nitric acid and nitrates, sulphuric acid and sulphates, and a little organic matter are also commonly present.

Spring-water. That portion of the rain-water which penetrates the ground exercises a powerful chemical action on the substances present in the soil and underlying rocks. This action consists of solution, hydration, oxidation, etc. Rain-water is an oxidizing agent on account of the considerable proportion of dissolved oxygen that it contains.

Springs may be divided into two classes: Common springs, yielding fresh, potable water; and mineral springs, yielding mineral, thermal, or medicinal waters, in which the dissolved mineral matters render them unfit

for ordinary domestic use, though of great value for therapeutic purposes.

Ordinary spring-water usually contains the gases of the atmosphere in solution. It also contains various mineral salts in solution, such as calcium carbonate and sulphate, magnesium carbonate and chlorid, sodium chlorid, alkaline sulphates and nitrates, and silicates. The amount of organic matter is usually small, and the content in free and albuminoid ammonia is low. The temperature of spring-water is usually lower than that of the surrounding air.

Well-water, if derived from a deep well, is similar in character to spring-water; but if derived from a shallow well it is contaminated with surface washings. Springwater is usually soft, while some well-waters are moderately hard because of the presence of calcium and magnesium salts in the rocks of the locality. Spring- and well-waters are usually not rich in bacteria unless specially polluted.

River-water.-The course of a great river may be divided into three portions-the mountain track, the valley track, and the plain track; and the composition of the water varies considerably in these three portions of its course. In the first part it is comparatively pure and partakes of the nature of spring-water; in the second and third parts it is usually more or less polluted, depending upon the density of the population along its course. The composition of river-water is complex, as in the case of spring-water, as the water of rivers is largely derived from springs. The proportion of organic matter and of free and albuminoid ammonia is usually higher than in spring-water; and if polluted with sewage the proportion of chlorin is also considerably higher. The character of the water varies greatly with the amount of rainfall and with the nature of the soil-covering of the valley and plain tracks. River-water is usually rich in bacteria, the number and variety of species varying

greatly with the season of the year and the amount of sewage pollution.

Lake-water.-Lake-water is of variable composition, the water of salt lakes being loaded with mineral constituents, while that of fresh lakes is usually of great purity. Fresh-water lakes act as settling basins for the inflowing water. The sudden diminution in the velocity of the current causes the subsidence of suspended matters, while oxidation of organic matters takes place from exposure of so large a surface to the atmosphere, and from the action of microscopic plants and bacteria.

Sea-water.-Sea-water is appreciably alkaline from the presence of carbonates. The proportion of solids in solution is about 3.5 per cent.; chlorin being the chief constituent, while sodium, calcium, and magnesium are next in amounts. It also contains considerable amounts of atmospheric gases, even at great depths, the average amount being from 2 to 3 per cent. by volume.

Comparison of Natural Waters.-The Rivers' Pollution Commission of England, in their sixth report, classify waters as follows:

I. In respect of wholesomeness, palatability, and general fitness for drinking and cooking:

1. Spring-water.

a. Wholesome.

2. Deep-well water.

}

Very palatable.

3. Upland surface-water.} Moderately palatable.

b. Suspicious. 4. Stored rain-water.

c. Dangerous.

5. Surface-water from cultivated land.

6. River-water to which sewage gains access. . Palatable. 7. Shallow-well water.

II. In respect to softness:

I. Rain-water.

2. Upland surface-water.

3. Surface-water from cultivated land.

4. Polluted river-water.

5. Spring-water.

6. Deep-well water.

7. Shallow-well water.