The chemistry of farm practice

Front matter

Class

BooL__

fopyriglitW

Preface

Life, food, and raiment are directly or indirectly dependent upon agricultural products. In the settlement of our country, land was abundant and people were few, consequently little thought was given to the needs of the increasing numbers of succeeding generations. As the land first cultivated lost its fertility, the tide of population turned westward where unlimited areas of virgin soil awaited the herds and plowshares of the settlers. But this fresh area has been occupied, and to maintain the fertility of those fields that are still productive and to restore those that have become exhausted is the problem now facing agriculture.

The maintenance of the fertility of a productive soil demands the intelligent application of the principles of agricultural chemistry. The restoration of wornout fields is a difficult and costly undertaking. The successful farmer must reinforce his art by the application of the fundamental information derived from the study of chemistry, geology, botany, bacteriology, and entomology.

Chemistry aids agriculture in many ways. By means of it, exact data are collected and the fundamental reasons for practical results are explained. Then, too, chemistry invents new or improves old methods of fertilization. The chemist analyzes soils, manures and vegetable products. The value of soil analysis to the practical farmer, perhaps formerly overrated, in more recent years has been underrated. From a soil analysis the farmer can at least learn if his soil is unusually deficient in any important element. Chemistry also protects the agriculturist from the impositions of the unscrupulous fertilizer manufacturer.

The thorough analysis of farm products enables the

iii iv PREFACE

farmer to know their composition, and how much of each element they contain. This analysis serves a two-fold purpose: First, the composition of a plant shows what elements, and what quantity of each, have been removed from the soil. This, in turn, determines what the soil must contain to grow plants in a healthy condition. Second, in feeding vegetable products to livestock, the composition of these vegetables must be known in order that the rations may be compounded correctly.

Furthermore, chemistry explains how plants grow and are nourished. It shows the kind and the quantity of foods which plants require at various stages of their growth, and this guides the farmer in properly handling his crops. It teaches what purposes the different elements in the food supplied serve in animal economy and how the best results in animal feeding may be obtained with the least outlay of time, labor, and expense.

The purpose of this text is to furnish the knowledge of the fundamentals of chemistry required for intelligent agriculture and to apply this knowledge to the art of agriculture and to the problems of the agriculturist. No attempt has been made to limit its scope to the study of soils, fertilizers, and manures, although these subjects are given careful consideration. In addition, such subjects as feeds, nutrition, sanitary water, boiler water, and insecticides, subjects in which not only the farmer, but the suburban resident is interested — are discussed in as nontechnical language as possible.

The student of this book is urgently requested to make a careful study of the first chapters; for in them has been given in as concise and elementary form as is practicable the chemistry applied in the chapters which follow. Each succeeding chapter requires a knowledge of the preceding chapter, consequently they should be studied in sequence.

The author wishes to express here his indebtedness to Professor Charles M. Allen, Pratt Institute, for careful

PREFACE V

revision of the chapters on General Chemistry and the critical reading of the whole text, and to Dr. C. A. Peters, Amherst Agricultural College, for many helpful suggestions and criticisms. Acknowledgment is made to Mr. J. Ross Hanahan of Planters' Fertilizer Company for Figs. 37, 38 and 39, to Mr. John S. Carroll of the German Kali Company for Figs. 40, 41 and 42 and 52-60, and to Dr. Wm. S. Myers of the Chilean Nitrate Propaganda for Figs. 45-51. The following members of the faculty of Clemson College furnished photographs: Director J. N. Harper, Dr. F. H. H. Calhoun, Prof. W. A. Thomas, and Mr. F. G. Tarbox. Mr. T. C. Hough furnished valuable aid by making drawings. Director Thorne of the Ohio Experiment Station and Director Hartwell of the Rhode Island Experiment Station very kindly gave permission for the reproduction of cuts used, to illustrate bulletins of their respective stations. Cuts from Farmers' Bulletins of the United States Department of Agriculture were reproduced as well as cuts from bulletins of the South Carolina Experiment Station.

Standard books that bear on the subject have been freely consulted. Grateful acknowledgment is also due my father, Thomas W. Keitt, for valuable assistance in reading proof.

T. E. Keitt.

Clemson College, 1916.

EDITOR'S NOTE

This little text has been prepared in the belief that boys attending high schools in farming communities and those taking short courses in agricultural colleges should receive instruction in the chemistry applying to farm practice. For such students, there is neither time nor opportunity for the usual formal course in General Chemistry, followed by technical Agricultural Chemistry. A single elementary course combining the two is possible, in which the information furnished is definite, practical and reasonably adequate.

In such a combined course, the essential principles of Chemistry naturally come first, to be followed by their applications to the problems which arise in the life on a farm, in the growing of farm crops, or in the feeding and care of farm animals. This order has been followed by the author in the present text. It is believed that teachers in high schools attended by boys living on farms and in agricultural colleges giving short courses will find in The Chemistry of Farm Practice the text-book best suited to their needs. It should prove especially valuable, also, as a reference book for those interested in farming.

The Editor.

Vll

CHAPTER I Elements, Atomic Weights, Molecules, Symbols, Molecular

Weights, Oxidation, Reduction 1

Chemistry — Elements — Composition of Matter — Atomic Weights — Molecules of Elements — Symbols — Molecular Weights — Oxidation — Combustion — Kindling Temperature — Spontaneous Combustion — Reduction .

Chapter V

Water, Springs, Wells, Hardness and Household Water. ... 44 Properties of Water — Solvent Action of Water — Availability of Plant Food — Drinking Water — Hardness in Water — Filtered Water — Boiled Water — Distilled Water — Boiler Water.

ix x TABLE OF CONTENTS

PAGE

The Assimilation of Plant Food 70

Sources of Plant Food — Osmosis — Function of the Leaves of Plants — Leaching.

CHAPTER IX The Formation, Composition, and Fertility of Soils 76

Formation of Soil — Composition of Soils — Gain and Loss of Plant Food — Importance of the Rotation of Crops — Proper Sequence of Crops — Use of Manures — Keeping the Land Covered.

CHAPTER X Animal Manures 92

Quality — Liquid Manures — Rotted Manures — Effect of Exposure to the Weather — Rate of Application.

Chapter XI

Sources of Lime — Effects of Lime on the Soil — Shipping Lime — Applying Lime to the Soil — Machine for Applying Lime — Gypsum .

Chapter XII

Presence in the Soil — Commercial Sources — Phosphate Rock — Acid Phosphate or Super Phosphate — Thomas Phosphate or Basic Slag — Bone — Mineral Phosphate — Guano — Purchase and Application of Phosphorus.

TABLE OF CONTENTS Xl

PAGE

Chapter XIII

Importance of Nitrogen — Commercial Nitrogen Profitable — Selection of Source of Nitrogen — Inorganic Sources of Nitrogen — Saltpeter — Sodium Nitrate — Calcium Nitrate — Ammonium Sulphate — Organic Sources of Nitrogen — Vegetable Sources.

Chapter XIV

Occurrence — Wood Ashes — Organic Sources of Potash — Minor Sources — Commercial Salts of Potash — Functions of Potash — Use of Potash on Different Soils — Selection of the Source of Potash — Tendency to Use too Much Potash.

CHAPTER XVIII Feeds and the Calculation of Rations 184

Corn — Oats — Barley — Dried Brewers' Grain — Rye — Wheat — Rice — Cottonseed Meal — Linseed Meal — Meat Scraps — Dried Fish — Blood Meal — Soy-bean Meal — Peanuts — Timothy — Cereals — Legumes — Composition and Digestibility of Feeds — The Calculation of Rations.

Chapter XIX

Milk — Danger from Infected Milk — Preservatives — The Detection of Formaldehyde in Milk — Detection of Boracic Acid — Testing Milk for Per Cent of Fat — Determination of Specific Gravity — Ash — Butter — Cheese — Condensed Milk.

xii TABLE OF CONTENTS

Chapter XX

Two Classes of Injurious Insects — Injurious Fungi — Insecticides for Biting Insects — Insecticides for Sucking Insects — Fungicides — Common Disinfectants.

CHAPTER XXI Paints and Whitewashes 225

Paints — Drying Oils — Driers — White Pigments — Green Pigments — Blue Pigments — Red Pigments — Yellow Pigments — Brown Pigments — Black Pigments — Mixing Paints — Whitewashes — Calcimine — Varnishes — Shellac — Glue.

Chapter XXII

Materials Producing Heat and Light. Fire Extinguishers 233 Petroleum — Kerosene — Gasoline — Acetylene — Fire Extinguishers.

Chapter XXIII

Use — Cement Manufacture — Setting of Cement — Sand — Gravel — Quantity of Material for Different Mixtures — Mixing Concrete — Placing the Concrete.

THE CHEMISTRY OF FARM PRACTICE

Chapter I

ELEMENTS— ATOMIC WEIGHTS— MOLECULES— SYMBOLS- MOLECULAR WEIGHTS— OXIDATION— REDUCTION

1. Chemistry. Chemistry deals with the composition and properties of substances and the changes which substances undergo. Agricultural chemistry has to do with the application of the knowledge gained through chemistry to the art of agriculture and to the problems which the farmer has to solve. To understand agricultural chemistry we must gain first a knowledge of some of the underlying principles of General Chemistry.

2. Elements. Matter is made up either of simple elements or of these elements combined into compounds of unvarying composition. An element is a simple substance which has certain definite properties and which has not been separated into substances having different properties. Iron is an element. However minutely the piece of iron may be divided, the smallest particle will always have properties identical with the iron before its division.

Somewhat more than eighty elements have been isolated which have resisted all the attempts of present chemical methods at further separation. Each element has certain distinctive properties that prevent it being classed with other elements, although certain elements which are closely related have some of their properties in common.

Only ten elements are necessary to sustain the life of the growing plant; these are carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, magnesium, calcium and iron. Some of these elements are derived from the atmosphere, some from water and some from the soil. Most of the substances of plants consist of the elements,

OXYGEN 49.8*

SILICON 26.156

ALUMINIUM 7.3*

IRON 4.1*

SODIUM 2.3^

POTASSIUM 2.3*

MAGNESIUM 2.2*

ALL OTHER ELEMENTS 2.7*

Fig. 1. — Percentage of elements in the combined mass of atmosphere, waters and crust of the earth.

carbon, hydrogen, and oxygen, which are obtained from the atmosphere, or from water. Nitrogen is the most expensive and the most elusive of the elements required by plants. A part of the nitrogen may be derived from the atmosphere by certain plants under conditions we shall study later, but most of the nitrogen which serves as plant food comes from the decomposition of organic substances in the soil. The other six elements necessary for plant growth are required in comparatively small amounts. With the exception of phosphorus, soils usually contain an abundant supply of these " ash elements."

In the diagram, Fig. 1, is shown the relative proportion of eight of the most abundant of the elements as found in the atmosphere, all waters, and the solid parts of the earth's crust which have been examined. It will be noticed that the seventy-five elements not mentioned, altogether, comprise but 2.7 per cent of the earth's constituents. With the exception of a comparatively small quantity of oxygen existing in a free condition in the air, this figure represents the percentage of the elements as they exist in compounds.

3. Composition of Matter. Matter may be divided and subdivided till definite parts called molecules are reached. Finally the molecule, by chemical means, may be separated into invisible particles called atoms. The atom may be defined as the extremely minute particle of matter that enters as a unit into chemical combinations with other atoms. A molecule is the smallest part of matter that can exist by itself. An atom does not remain free or uncombined; it unites either with other atoms of the same kind to form a molecule of an element or it combines with atoms of a different kind and thus produces a compound.

4. Atomic Weights. Atoms combine chemically according to definite proportion by weight, The smallest amount of hydrogen that will enter into chemical reaction, i.e., the hydrogen atom, is less by weight than the atom of any other element. For this reason hydrogen may be taken as a convenient unit of comparison and its smallest combining weight, which is the weight of its atom, may be assumed to be one. The gases hydrogen and chlorine, when mixed and exposed to light, will form a new substance, hydrogen chloride, which in its water solution is called hydrochloric acid. When one part by weight of hydrogen is allowed to unite with an excess of chlorine, it is found that it will, in every case, combine with 35.46 times its own weight of the chlorine. Hydrogen chloride therefore contains hydrogen and chlorine in the proportion of 1 to 35.46. There are reasons for believing that the molecule of hydrogen chloride consists of one atom of hydrogen combined with one atom of chlorine. If this is true, then, taking the hydrogen atom as a unit of weight, the chlorine atom weighs 35.46 and the molecule of hydrogen chloride, HC1, weighs 36.46. It has been found that the elements which compose a compound always combine in proportion to their atomic weights or to some multiple of their atomic weights. It will be seen that this must be the case if an atom is indivisible.

The atomic weight of an element is fixed by determining with great care the weight of the element that will unite with another element whose atomic weight has previously been determined. As nearly all the elements form compounds with oxygen, while comparatively few unite with hydrogen, the atom of oxygen with an assigned weight of 16 is really used as the standard instead of the atom of hydrogen, which it will be observed in the table on page 6, weighs 1.008.

5. Molecules of Elements. The common elementary gases, such as oxygen, hydrogen, nitrogen, and chlorine, have molecules consisting of two atoms of each element. The molecules of some of the elements, such as phosphorus, arsenic, and antimony, have four atoms in each molecule, when they are at a temperature slightly above vaporization. In most cases the number of atoms in a molecule of an element depends upon its temperature. Thus sulphur vapor at 468° C. has eight atoms to the molecule; at 830° C. its molecule has but two atoms.

Some elements, such as sodium, potassium, mercury, and zinc, when in a vaporous state, have but one atom to each molecule. Such molecules differ from atoms in that they seem to have lost the chemical affinities which are characteristic of atoms. The number of atoms in a molecule of an element when it is in the solid condition is not known.

6. Symbols. For the sake of convenience, each element is represented by one or more letters. This abbreviation of the name of an element is called its lymhcl. The letter used to represent an element is often the first letter of its name. It is written as a capital, but unlike other abbreviations, it is not followed by a period. The symbol of oxygen is 0, of hydrogen is H, of nitrogen is N, and P is the symbol of phosphorus. When the name of more than one element begins with the same letter, the most important or the first discovered of these elements has the single letter for a symbol and to the others an additional letter is assigned. Thus C is the symbol for carbon, Ca the symbol for calcium, CI the symbol for chlorine, Cr for chromium. It will be noted that the second letter is not written as a capital. Sometimes the symbol is derived from the foreign word which means the element, as Fe from the Latin ferrum, meaning iron, K from Kalium, the German word meaning potash, which contains potassium, Hg from the Greek, hydrargyrum, meaning " water silver," a good description of mercury. Many symbols are taken from the names of countries, as Cu, copper, from the island of Cyprus, Mg, magnesium, from Magnesia in Asia Minor. The symbol of an element represents not only the name of the element, but it means also one atom of the element and consequently is a definite weight. S stands not only for sulphur, but for one atom of sulphur, which weighs 32, or two times the weight of an atom of the standard, oxygen.

7. Molecular Weights. One method for determining molecular weights depends upon an hypothesis proposed by Avogadro, which has been quite universally accepted. This hypothesis states that " under the same conditions

TABLE I.— THE COMMON ELEMENTS

Elements.

Symbols.

Atomic Weights Usual Valence

Aluminium .

Antimony. .

Arsenic

Barium ....

Bismuth.. . .

Boron

Bromine. . . .

Calcium. . . .

Carbon

Chlorine. . . .

Chromium. .

Cobalt

Copper

Fluorine. . . .

Gold

Hydrogen. .

Iodine

Iron

Lead

Lithium. . . .

Magnesium.

Manganese .

Mercury. . .

Nickel

Nitrogen. . .

Oxygen. . . .

Phosphorus.

Platinum. . .

Potassium . .

Radium. . . .

Silicon

Silver

Sodium. . . .

Sulphur

Tin

Zinc

Al

Sb

As

Ba

Bi

B

Br

Ca

C

CI

Cr

Co

Cu

F

Au

H

I

Fe

Pb

Li

Mg

Mn

Hg

Ni

N

O

P

Pt

K

Ra

Si

Ag

Na

S

Sn

Zn

27.1

120 . 2

3 or 5

74.96

3 or 5

137.37

208.0

11.0

79.92

40.07

12.0

35.46

52.0

58.97

63 . 57

19.0

197.2

126.92

55.84

2 or 3

207.1

6.94

24.32

54.93

200.6

1 or 2

58.68

2 or 3

14.01

3 or 5

16.0

31.04

3 or 5

195.2

2 or 4

39.1

226.4

28.3

107.88

23.0

32.07

119.0

2 or 4

65.37 of temperature and pressure, all gases have the same number of molecules in equal volumes." While it is quite impossible to isolate and weigh a single molecule, yet if we select as a standard of molecular weight a molecule of oxygen, consisting of two atoms, and assign to it a weight of 32, we may, by assuming the Avogadro hypothesis, obtain the weight of the molecule of any gas. This is done in the case of ammonia by weighing equal volumes of ammonia and of oxygen under the same conditions of temperature and of pressure, and solving for the molecular weight of ammonia in the proportion:

Weight of volume of oxygen : weight of same volume of

ammonia = 32 : molecular weight of ammonia.

In this manner we may determine accurately the weight of the molecule of any element or of any compound which may be weighed in a gaseous condition.

8. Oxidation and Reduction. An understanding of the principles of chemistry involved in the processes of oxidation and of reduction is important. It is impossible to obtain a working knowledge of chemistry without gaining a mastery of these processes. A simple case of oxidation takes place when a bright strip of iron is heated over a flame. The oxygen of the air, more active at the high temperature, combines with the metallic iron, rusting it into a black oxide of iron. If a silver spoon is exposed to the fumes of sulphur, the silver combines with the sulphur and black silvsr sulphide is formed on the surface of the metal. This is also a case of oxidation, although here there is no oxj^gen involved.

Oxidation may be defined as the combination of oxygen, or of other elements that act chemically in the same way as oxygen, with other material. Sometimes oxidation takes place in two or more stages. Thus metallic mercury will be oxidized by combining with one atom of chlorine, forming white mercurous chloride or calomel, which, in turn, s may be further oxidized by adding to itself another atom of chlorine, forming mercuric chloride or corrosive sublimate.

9. Combustion. Oxidation produces heat as a result of the chemical action taking place. Should oxidation be sufficiently rapid to produce enough heat so that light is evolved, the process is termed combustion. When the materials entering into combustion are gases, a flame is produced. Burning wood is undergoing combustion, this process of oxidation being supported by the oxygen drawn from the atmosphere entering into rapid union with the hydro-carbon gases produced from the heated wood. The three conditions necessary for combustion are first, a combustible substance; second, a supporter of combustion; third, a kindling temperature. Usually carbon or hydrogen or some of their numerous compounds or the metals that are easily oxidized are regarded as the combustibles, while oxygen or some of the elements that act chemically like oxygen are considered the supporters of combustion. The material entering into chemical reaction which is the more abundant is likely to be considered the supporter of combustion, and thus the combustible and the supporter of combustion may exchange places.

This is seen in the reversal of flames effected by the apparatus in Fig. 2. The lamp chimney is fitted with corks and tubes as in the figure. The straight glass tubes at A and C are at least ■£$ of an inch in internal diameter. The glass elbow D is connected with the illuminating gas supply. While the finger is placed at C, closing the tube, the gas is turned on and allowed to flow till the chimney

Fig. 2. — Apparatus for showing reversal of flames.

is surely full of gas, which is escaping at A. The gas at A is now ignited and the gas cock turned down till a small inverted flame is produced at A. The finger is now removed from C and the flame at A will be seen to ascend the tube and to burn at B. The gas issuing at C is immediately ignited. If we consider these two flames, we shall see that the one at C is composed of illuminating gas burning in an atmosphere of air, and naturally the oxygen of the air would be considered the supporter of combustion, while the gas would be considered the combustible. The flame at B is composed of air coming up the tube from A which is burning in an atmosphere of illuminating gas, and in this case we should naturally consider the illuminating gas as the supporter of the combustion while the oxygen of the air is the combustible.

10. Kindling Temperature. While oxidation may take place at any temperature, in order to start and to continue combustion, it is necessary for the combustible and the supporter of combustion to be at a certain temperature, known as the kindling temperature. Each substance, under the same conditions, has its own definite kindling temperature. Phosphorus has a very low kindling temperature, taking fire spontaneously in the air. This is due to the fact that oxidation raises the temperature of the phosphorus to its kindling point. If a bit of phosphorus as large as a wheat grain is dissolved in a small amount of carbon disulphide and the solution poured upon a filter paper placed in an iron ring, as soon as the carbon disulphide evaporates, the phosphorus will burst into flame. In this case the finely divided condition of the phosphorus exposes a relatively large surface to oxidation. When a candle flame is extinguished by blowing upon it, the blast of air cools the flame below its kindling point.

The quantity of heat produced by combustion will depend upon the quantity and the character of the gases entering into reaction, while the degree of heat will depend upon the nature of the combining substances and upon how intimately the combustible and supporting gases may be mixed at the point of ignition. Hence a blast is used to provide a large quantity of oxygen which is introduced into the interior of the combustible gas, which is then forced to combine internally with the air of the blast and externally with the oxygen of the atmosphere. The hole near the base of the Bunsen burner, Fig. 3, is required to supply air for combustion. The blacksmith's forge, Fig. 4, must be equipped with a " blower " to furnish |M| admiTlir enough air for the complete combustion

Hjfesaiia of the large amount of gas. ^IIIJjjL Gas Iutake In many cases the oxidation is a

^fcJBIP slow process. This may be seen in the

„ rusting of iron, the tarnishing of copper,

Fig. 3.— Bunsen & ' . ° + w •

burner tne dulling °* zinc - When ink that is

pale when first applied becomes dark

upon exposure, the ink has undergone oxidation. When

paper turns yellow with age, it is being slowly oxidized.

Almost everything we see about us has been oxidized.

Besides the noble metals, such as gold and platinum, it

is only thoss substances that have been artificially deprived

of their oxidizing constituent that exist in any other than

an oxidized condition.

The rapidity with which oxidation takes place does not affect the total quantity of heat produced. An iron wire may be burned in a jar of oxygen and the combustion may last but a few seconds, or the wire may be rusted by exposure to moist air, and the oxidation may take a month to complete itself, but when the two actions are complete, if the same iron oxide is finally produced and in the same quantity, the total amount of heat will be identical in the two cases. Although combustion cannot take place without light, yet light may be produced by other means than combustion, as in the case of the carbon filament of an electric light bulb.

11. Spontaneous Combustion. If the heat produced in slow oxidation is not allowed to be dissipated, the temperature of the combustible which is being oxidized may gradually be raised till it attains its kindling temperature

Blower to Supply Air

Fig. 4. — A down-draft forge.

and it will then burst into flame. This is quite likely to happen in the case of vegetable or animal oils with which cloths may have been saturated. These oils are " drying oils," that is, they are easily oxidized as shown by the necks and stoppers of their containers, which are gummed upon standing. This combustion is produced spontaneously, that is, by the internal development of heat without the action of an external agent other than the Oxidizer. Heaps of finely divided coal may in the same way be set on fire by the spontaneous combustion occasioned by the oxidation of their sulphur or oil content.

12. Reduction. This process is the reverse of oxidation. It is the subtraction from a compound of oxygen or of the element which plays the role of oxygen. The extraction of the common metals from their ores is a process of reduction. This process is brought about by subjecting the material to be reduced to certain reducing agents which have a stronger affinity for the oxygen part of the compound than does the metal originally combined with that part. The most useful of these reducing agents are carbon, in the form of charcoal or coal, hydrogen and the hydro-carbon compounds and certain active metals, such as sodium or aluminium. Carbon monoxide gas, which needs more oxygen so as to make itself into carbon dioxide, a more stable compound, is a very valuable reducing agent. In the highly heated areas of the blast furnace in the process of iron reduction, the carbon monoxide takes away the oxygen of the iron ore and leaves metallic iron.

Chapter II Compounds— Mixtures— Valence— Formulas- Equations

13. Conservation of Matter. Chemical changes neither create nor destroy matter. By chemical means matter may be converted into new forms having different properties, but the total weight of the substances before chemical action will equal the total weight of the substances after the

Fig. 5. — The black solid, carbon, combined with the yellow solid, sulphur, forms the colorless liquid carbon disulphide.

action has been completed. Sulphur, when burning, combines with the oxygen of the air, and when the sulphur is entirely consumed, the weight of the choking fumes of sulphur dioxide produced will exactly equal the sum of the weights of the sulphur burned and the oxygen with which it combined.

14. Compounds. Two or more elements when chemically combined form compounds. A chemical compound is composed always of the same elements combined in the same proportion by weight. A compound possesses properties which differ from those of the elements of which it is composed. Chemists have recognized many thousands of compounds, each of which has a characteristic set of properties which is constant and differs from that of other compounds.

The element carbon, a black solid, will combine chemically under certain circumstances with the jtIIow solid, sulphur, and form a compound, carbon disulphide, which is a transparent liquid with a characteristic odor, and very volatile at ordinary temperatures. The formation of compounds is due to the chemical attraction existing between the atoms of the elements which causes them . to combine to form the molecules of the compound.

15. Mixtures. Two or more substances may be ever so intimately blended together, but, if no chemical combination takes place, there is no compound produced. Such a mingling is termed a mixture. A mixture differs from a compound in two ways: First, it possesses the characteristic properties of all of its ingredients; second, the proportion of its constituents may vary each time the mixture is made. In this respect a mixture differs from a compound in which the percentage content of its constituents does not vary.