148 



CHEMISTRY 



point of view is the fact that since 1870 three new 

 elements have been discovered gallium, scandium, 

 and germanium the existence of all of which had 

 been predicted, and the properties of which had 

 to a certain extent been described beforehand by 

 Mendeleeff. (See periodic law in article ATOMIC 

 THEORY. ) 



Of late much attention has been given to 

 measurements of the quantity of heat produced 

 in various chemical changes, notably by Betthelot 

 and Thomsen. 



Elementary Principles of Chemistry. The science 

 of chemistry deals with a certain class of changes 

 which matter undergoes when subjected to par- 

 ticular conditions. Similar treatment may produce 

 very different effects upon different substances, as, 

 for instance, the effect of strong heat upon a piece 

 of quartz, a piece of limestone, and a piece of sugar. 

 The quartz does not suffer any permanent change, 

 that is, it has the same properties after it is cold 

 again as it had before the action of heat. The 

 limestone, although not necessarily much altered in 

 appearance, has its properties entirely changed, and 

 what remains is a new kind of matter quicklime. 

 The sugar melts, darkens, and chars, and becomes 

 quite manifestly changed into more than one new 

 kind of matter, for gaseous products, having the 

 smell characteristic of 'burnt sugar,' go off, whilst 

 a black coaly mass remains. 



The first of the above changes is merely a 

 physical change, from cold to hot ; the other two 

 are chemical changes, which result in the pro- 

 duction of new kinds of matter having properties 

 entirely different from those of the kinds of matter 

 from which they were obtained. The existence of 

 chemistry depends upon the existence of different 

 kinds of matter, and it is with such different kinds 

 of matter and the change from one kind to another 

 that chemistry has to do. 



When the properties of matter are studied, it is 

 found that for chemical purposes all kinds of 

 matter may be divided into two great classes, 

 which are called respectively elements and com- 

 pounds. The name element is applied to any 

 kind of matter that has not been proved to be com- 

 posed of more than one simpler kind of matter. 

 This conception of an elementary substance we 

 owe to Boyle, and it will be noted that some of 

 those substances which are now looked upon as 

 elements ( see article ATOMIC THEORY for a list of 

 the 68 known elements) may hereafter be proved 

 to be compounds, or kinds of matter composed of 

 more than one simpler kind, just as some sub- 

 stances which were at one time rightly classed as 

 elements ( according to Boyle's definition ) are now 

 known to be compounds of two or more elements. 



The compound nature of a specimen of matter 

 may be proved in one or other (or both) of two 

 ways. One of these methods is called Synthesis 

 (q.v. ), and consists in building up the compound 

 from the component simpler kinds ; the other is 

 called Analysis (q.v.), and consists in separating 

 more than one simpler kind from the compound 

 kind. 



The distinction between chemical compounds 

 and mere mechanical mixtures is a fundamental 

 one, and must be fully understood. The substance 

 gunpowder, for instance, is an intimate mixture of 

 finely powdered sulphur, charcoal, and saltpetre 

 (potassium nitrate), certain precautions being 

 observed during the mixing in order to avoid 

 explosion. These substances are not combined 

 together chemically in gunpowder, but are only 

 mixed, a fact as to which we can easily satisfy 

 ourselves in various ways. We may examine the 

 gunpowder under the microscope and identify the 

 separate particles of the ingredients ; or, by the 

 use of appropriate solvents, we may dissolve out 



first the saltpetre and then the sulphur, and thus 

 recover all three ingredients separately. The explo- 

 sion of gunpowder when heated to a sufficiently 

 high temperature is due to the occurrence of a series 

 of changes of the kind we call chemical, for these 

 changes result in the production of new kinds of 

 matter, gaseous and solid, which possess properties 

 in no way resembling those of sulphur, charcoal, or 

 saltpetre, and from which these substances cannot 

 now be dissolved out. 



A mixture possesses to a greater or less extent 

 the properties of its respective ingredients ; a com- 

 pound, on the other hand, has not as a rule any 

 properties resembling those of its constituents. A 

 piece of magnesium wire heated in the air to a 

 sufficiently high temperature takes fire and burns. 

 This is a chemical change in which the metal mag- 

 nesium combines with the oxygen of the air to form 

 a white, brittle, solid compound called magnesia or 

 magnesium oxide. This magnesia does not in the 

 least resemble either magnesium or oxygen in its 

 properties, and the most powerful microscope fails 

 to reveal particles of either of these substances to 

 our vision. 



The Atomic Theory (q.v.) is based upon the 

 assumption that matter of every kind is made 

 up of extremely minute indivisible particles called 

 atoms. The atoms which exist in a substance may 

 be all of the same kind, as in elements, or of differ- 

 ent kinds, as in compounds. Chemists believe that 

 the element hydrogen consists of molecules or 

 aggregates of atoms each molecule consisting of 

 two atoms ; further, that the compound substance 

 water consists of molecules, each composed of two 

 atoms of hydrogen and an atom of oxygen united 

 to each other by that force which is called Chemi- 

 cal Affinity (q.v.) ; and that similarly every other 

 compound substance is composed of molecules, 

 each molecule consisting of two or more different 

 kinds of atoms united by chemical affinity. The 

 weight of a new compound formed by the union of 

 two or more substances is in every case equal to 

 the sum of the weights of its constituents. In 

 chemical actions it is only the kind of matter 

 which is changed, whilst, as in every physical 

 change, the quantity of matter concerned remains 

 constant and unalterable. 



It has already been seen that one of the char- 

 acteristics of the chemical combination of two sub- 

 stances is that the properties of both disappear and 

 are not observable in the compound. Another and 

 a most important characteristic is the evolution of 

 heat, which is a very frequent although not an in- 

 variable accompaniment of chemical action. The 

 best examples of this may be seen in the ordinary 

 phenomena of Combustion (q.v.). All combustion, 

 whether it be of magnesium wire, coal, phosphorus, 

 paraffin oil, or a candle, is nothing more than a 

 chemical action accompanied by the evolution of 

 heat and light, oxygen gas of the atmosphere being 

 almost invariably one of the substances taking 

 part in such action. 



The conditions under which substances act chemi- 

 cally upon each other are very various for different 

 substances. In the first place, certain substances 

 cannot be got to act upon each other at all. Such 

 substances may have little affinity for each other, 

 as chlorine and oxygen, or no affinity, as fluorine 

 and oxygen. Other substances, again, only act upon 

 each other with difficulty. The main conditions 

 upon which action of one substance upon another 

 depends are the state of physical aggregation and 

 the temperature. Certain chemical actions take 

 place at ordinary temperatures, as, for instance, 

 the combination of chlorine with metallic antimony 

 or copper, or the spontaneous ignition of one of the 

 compounds of phosphorus and hydrogen when 

 brought into contact with oxygen. Other actions 



