150 



CHEMISTRY 



grammes), each occupy, when measured at C. 

 and 760 mm. pressure, 22 '33 litres. This rule 

 holds for other gases, and also, with a certain 

 qualification, for the vapours of volatile liquids. 

 In the case of the latter, of course, conditions of 

 temperature and pressure must be chosen such that 

 the substance is in the state of vapour ; and the 

 quantity in grammes which, as a vapour, occupies 

 the same volume as 2 grammes of hydrogen under 

 the same conditions, is the quantity whicfh the 

 formula is chosen to represent. Thus, the formula 

 HLjO informs us that 18 (=2 + 16) grammes of 

 water occupy, in the form of steam, the same 

 volume as 2 grammes of hydrogen when both are 

 measured at the same temperature and pressure. 

 It must, of course, be understood that the formula 

 for a substance is chosen so as to represent the 

 observed facts. The formula of a volatile liquid is 

 deduced from the determination of the vapour 

 density of the liquid ; this determination is made 

 by ascertaining the weight of that quantity of the 

 liquid which, when converted into the state of 

 vapour, occupies the same volume as a given weight 

 of hydrogen, both being measured at the same 

 temperature and pressure. 



Returning to the equation already given, it will 

 be seen that from it we learn that 48 ( = 2 x 24 ) 

 grammes of magnesium unite to form magnesium 

 oxide with a quantity of oxygen (32 grammes) 

 which at C. and 760 mm. occupies 22 '33 litres. 

 What volume this quantity of oxygen would occupy 

 under other conditions of temperature and pressure 

 can be calculated from formulae deduced from the 

 laws of Charles ( relation of the volume of a gas to 

 the temperature ) and Boyle ( relation of the volume 

 of a gas to the pressure). See further in article 

 GAS AND GASES. 



As there are certain conditions under which chemi- 

 cal combination takes place, so there are definite laws 

 which regulate combination. The first of these has 

 been called the law of constant proportions, and it 

 states that any chemical compound always contains 

 the same constituents and in the same proportions. 

 Thus magnesium oxide, MgO, always consists of 

 magnesium and oxygen in the proportions by 

 weight of 24 to 16 one atom of magnesium weigh- 

 ing 24, being combined with one atom of oxygen 

 weighing 16. No compound of magnesium and 

 oxygen containing these elements in any other 

 proportion has e.ver been obtained. If in preparing 

 magnesium oxide quantities of magnesium and 

 oxygen were employed differing from this propor- 

 tion, then some either of the magnesium or of the 

 oxygen would remain over after the action, accord- 

 ing as the former or the latter had been employed 

 in excess of the right quantity. 



Intimately connected with the foregoing law is 

 the law of multiple proportions. Whilst certain 

 elements combine with each other in only one pro- 

 portion by weight, others combine in two, and 

 sometimes more than two different proportions. 

 The law of multiple proportions states that when 

 elements combine in two or more proportions these 

 various proportions can be expressed by simple 

 multiples of the atomic weights of the elements 

 concerned. Thus carbon and oxygen unite with 

 each other to form two different compounds : 12 

 parts by weight of carbon unite with 16 parts by 

 weight of oxygen to form carbonic oxide, CO ; 12 

 parts by weight of carbon unite with 32 parts by 

 weight of oxygen to form carbonic anhydride, CO 2 . 

 Here the relation is of the simplest kind, for the 

 one compound contains exactly twice as much 

 oxygen for the same quantity of carbon as the 

 other. Again, iron ana oxygen unite with each 

 other to form three different compounds : 56 parts 

 by weight of iron unite with 16 parts by weight of 

 oxygen to form ferrous oxide, FeO; 112 parts by 



weight of iron unite with 48 parts by weight of 

 oxygen to form ferric oxide, Fe 2 O 3 ; 168 parts by 

 weight of iron unite with 64 parts by weight of 

 oxygen to form ferroso-ferric oxide, Fe 3 O 4 . This 

 case is not quite so simple as that of the oxides 

 of carbon, for here it is necessary to employ 

 multiples of the atomic weights of both elements 

 concerned in order to see the simplicity of the 

 quantitative relations existing amongst these 

 oxides of iron. The law of multiple proportions 

 is, however, fully illustrated by both series of 

 oxides. 



It may be useful to call attention here to the 

 simple explanation furnished by the Atomic Theory 

 ( q. v. ) for the occurrence of compounds illustrating 

 this law of multiple proportions. There is no com- 

 pound intermediate in composition between car- 

 bonic oxide and carbonic anhydride. The atomic 

 theory explains this very simply. Under one set 

 of conditions we can obtain a compound of one 

 atom of carbon with one atom of oxygen, whilst 

 under other conditions we obtain a compound of 

 one atom of carbon with two atoms of oxygen, or 

 exactly twice as much. This is why we find such 

 marked intervals in composition between two or 

 more compounds of the same elements. The mole- 

 cule of one compound cannot differ from that of the 

 other by less than an atom, and the addition of an 

 atom to a molecule necessarily forms a new mole- 

 cule differing in weight from the old one by the 

 weight of the added atom. 



The last law of combination has been called the 

 law of volumes. It states that when gases combine 

 to form new compounds, the volumes taking part 

 in the action bear a very simple relation to each 

 other and to the volume of the product if gaseous 

 when all the volumes are measured at the same 

 temperature and pressure. Thus, one volume of 

 hydrogen combines with one volume of chlorine to 

 form two volumes of hydrochloric acid gas ; two 

 volumes of hydrogen combine with one volume of 

 oxygen to form two volumes of water vapour ; two 

 volumes of carbonic oxide combine with one volume 

 of oxygen to form two volumes of carbonic anhy- 

 dride, and so forth. The very simple relations of 

 the volumes concerned in these examples are suffi- 

 ciently manifest, and much greater complexity is 

 not frequently met with. 



Chemists divide the elements into two great 

 classes, the typical members of which are very 

 different in their physical and chemical characters. 

 These are metals and non-metals, and as representa- 

 tive of each class may be mentioned copper and 

 sulphur. The more prominent physical character- 

 istics of metals are the metallic lustre, malleability, 

 ductility, and the property of conducting heat and 

 electricity, all of which are possessed to a more or 

 less marked degree ; whilst non-metallic elements 

 as a rule possess these properties to a very limited 

 extent, if at all. Differences in chemical behaviour 

 are also very striking in typical representatives of 

 each group. It must be borne in mind, however, 

 that all the members of each group are not typical, 

 but that there is a gradual transition from one 

 group to the other, and certain of the transition 

 elements possess some of the properties of both 

 groups, as in the cases, for instance, of arsenic and 

 antimony. 



With the exception of bromine and fluorine, all 

 the elements enter into combination directly or 

 indirectly with oxygen to form oxides. The oxides 

 produced from metallic elements are quite different 

 in chemical character from those produced from 

 non-metallic elements. We shall look first at the 

 oxides of the metals. Every metal forms one or 

 more oxides, and at least one oxide of every metal 

 is a basic oxide i. e. an oxide which has the pro- 

 perties of a Base (q.v. ). A distinction is made 



