r i IK MIST it v 



149 



only take place when the temperature of the sub- 

 stances which are to take part in them has been 

 siillii-iently raised. Thus magnesium requires to 

 IK* strongly heated in air before it takes fire ; once 

 tin- notion is started, however, the heat given out 

 liy i lie combustion of one part of the magnesium is 

 Millicient to raise another part to the temperature 

 necessary for combustion to go on, and HO the 

 change is propagated. Coal-gas only burns in air 

 when it is raised to a bright-red heat. A jet of 

 coal-gat* escaping into the air may be easily ignited 

 liy applying a brightly red-hot poker, but when the 

 poker cools to dull redness it will no longer ignite 

 the jet. A bar of metallic iron does not undergo 

 any chemical change on exposure to dry air at 

 ordinary temperature, but it iron in the state of 

 very fine powder (a form in which it can easily be 

 obtained by appropriate methods) be thrown into 

 the air, combination at once takes place with the 

 evolution of heat and light. When a piece of iron 

 ( say a moderately fine iron wire ) is heated to red- 

 ness in air, combination with the oxygen of the air 

 takes place with the formation of a scale composed 

 of a black oxide of iron, but the quantity of heat 

 given out during the combination is not sufficient 

 to propagate the combustion from particle to 

 particle of the iron after removal of the source of 

 neat. If, however, iron wire be raised to a red 

 heat in an atmosphere of oxygen, it takes fire and 

 burns with great brilliancy. The difference noticed 

 here is due to the presence in the one case, and the 

 absence in the other, of the diluting nitrogen which 

 forms nearly four-fifths of the air by volume. 



There are certain chemical actions which in tak- 

 ing place are accompanied, not with evolution, but 

 with absorption of heat. In such cases heat has to 

 be supplied throughout the action, and not merely 

 to start it. This is frequently noticed in the com- 

 bination of substances which have feeble affinity 

 for each other, and the compounds produced are 

 less stable, or more readily break up into their con- 

 stituents, than those which are produced with the 

 evolution of heat. In general terms it may be 

 stated that the quantity of heat given out in the 

 formation of a compound is a measure of the 

 stability of the compound. When a given weight 

 of magnesium unites with oxygen to form magnesia, 

 a quite definite and measurable quantity of heat is 

 given out. In order to separate the magnesium 

 from the oxygen again, exactly the same quantity 

 of heat must be supplied. In the case of those sub- 

 stances in the formation of which heat is absorbed, 

 we find, as we should expect, that heat is given out 

 during their decomposition, and that its quantity 

 is exactly that which was absorbed during their 

 formation. 



Chemical Notation. For the purpose of shortly 

 expressing the composition of chemical substances, 

 and for representing chemical changes, chemiste 

 employ a system of notation which is in extremely 

 common use. In the table of Atomic Weights (see 

 ATOMIC THEORY ) it will be noticed that after the 

 name of each element is placed its symbol, which 

 usually consists of the first, or of the first and another 

 letter of the Latin name of the element. Each 

 symbol distinctly indicates the element which it is 

 intended to represent, but it must always be borne 

 in mind that the symbol for an element is not merely 

 a contracted form of its name, but that it stands 

 for a definite quantity of that element, this quantity 

 being the atomic weight expressed in terms of the 

 unit of weight employed. The unit of weight 

 almost universally employed by chemists and scien- 

 tific men in general is the gramme (see METRE), 

 and that unit will be adopted for illustrations- 

 throughout this article. With the gramme as 

 unit, H stands for 1 gramme of hydrogen, Cl for 

 35'4 grammes of chlorine, O for 16 grammes of 



oxygen, Mg for 24 grammes of magnesium, and no 

 on. In order to represent the composition of a 

 compound, the symbols of the various element* 

 which occur in the compound are written side by 

 side, and this collection of symbols is called a 

 formula. Thus, MgO represent* 40 ( = 24+ 16) 

 grammes of magnesium oxide, and HC1 in 36*4 

 (= 1 + 35 '4) grammes of hydrogen chloride. When 

 ;i oompOUM contains more than one atom of the 

 same element the symbol for that element is not 

 repeated, but the number of atoms is indicated by 

 a subscribed numeral. Thus the formula for water 

 is written H a O, which indicates that the molecule 

 of water contains two atoms of hydrogen and one 

 of oxygen ; and the formula for sulphuric acid is 

 written 1LS< ) 4 , which indicates that the molecule 

 of sulphuric acid contains two atoms of hydrogen, 

 one of sulphur, and four of oxygen (besides the 

 quantitative signification of these formula' already 

 mentioned). A number subscribed to a portion 

 of a formula inclosed in brackets multiplies the 

 portion so inclosed. Thus the formula Ba(NO,) a 

 represents one atom of barium united to twice the 

 quantity of the group NO 3 , which is represented as 

 united to one atom of potassium in the formula 

 KNO. A number prefixed to a formula multiplies 

 the whole of the formula that follows. Thus 2H 2 O 

 represents twice the quantity of water represented 

 by H 2 0. 



Chemical symbols and formuhe are used to repre- 

 sent shortly chemical changes. A simple illustra- 

 tion of the method of using them may DC given to 

 represent the case of the Duming of magnesium. 

 The symbols for the magnesium and the oxygen 

 entering into combination ( connected by the sign 

 + ) are written on one side of what is called a 

 chemical equation, whilst the product is written on 

 the other side, thus : 



2Mg + O 2 = 2MgO. 



The formula for free ( or uncombined ) oxygen is 

 written Oi>, because a molecule of oxygen is believed 

 to consist of two atoms (see ATOMIC THEORY). 

 In order to represent the element magnesium, the 

 simplest possible formula (Mg) is employed be- 

 cause there is no evidence for writing a more com- 

 plicated one. 2Mg simply represents twice as 

 much magnesium as Mg does. 



The above equation when fully interpreted gives 

 a great deal of information about thp change which 

 it is intended to represent. It shows that mag- 

 nesium and oxygen unite with each other ( under 

 conditions which are not expressed) to form an 

 oxide of magnesium, and that these elements are 

 united in the compound in the proportions by 

 weight of 24 of magnesium to 16 of oxygen ; 

 and, further, it enables us, by applying a simple 

 and easily remembered rule, to calculate the volume 

 of oxygen taking part in the action as well as its 

 weight. This rule for ascertaining the volume 

 may be conveniently stated here. From certain 

 theoretical considerations, as well as for con- 

 venience in calculations concerning the volumes of 

 gases, chemists write the formulie of gaseous sub- 

 stances in such a way that the quantity 'of a gas 

 represented by its formula, in terms of any nnit of 

 weight, shall occupy, under similar condition* .t 

 temperature and pressure, the same volume as 

 two units weight of hydrogen. Thus, the unit 

 being the gramme, H 3 represents 2 grammes of 

 hydrogen, and 2 grammes of hydrogen at stand- 

 ard temperature (0 C.) and pressure (760 milli- 

 metres or mercury) occupy a volume of 22;33 litres. 

 See (under Metre) METRIC SYSTEM. Similarly, 

 the quantities in grammes of oxygen, carbonic 

 anhydride, mid nitrous oxide, represented by their 

 respective formula-, U.. ( 16 x 2 = 32 grammes), CO, 

 (12 + 32 = 44 grammes), and N 8 O (28 + 16 = 44 



