CHEMISTRY 



1296 



CHEMISTRY 



a mixture as the cake, where each article put 

 in keeps its own properties, except as these 

 are modified by heat. An easily-tried experi- 

 ment will show more clearly than description 

 can do the difference between the two kinds 

 of composite bodies. 



If a small quantity of very fine iron-filings 

 be mixed thoroughly with a small quantity of 

 powdered sulphur, the iron remains iron and 

 the sulphur remains sulphur. They may be 

 distinguished from each other when looked at 

 through a microscope, and a magnet held over 

 the mixture will quickly draw out the iron, 

 leaving the sulphur. But if the mixture is 

 placed in an iron spoon (or a glass test tube) - 

 and held over a hot flame, something curious 

 results. Something is formed which is neither 

 iron nor sulphur which is not like either iron 

 or sulphur. The new substance may be 

 pounded to a powder, but no magnet, however 

 strong, can now draw out the iron, for the 

 simple reason that, as iron, it is not there. 

 The new substance is just as real and has just 

 as distinct properties of its own as had the two 

 elements which combined to make it, but there 

 is one difference. Any person who knows the 

 proper chemical means for decomposing the 

 new substance could reduce it again to iron 

 and sulphur, while no means known to man 

 could have divided either of the original ele- 

 ments. 



The iron and sulphur before they were 

 heated formed what is known as a mechanical 

 mixture, each keeping its own properties; after 

 they were heated they formed a chemical 

 compound. Many of the very commonest 

 things, which seem as simple as anything could 

 well be, are in reality chemical compounds, 

 for water and salt, spoken of above, are of 

 this nature. Air, on the other hand, is a mere 

 mechanical mixture of gases. The piece of 

 cake we spoke of is a mechanical mixture of 

 the chemical compounds, water, salt, sugar, 

 starch, fats, proteins, etc. 



Atoms. There are definite ways in which 

 chemical compounds are made up. All mat- 

 ter which exists in the world, according to 

 chemists, is made up of inconceivably tiny 

 particles called atoms. These are far too small 

 for the most powerful microscope ever to dis- 

 cover, and no atom can be further divided. 

 Now when a certain number of atoms of one 

 element are brought close to atoms of an- 

 other element, various things may happen. 

 They may remain exactly as they have been, 

 neither substance showing the slightest inter- 



est in the other; one atom of one kind may 

 seize upon one or more atoms of the other 

 kind and unite with them to form a tiny par- 

 ticle of a new substance a chemical com- 

 pound, in which each of the original atoms 

 loses its identity; or both kinds of atoms may 

 wait until some force, as heat or electricity, 

 puts them in such a condition that they can 

 unite. 



Atoms which will thus unite with each other, 

 either with or without aid, are said to have a 

 chemical affinity for each other, and unless 

 two substances have such affinity they cannot 

 be forced to unite. No amount of mixing or 

 melting or heating will make of them any- 

 thing but a mechanical mixture. In the ex- 

 periment described above, the sulphur and iron 

 filings united to form a new substance with 

 properties of its own, not just because they 

 were melted together but because they also 

 have a chemical affinity for each other. 



In the very simplest form of a chemical 

 compound, one atom of one substance unites 

 with one atom of another. But often one 

 atom of one element will seize . upon two or 

 three or even four of another, or two atoms 

 of one may combine with three of another. 

 It is easier for some elements to enter into 

 combination than for others, because some 

 elements are gases and some are solids, and 

 the latter are much more dependent on out- 

 side forces to make it possible for them to 

 unite with substances for which they have 

 even the strongest chemical affinity. 



Chemical Symbols. Each element has its 

 own name, and also a "nickname," or abbre- 

 viation, which is known as its symbol. Usu- 

 ally this symbol is the first letter of its name, 

 as O for oxygen, N for nitrogen and H for 

 hydrogen. Some of the very common chem- 

 ical compounds have simple names names 

 which give no hint that the substance is a 

 compound, as water and salt. But this is not 

 enough for the chemists, v;ho have devised a 

 system of naming all compounds which not 

 only shows that they are such, but gives the 

 elements of which they are composed, as well, 

 and the number of atoms of each which enters 

 into the combination. By this simple system, 

 the symbols of the elements which make up a 

 substance are written together as a formula, 

 thus NaCl. Na stands for sodium, the Latin 

 name for which is natrium, and Cl for chlorine, 

 and the substance declares itself at once as a 

 compound of sodium and chlorine a com- 

 pound for which the common name is salt. 



