36 



THE POPULAR EDUCATOR. 



LESSONS IN CHEMISTRY. II. 



ELEMENTARY SUBSTANCES METALLOIDS METALS SALT 



ACID ALKALI BASE CHEMICAL NOMENCLATURE. 



ELEMENTARY SUBSTANCES. Seeing that "atoms" are indi- 

 visible, and that cohesion binds them into masses, whilst affinity 

 joins different atoms to form new substances, it naturally follows 

 that all bodies are either simple or compound. Simple or elemen- 

 tary substances cannot be split up into others which essentially 

 differ from the original body, whilst compound substances can. 



In the example of chemical combinations given in the last 

 lesson, the sulphur and copper were simple substances or 

 " elements," and the sulphide of copper, which was formed on 

 the application of heat, was a " compound " body. No efforts 

 of the chemist have proved sufficient to split up sulphur or 

 copper. They defy all power to alter or change them ; but, by 

 using certain means, the sulphide of copper can be made to 

 resolve itself once more into its components copper and sulphur. 

 The constitution of a body may be determined by two means : 

 either by analysis, which is separating a body into its compo- 

 nents or elements, or by synthesis, which is the putting together 

 of the components to form the body. 



By analysing the substances of which our earth is composed, 

 Bixty-three elements have been discovered ; and in after years, 

 when we possess more powerful means of analysis, we may find 

 that some of these " elements " are capable of further division. 

 The elements are usually divided into forty-nine metals and 

 fourteen non-metallic substances or metalloids. The distinction 

 between these two classes is not very satisfactory. The metals 

 are opaque, they possess metallic lustre, and are good conduc- 

 tors of heat and electricity. The metalloids either want these 

 properties or possess them only in a very low degree. This 

 division is not very accurate, for iodine and carbon, though 

 metalloids, both have metallic lustre; and the latter, in the 

 form of plumbago or graphite, is a good conductor of electricity. 



For the sake of convenience and shortness in writing, symbols 

 are used for the elements. These consist of the first or the 

 first and most characteristic letter of the Latin name of the 

 body. In the following table, the metals printed in italics are 

 of rare occurrence : 



THE METALLOIDS. 



The numbers in the third column are the combining weights, or, 

 as they are sometimes called, the equivalent numbers, or atomic 

 weights. A chemical compound is made up of molecules. Each 

 molecule, as we have seen, is composed of atoms, and each atom 

 is indivisible ; therefore, whatever proportion the atoms bear to 

 each other in the molecule whether in number, or weight, or 

 volume they will have that same proportion in the mass. 



For instance, if we analyse 18 grains of water, we shall find 

 that 16 grains are oxygen and 2 grains are hydrogen, but the 

 molecule of water is composed of 2 atoms of hydrogen and 1 of 

 oxygen ; therefore we conclude that one atom of oxygen weighs 

 16 times the atom of hydrogen, and in the 2 grains of hydrogen 

 there will be double the number of atoms that there are in the 

 16 grains of oxygen, because such is the case in the molecule. 



Because hydrogen is the lightest of all known bodies, its atom 

 is taken as the standard, and by careful analyses the compara- 

 tive weights of all other atoms have been determined. The 

 French take the equivalent of oxygen 100 as their standard. 



It is to be remembered, then, that the atomic weight of 

 any element signifies the relation in weight which the atom of 

 that element bears to an atom of hydrogen ; and since chemical 

 compounds are formed by the union of atoms, the atomic weight 

 will also represent the weight in which the element will enter 

 into combination. Sometimes one, two, or many atoms enter 

 into the compound, so that whatever may be the quantity of the 

 element, it must always be a multiple of the atomic weight; 

 hence the name combining weight. And it very frequently 

 happens that one element in a compound is replaced by another, 

 one atom taking the place of another, the weight of one atom 

 being equivalent to the weight of the other ; hence the name 

 equivalent number. The student will soon become familiar with 

 these laws of combination. In writing compounds, the symbols 

 of the elements are placed side by side, and the number of atoms, 

 if more than one, written beneath. 



CuO = copper and oxygen combined, to form the oxide of copper. 



H.,0 = water ; 2 atoms of hydrogen and 1 of 0. 



2NH 3 = 2 atoms of ammonia ; that is, 2 atoms of N and 6 of H. 



A salt is a compound of an acid and a base. 



An acid is a body which usually possesses a sour taste, and 

 will redden litmus paper and vegetable blues. Formerly it was 

 supposed to owe its properties to the presence of oxygen, but it 

 has been found that an acid can exist without that gas, but 

 hydrogen must always be present. Hence, an acid is now 

 defined to be a salt of hydrogen. 



An alkali neutralises an acid, and returns the blue colour to 

 the reddened litmus. Potash, soda, and ammonia are the chief 

 alkalies. 



A base is a body which will combine with an acid to form a salt. 



