CHEMISTRY. 



of monad atoms is termed 'atomicity;' thus, the 

 ^atomicity of hydrogen is one ; of oxygen, two ; of 

 nitrogen, three; of carbon, four. More complex com- 

 pounds can be represented in a similar way, thus : 



, O , 



Ca=O O=C=O Ca C=O 



Lime. Carbonic Acid. Carbonate of Lime. 



Chemists are divided in opinion, as to whether an 

 element always has the same atomicity some 

 holding that this is the case ; while others consider 

 that an element may have one atomicity in one 

 set of compounds, and another in another set. 

 The latter school regard, for instance, sulphur as 

 dyad in sulphuretted hydrogen (H S H) and 

 analogous compounds, tetrad in anhydrous sul- 

 phurous acid (SO 2 ; O = S = O), and hexad in 



O 

 II 



anhydrous sulphuric acid (SO 3 ; O = S = O) 

 .and the sulphates, such as sulphate of potash 

 O 

 II 



(K a SO 4 ; K O S O K). For further de- 

 ll 



O 



velopments of the Theory of Atomicity, the reader 

 is referred to any of the larger treatises on chem- 

 istry. 



DULONG AND PETIT'S LAW. 



We have hitherto considered the atomic -weight of 

 an element as a number adopted merely with a view 

 to the simple, consistent, and convenient represen- 

 tation of the composition of the compounds of that 

 element by means of formulae. It has been found, 

 however, that the atomic weights so selected have 

 an important relation to some of the physical 

 properties of the elements. The relation of the 

 atomic weight to the specific heat (see NATURAL 

 PHILOSOPHY, p. 205) of the elements has been 

 most fully worked out, and will alone be con- 

 sidered here. This relation is expressed by means 

 of a proposition, called, from its discoverers, 

 Dulong and Petifs Law, This is, that the specific 

 heat of an element in the solid state is inversely 

 proportional to its atomic weight ; or, that the pro- 

 duct of the atomic weight and specific heat in the 

 solid state of an element is constant. This law 

 is not absolutely true. The specific heat in the 

 solid state of an element multiplied by its atomic 

 weight is a number which, in the case of the great 

 majority of elements, is 6-6, or very nearly this 

 number, and this enables us in some cases to fix 

 the atomic weight of an element where purely 

 chemical evidence is doubtful ; but as there are a 

 few exceptional cases, and as even in those in 

 which the product approximates to 6-6 the di- 

 vergence is considerable, this law can only be 

 regarded as an indication of a relation, not as an 

 exact statement of it ; it is, however, too important 

 to be altogether passed over in such a sketch of 

 chemical principles as we have here endeavoured 

 to give. 



We have now gone over the main points of 

 chemical principle : in order to give the reader a 

 general view of the science, it is now necessary 

 that we should append a short statement of chem- 

 ical facts, to supplement what has necessarily been 

 interwoven into the preceding parts of this paper. 



SHORT STATEMENT OF THE PROPERTIES OF THE 

 ELEMENTS AND THEIR COMPOUNDS, OF THE 

 METHODS OF PREPARING THEM, AND THEIR 

 ACTION UPON EACH OTHER. 



OXYGEN. Oxygen is a colourless, tasteless, 

 odourless gas ; its specific gravity (that of air 

 being taken as unity*) is 1-1056. It is slightly 

 soluble in water, 100 volumes of water, at ordi- 

 nary temperatures and pressure, dissolving about 

 3 volumes of oxygen. It has not been reduced to 

 the liquid state by any amount of pressure even at 

 the lowest temperature hitherto attained. 



It exists free in atmospheric air, of which it 

 constitutes, by volume, about 20-9 per cent. It 

 may be obtained pure 



1. By heating certain metallic oxides, such as 

 oxide of silver (Ag 2 O), oxide of mercury (HgO), 

 which simply decompose into oxygen and metal ; 

 or peroxide of manganese (MnO 2 ), peroxide of 

 barium (BaO 2 ), anhydrous chromic acid (CrO,), 

 which decompose into oxygen and an oxide of the 

 metal containing less oxygen than the original 

 substance peroxide of manganese yielding oxy- 

 gen and an oxide having the formula Mn 3 O 4 , con- 

 taining two-thirds of the oxygen in the peroxide 

 peroxide of barium and anhydrous chromic acid 

 losing half their oxygen, and leaving oxides having 

 the composition expressed by the formulae BaO 

 and Cr 2 O 3 respectively. 



2. By the decomposition by heat of certain 

 salts containing oxygen. The most practically 

 important of these are : (a) Chlorate of potash, 

 K 2 O,C1 2 O 6 or KC1O 3 this salt, when strongly 

 heated alone (or by the action of a more moderate 

 heat when mixed with a small quantity of peroxide 

 of manganese, red oxide of iron, or black oxide of 

 copper), decomposing into oxygen and chloride of 

 potassium (KC1), thus yielding all its oxygen as 

 oxygen gas ; (ff) Sulphate of zinc, ZnO,SO 3 or 

 ZnSO 4 , which, when heated, yields oxide of zinc 

 (ZnO), sulphurous acid gas (SO.,), and oxygen 

 gas. As sulphurous acid gas is readily soluble in 

 water, we can obtain the oxygen pure by washing 

 the mixed gas. 



3. By the electrolysis of water. See p. 311. 



4. By the action of the green parts of living 

 plants upon carbonic acid gas and water, under 

 the influence of sunlight. This action cannot be 

 conveniently used for the preparation of pure 

 oxygen, but it plays a most important part in the 

 economy of nature. See VEGETABLE PHYSIOLOGY. 



Oxygen is a supporter of combustion ; that is, it 

 enters into combination with combustible sub- 

 stances, and in doing so produces a great deal of 

 heat. The conditions under which such combus- 

 tions take place, and the products formed, will be 

 given under the heads of the combustible sub- 

 stances themselves. 



Oxygen is essential for the support of animal 

 life. In the process of respiration, oxygen is 

 absorbed by air-breathing animals in the lungs, by 

 water-breathing animals in the gills from the oxy- 

 gen dissolved in the water, in both cases the red 

 colouring-matter of the blood entering into a 

 feeble combination with oxygen. Oxygen is thus 

 carried by the blood to all parts of the body, where 



Throughout this paper, when it is not otherwise stated, the 

 specific gravity of a gas is referred to that of air as unity. 



319 



