CHEMISTRY. 



it yields samples identical with one another. 

 But if we weigh a piece of marble in a crucible, 

 the weight of which is known, and then raise it to 

 a bright red-heat, we find that, upon cooling, it 

 weighs less than it did before. If we continue 

 the heating until no further change of weight is 

 produced, we find that one hundred parts of marble 

 are reduced to fifty-six. Something must have 

 been lost in this process, and we are able, by 

 methods the details of which we cannot at pres- 

 ent describe to shew that this something is a 

 gas, or kind of air, which the marble gives off 

 when strongly heated. By appropriate means we 

 can catch this air, and shew that one hundred 

 parts of marble give off exactly forty-four parts 

 -of it by weight. What is left in the crucible is 

 not marble it differs entirely from it in appear- 

 ance and properties it is quicklime. We have 

 thus decomposed marble into two substances, 

 quicklime, and a gas, which its discoverer, Dr 

 Black, called ' fixed air,' because it had been in a 

 fixed or solid condition in the marble. We shall 

 .afterwards see what this gas is in the meantime 

 we shall call it fixed air. By heating marble we 

 have then obtained from it two different sub- 

 stances, lime and fixed air : but it will be at once 

 seen that this decomposition is quite another thing 

 from the separation of a mixture into ingredients. 

 Marble is not a mixture of lime and fixed air ; 

 it is a pure substance, having properties of its 

 own ; thus, lime and fixed air are both soluble in 

 water, marble is not ; marble is tasteless, while 

 both of the bodies formed from it have well- 

 marked tastes. The space occupied by a piece 

 of marble is not the same as that occupied by 

 the lime and fixed air which can be obtained 

 from it. On the other hand, the properties of the 

 ingredients of a mixture remain unchanged that 

 is, are the same in the separate and in the mixed 

 state ; and the space occupied by the mixture is 

 the same as that occupied by the ingredients 

 unmixed. 



A large number of substances can be directly 

 decomposed by the action of heat, light, or elec- 

 tricity, and thus proved to be compounds, as we 

 have seen is the case with marble ; and in many 

 cases we can reverse the process, and prove that a 

 substance is compound by forming it from its com- 

 ponents ; thus, quicklime and fixed air unite and 

 .form a substance identical in chemical characters 

 with marble.* 



By this process of ' synthesis ' (or putting to- 

 gether) we can often prove that substances which 

 .we are unable to decompose directly are really 

 compounds. Thus, quicklime cannot be decom- 

 posed without the addition of some other substance, 

 but we can prove it to be a compound by effecting 

 its synthesis. The metal calcium, which can be 

 .obtained from lime by a series of processes which 

 will be described in a later part of this paper, unites 

 with oxygen gas, and the resulting compound is 

 quicklime. The compound character of quick- 

 lime, and the nature of its components, are as 

 clearly proved by this synthesis as they could be 

 -by the direct decomposition of quicklime into 

 calcium and oxygen. There are, however, a con- 

 siderable number of substances (sixty-three are at 



* Sir James Hall has shewn that if this substance (amorphous or 

 Mttcrystailised carbonate of lime) is heated under great pressure, it 

 does not decompose, but fuses, and, on cooling, forms a substance in 

 . all respects the same as white marble. 



present known) which chemists have not as yet been 

 able either to decompose, or to produce by syn- 

 thesis. Such substances have, therefore, not been 

 proved to be compound, and are provisionally 

 styled 'elements,' or simple bodies. All com- 

 pounds are composed of two or more of these 

 elements united together; and compounds are 

 sometimes classified as binary, ternary, quater- 

 nary, &c. according as they consist of two, three, 

 four, &c. elements. 



A more generally useful system of classification 

 of pure substances is to arrange them in groups or 

 natural families according to their resemblances in 

 chemical action. This system will be best illus- 

 trated by means of examples, and for -this purpose 

 we shall select five groups, which include the most 

 important and useful substances, and the relations 

 of which exhibit most strikingly the nature of 

 chemical action. These five groups are (i) Acids, 

 (2) Bases, (3) Salts, (4) Metals, and (5) Salt- 

 radicals. 



As particular examples of these groups, we may 

 here mention the following substances (i) Acids : 

 Sulphuric Acid (oil of vitriol), Nitric Acid (aqua- 

 fortis), Hydrochloric Acid (spirit of salt), Acetic 

 Acid (vinegar). (2) Bases : Caustic Potash, Caustic 

 Soda, Lime, Magnesia, Oxide of Lead (litharge), 

 Ammonia (spirit of hartshorn). (3) Salts : Sulphate 

 of Soda (Glauber's salt), Sulphate of Magnesia 

 (Epsom salt), Chloride of Sodium (common salt), 

 Chloride of Ammonium (sal-ammoniac), Acetate 

 of Lead (sugar of lead), Nitrate of Potash (salt- 

 petre). (4) Metals : Potassium, Sodium, Calcium, 

 Magnesium, Zinc, Iron, Copper, Silver. (5) Salt- 

 radicals : Chlorine and Iodine. 



If we examine the acids above mentioned, we 

 find that they possess certain well-marked charac- 

 ters in common. They have all a sour taste; they 

 all act in the same way upon blue and purple 

 vegetable colours (such as the colouring-matter 

 of the violet, or of the red cabbage, or the colour- 

 ing-matter called litmus, obtained from certain 

 lichens), turning them red. These two characters 

 the sour taste, and the * acid reaction,' as the 

 reddening of vegetable blue colouring-matters is 

 called belong to most acids which are soluble in 

 water. The taste of the soluble bases is not so 

 generally characteristic, although many of them 

 have a peculiar so-called ' alkaline ' taste resem- 

 bling that of soap well marked in potash, soda, 

 and lime. The change which they produce upon 

 the above-mentioned vegetable colouring-matters 

 is exactly the reverse of that of the acids, and is 

 called the ' alkaline reaction.' Thus, if we take a 

 purple infusion of red cabbage, and add potash, 

 soda, lime, or ammonia to it, it becomes green ; if 

 to this green liquid we cautiously add an acid, it 

 becomes purple, and then red ; an addition of a 

 soluble base (alkali) to this red liquid renders it 

 first purple, then green. Similarly, blue litmus is 

 reddened by acids, and reddened litmus has its 

 blue colour restored by the addition of an alkali. 

 Another substance which is used as a test for the 

 alkaline reaction is the yellow colouring-matter of 

 the turmeric root this is rendered brown by 

 alkalies, and the yellow colour is restored on add- 

 ing an acid. These characters (taste and reaction) 

 belong to well-marked acids and bases which are 

 soluble in water; they do not belong to all acids 

 and bases. The distinguishing character of these 

 groups is their action upon each other. 



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