CHEMISTRY. 



/CHEMISTRY is a department of physical 

 V science, and therefore treats of the prop- 

 erties and changes of matter. 



As the division of physical science into various 

 departments depends more upon the various 

 methods which we use in observing the phe- 

 nomena of nature, than upon any real natural 

 distinction, it is impossible to give a logically 

 complete definition of any of these departments ; 

 and what now belongs to one, it may be found, 

 at some future time, more convenient to transfer 

 to another. We shall, therefore, neither quote 

 any of the numerous definitions of chemistry 

 already given, nor add one to the number, but 

 leave the reader to form an idea of its scope and 

 purpose from the outline of the facts and prin- 

 ciples of the science contained in the present 

 paper. 



Matter, whether as it occurs in nature, or as 

 modified by art, is obviously of various kinds, and 

 any specimen of matter that we may examine is 

 either all of one kind, or a mixture of several 

 different kinds. There are various means by 

 which we can determine whether a specimen of 

 matter is pure (that is, all of one kind) or not. 



We can often prove that a substance is a mix- 

 ture by simple inspection. Thus, any one who 

 looks at a piece of granite at once sees that it 

 consists, not of one substance, but of a mixture of 

 several, the particles of each being large enough 

 to be distinctly visible. The smaller the particles, 

 and the more intimate the mixture, the more 

 difficult does it become to distinguish it, by simply 

 looking at it, from a pure, single substance. 

 Where the eye cannot settle the question, we 

 must resort to other means. These may be 

 described generally as ways of ' taking samples.' 

 If samples taken in many different ways agree 

 with one another, and with the original substance, 

 we conclude that we have to deal with a pure 

 body ; if, on the contrary, the samples differ from 

 one another, we learn that the substance is a 

 mixture, and, further, find out how to separate it 

 into its several ingredients. We shall describe a 

 few of these ' tests of purity,' or ways of taking 

 samples. 



I. Elutriation. In the first case, we shall sup- 

 pose that we have a substance, no part of which 

 dissolves in water, and which consists of very 

 small particles of two or more bodies of different 

 specific gravity. The substance is reduced to a 

 fine powder, and stirred up with water. This is 

 left for a time at rest, and gradually a part of the 

 powder separates if heavier than water, as a 

 sediment at the bottom ; if lighter than water, as 

 a scum at the top. The scum is then removed, 

 or the muddy water poured off from the sediment 

 and again left at rest, when a further portion 

 separates ; and by repeating this process, we 

 obtain a series of samples differing from one 

 another in the rate of deposition. If the sub- 

 stance were all of one kind, these samples would 

 only differ from one another in the size of the 

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particles, the largest separating most rapidly ; but 

 if we have a mixture, we find that (supposing the 

 particles of the different ingredients to be of the 

 same average size) the particles which are denser 

 than water go down the faster the denser they are, 

 and the particles lighter than water rise to the 

 surface the faster the lighter they are, so that the 

 first sample differs in kind from the last. This 

 method of ' elutriation' is exemplified in the wash- 

 ing of ores ; the lighter clay or earth with which 

 the ore is mixed, falling slowly through the water, 

 is washed away as mud, while the heavier ore 

 falls quickly to the bottom of the vessel in which 

 the washing is performed, and is thus retained. 



2. Solution, Some substances, such as sugar, 

 salt, &c. dissolve in water ; others, such as sand, 

 charcoal, &c. do not. We can therefore detect 

 a mixture of a soluble and an insoluble substance, 

 and separate them from one another, by treating 

 the mixture with water until no more will dissolve : 

 we have then the insoluble substance left, and can 

 recover the soluble one from the liquid by evapor- 

 ating away the water. This process, sometimes 

 called ' lixiviation,' is used in extracting saltpetre 

 from earth containing it, in obtaining potash from 

 wood-ashes, and in many other important opera- 

 tions. Other liquids besides water may be used 

 for similar purposes, a liquid being selected which 

 will dissolve some of the ingredients in the mix- 

 ture and leave others undissolved. The solution 

 is separated from the insoluble residue, either by 

 allowing the latter to settle and then decanting, or 

 by filtration. 



3. Crystallisation. Substances soluble in water 

 require very various proportions of water to dis- 

 solve them ; thus 100 parts of water will, at the 

 ordinary temperature, dissolve 33 parts of Epsom 

 salts, 20 parts of washing-soda, 10 parts of bicar- 

 bonate of soda (common 'baking- soda'), and 

 about one-third part of plaster of Paris. If, then, 

 a mixture of soluble substances be dissolved in 

 water, and the water gradually evaporated, the 

 least soluble of these substances will separate 

 first ; and by collecting separately the successive 

 crops of crystals, we shall have a series of samples, 

 the first consisting almost exclusively of the least 

 soluble, the last of the most soluble. Such a 

 method of separation is seldom perfect, each crop 

 generally containing some admixture of the ingre- 

 dients prevailing in the crop before and in that 

 after it. It is in this way that common salt is 

 obtained from sea-water. Sea-water contains, 

 besides common salt, smaller quantities of various 

 other salts. Of these the least soluble is sulphate 

 of lime, and this is the first to appear on evapor- 

 ating the water ; the common salt crystallises 

 next ; and the ' mother-liquor,' as the solution left 

 after the common salt has been removed is called, 

 contains salts of magnesia, which are very soluble. 



4. Dialysis. If a solution of common salt be 

 securely tied up in a bladder, and the bladder be 

 hung in a vessel filled with pure water, it will be 

 found after a time that the water outside the 



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