ANALYSIS SOLUTION, ETC.] 



UHE811STRY. 



305 



We shall first give some preliminary and practical 

 directions, and then proceed to point out a systematic 

 course of qualitative chemical analysis. 



Before commencing, it is well to provide a sufficiency 

 of glass vessels of various sorts, for the operations which 

 have to be pursued. And, to the young beginner, test- 

 tubes are of great value. He can, by meaus of them, 

 boil solutions, test them, and perform many operations 

 on the small scale. To commence with a few test- 

 tubes, Florence flasks, two or three evaporating dishes, 

 a spirit-lamp, pestle and mortar, some glass tubing, a 

 funnel, and some filtering-paper, are the chief re- 

 quisites. A book should be kept, in which every experi- 

 ment should be registered ; and small pieces of paper, 

 marked with the letters of the alphabet, and others with 

 the numerals, must also be at hand. Such are employed 

 to distinguish the results of the different tests. Thus, 

 one series of experiments may be called A ; whilst each 

 separate addition of a test in the series may be marked 

 1, 2, <fcc. , on the test-glasses. By such means the experi- 

 ments are classified ; and they should be registered after 

 the same order in the note-book. To illustrate this, 

 we may refer to our directions for arsenical analysis ;* 

 in which we supposed, that after the arsenious acid had 

 been brought into solution, three different methods, 

 viz., Marsh's, Reiusch's, and the liquid tests, hid 

 been employed. The original liquid aud the results 

 might have been marked as follows : 



- >hition of Arsenious Acid. Results. 



1. Marsh's test Arsenic deposited. 



2. Reinsch's do Metallic arsenic reduced. 



3. Liquid Precipitates afforded. 



By such a plan the progress of the analyst is mate- 

 rially assisted, errors are prevented, aud the results kept 

 for reference. It is also of great importance that the 

 notes of experiments should be made at the moment 

 the results are obtained. Young chemists, and even old 

 beginners, are so eager to arrive at the end of their 

 examinations, that they neglect these intermediate pre- 

 cautions. After they have thus proceeded for some time, 

 t'iuy become perfectly bewildered, and are quite at a loss 

 to distinguish one result from another, simply because 

 they will not take the trouble to note each stop as they 

 proceed. In most laboratories for young pupils, it would 

 be an advantage if the old fable of the tortoise and the 

 hare were printed in capitals anund the room. We 

 proceed to practical details. 



SOLUTION. The first step in an analysis is to bring the 

 body into solution, if it be not already in that state ; and 

 this may be effected in various ways. Some bodies are 

 iv;idily soluble in water ; and the substance, in that cose, 

 may be dissolved by that fluid alone. It must be remem- 

 bered, that most substances are more soluble in hot 

 liquid* than at ordinary temperatures ; and hence the 

 application of heat is required. Sometimes long boiling 

 is requisite ; and for such purposes Florence flasks auj 

 test-tubes may be employed. No more liquid should be 

 used than is absolutely necessary, because this increases 

 tho bulk of the solution, and also diminishes its strength 

 As the water evaporates, more should be added, or 

 otherwise a portion of the solid will be deposited on 

 cooling, and crystallisation may also take place. All 

 Ixxlies more soluble in hot than in cold water will 

 deposit some portion on cooling, if a hot saturated solu- 

 tion be made that is, when the hot liquid has dissolved 

 as much as it can. 



It is always best to reduce a solid substance into 

 powder, because, by that means, more particles are 

 brought into contact with the fluid in the same time. 

 The operation is thus facilitated. Fur this purpose, the 

 pestle and mortar are to be used ; the mass being first 

 broken up into fragments, a little of which at a time is 

 to be reduced to powder in the mortar. If too much 1m 

 employed at once, the pulverisation will be incomplete. 

 In pulverising some substances great difficulty is expe- 

 rienced ; but this can be overcome by various expedients. 

 Thus it is impossible to reduce camphor to a powder 



See ante, p. 3^2, el leg. 



by itself ; but the addition of a few drops of spirits of 

 wine renders the operation quite easy. Flints, again, 

 are difficult to pound ; but if they are heated red-hot, 

 and then quenched suddenly in cold water, their pul- 

 verisation may be readily performed. 



Some substances are quite insoluble in water ; and we 

 may here state that we are confining our remarks, for 

 the present, entirely to inorganic or mineral bodies. 

 With such, a different course of treatment must be 

 adopted. Take, for instance, a piece of chalk, which is 

 entirely insoluble in distilled water. If it be placed in 

 dilute sulphuric acid, no solution would be effected for 

 the purpose of analysis, because the insoluble sulphate 

 would be afforded. If, however, nitric or hydrochloric 

 acid were employed, a solution would be at once pro- 

 duced. But such a solution would differ considerably 

 from an aqueous one ; for, in the latter, the substance is 

 merely suspended as sugar dissolved in water ; which 

 may be restored, without chemical change, to its previous 

 solid state ; whilst a solution of chalk in nitric acid is 

 attended with decomposition, the nitrate of lime being 

 produced in place of the carbonate. Still, for all analyti- 

 cal purposes, the result is the same, so that the body be in 

 a state of solution at last. C'ay may be taken as another 

 example ; for all except its silicious portion may be dis- 

 solved by acids, with the aid of heat, and thus a solution 

 is afforded which can afterwards be conveniently tested. 



The mention of silicious matter such as sand, flint, 

 quartz, <kc. brings us to remark on a class of bodies 

 which are insoluble in all menstrua, with the exception of 

 hydro-fluoric acid ; and which, from its nature, forbids its 

 employment, except in very peculiar cases. Supposing 

 it were required to analyse a piece of flint, it would be 

 necessary to fuse it with a flux, by means of a red heat 

 at least. The plan to be adopted in such cases is as 

 follows : The body having been reduced to a fine powder, 

 is to be mixed in a mortar with three or four times its 

 weight of carbonate of potass, or soda. The mixture is 

 to be put into a Berlin porcelain, or, in rough experi- 

 ments, a Hessian or English crucible. It is then to be 

 exposed to a red heat for two or three hours. By this 

 process the silicious matter will forma soluble glass, and 

 so a solution will be afforded, which may be dealt with 

 in a manner we shall presently describe. 



The metals and their alloys are all more or less soluble 

 in some acid, but their ores have sometimes to be dis- 

 posed of differently. At times they must be roasted, to 

 drive off the combined sulphur, (fee. This is effected by 

 placing tho substance in a common crucible, and exposing 

 the contents to a red heat for some time. The residual 

 mass may then be generally dissolved in an acid or other 

 solvent. Some means may be occasionally tried, which 

 produce secondary but required results : thus, the lead 

 ore called galena is decomposed into sulphate of lead by 

 long boiling with nitric acid. 



Supposing solution to have been effected by any of 

 the foregoing means, or others, the next step is that of 

 examining or testing the solution, which affords either a 

 precipitate or a change of colour. In some cases, neitlier 

 of these results may follow. In this case, our next re- 

 source is crystallisation. Each of these cases we shall 

 proceed to examine. 



PRECIPITATION. When a solid body is removed by 

 means of a test from a solution, the substance so sepa- 

 rated, whether it fall down or not, is called a preci- 

 pitate, of which so many examples have already been 

 noticed. We shall not repeat here the numerous instruc- 

 tions already given on this point, but rather mention 

 minutiae of practice to which we have not yet alluded. 

 Some precipitates will not fall down or separate, owing 

 to the extreme fineness of their particles ; in such cases, 

 heat is to be applied to the vessel containing them, 

 which will generally cause the particles to coalesce. 

 During precipitation, the liquid ought to be constantly 

 stirred to assist the process ; and an abundance of fluid, 

 if not otherwise objectionable, is, generally speaking, an 

 advantage. If it bo desired to preserve the precipitate, 

 the contents of the vessel, after being well stirred, are 

 to be poured into a funnel holding filtering-paper. The 



