QUALITATIVE ANALYSIS.] 



CHEMISTRY. 



397 



which rings of platina wire are to be employed. The 

 flux is first melted so as to form a little bead, which will 

 fill up the ring. On this the oxide is placed, aud fused 

 with the flux. The oxides of cobalt, manganese, and 

 chromium are useful for initiatory experiments. In all 

 cases the heat should be employed gently at first, until 

 the required temperature be obtained. In the absence 

 of this precaution, charcoal supports and refractory 

 minerals will fly into pieces ; and it is by no means un- 

 likely that the first intimation of this result will be 

 feelingly conveyed to the operator by the lodgment of a 

 red-hot fragment in the eye, by which he will be forcibly 

 impressed with the necessity of care for the future. 



Presuming that the reader will have become ac- 

 quainted with the various processes involved in general 

 analysis, we shall proceed to give details of the methods 

 adopted in laboratory practice. 



QUALITATIVE ANALYSIS. 



Ix the following pages we shall not follow the usual 

 plan of giving directions for a systematic course, by 

 means of which every body which can possibly be met 

 with may be detected, but confine ourselves to instances 

 U will afford types of the methods to be employed. 

 'iose of our readers who may be desirous of entering 

 extensively into the subject, we recommend the works of 

 Frescnius, Parnell, and Rose ; in which may be found 

 directions for almost every analysis which can be under- 

 taken. We shall adapt our directions chiefly for such 

 who have never attempted to analyse, and so fit them 

 for a perusal of the works we have named, and for the 

 subsequent pr< locution of more elaborate researches. 



The external qualities of a body will often give a 

 general insight into the nature of its composition. The 

 value of this, however, will depend on the extent of 

 general information which the operator may possess. An 

 experienced chemist can often give an approximate idea 

 of the composition of Jxxlies by a few preliminary expe- 

 riments, or even by simple inspection. The physical 

 characters of specific gravity, colour, form, whether 

 crystalline or amorphous, suggest the connection of any 

 substance with others of a similar class. Thus, no one 

 having any acquaintance with mineralogy would mistake 

 quartz for Iceland spar ; for one U hard, crystallises in 

 prisms having six sides, will scratch glass, ic. ; whilst the 

 latter U soft, splits into rhomboids, and may therefore be 

 readily scratched by glass, iron, or almost any other 

 body. Metallic alloys, oxides, &c., are generally to be 

 distinguished by their specific gravity, lustre, and colour ; 

 and hence, for them, the usual tests for metals may be 

 employed. 



Almost every inorganic substance is composed of oxides 

 of metals proper, or those of the alkaline and terrigeneous 

 metals ; and maintaining the distinction incident to the 

 individual character of each class, the student may often 

 make a rough and tolerably accurate guess of the sub- 

 stance under examination. 



In illustrating the different methods of qualitative 

 examination, we shall take instances of well-known sub- 

 stances, progressing from such as afford comparatively 

 little difficulty, to others of a more complex character. 



Our first attempt shall be a qualitative analysis of a 

 piece of marble, by which . we shall show that it mainly 

 consists of carbonic acid and lime. The student will thus 

 learn to detect the presence of a gas and an earth, aud 

 so take a fir.it step in his analytical progress. 



' 82. Take a small piece of marble, of the 

 size of a pea ; and having reduced it into powder, boil it 

 for some ti-.ny in distilled water. Pour some of the 

 liquid on to a piece of clean window-glass, and hold it 

 over the spirit-lamp flame until the glass be dry. No 

 deposit will take place, because the marble is insoluble 

 in water. 



(a.) The next step is to find s .,me acid which will dis- 

 olve th " ! dilute hydro- 



chloric acid may be poured ,, tl,., powder, which should 

 be placed in a test Hize jj^ ^ 



should be added little by littl-, waking till all ollcrvt-i- 



1 cenee has ceased before more is poured into the tube. Tt 

 will be evident that a gas, which formed part of the 

 marble, is escaping ; but this we will consider hereafter. 



(&.) Having effected the solution, we must next test it, 

 and discover, if possible, what the solid substance which 

 has dissolved consists of. It is evident that that has 

 united with the hydrochloric acid, and the substance 

 with which it was previously in combination has been 

 removed. The very appearance and source of the marble 

 indicate that it belongs to the earths, and it must there- 

 fore consist of one of these. But as all the earths are 

 soluble in hydrochloric acid, we must add some test 

 which shall detect that really present ; and herein lies 

 the art of the analyst. If we add alkalies or their car- 

 bonates, in solution, we uiay have any of the earths pre- 

 cipitated as a white powder ; hence, for the present, they 

 will not assist us. But we find that sulphuric acid will 

 not precipitate all the earths, for the sulphate of mag- 

 nesia and alumina are soluble in water. We accordingly 

 add a little dilute sulphuric acid to the solution made in 

 a, and find a copious white precipitate tlirown down. 



(c.) The precipitate obtained in 6 may either be the 

 sulphate of baryta, strpntia, or lime ; and for all practical 

 purposes we may consider one of these to be present. A 

 portion of the precipitate may accordingly be taken, and 

 having been washed in a test-tube with distilled water, 

 the washings being thrown away, the powder is first to 

 be agitated in abundance of distilled water, and a little 

 of the liquid is then to be poured on a piece of clean 

 window-glass, or platina foil This is to be held over the 

 spirit-lamp until quite dry. If any deposit be effected, 

 it must either be sulphate of stroutia or sulphate of lime, 

 ise the sulphate of baryta is insoluble in water. 

 \Ve therefore conclude that the solid substance dissolved 

 in 6 must be one of the two former earths. 



(d.) As we may infer that either the sulphates of 

 strontia or lime alone are present, we add to the solution 

 obtained in c one of sulphate of lime, which may be 

 made by agitating that substance with plenty of cold 

 water, and filtering off the clear solution. A portion of 

 this being added to solution c, if a precipitate fall, we 

 know that it cannot be sulphate of lime, for that would 

 not be thrown down by a solution of itself. We there- 

 fore at once infer that strontia is present. For an 

 equally good reason, we are assured that strontia is not 

 present if no precipitate take place. In such a case we 

 conclude that lime alone could be present, and therefore 

 that earth must have been the solid matter dissolved in a, 

 by the hydrochloric acid. 



Having so far gained a knowledge of one of the con- 

 stituents of the marble, we proceed to examine the 

 nature of the gas given off whilst the solution was being 

 effected ; and this may easily be done as follows : 



(e.) Take a small piece of the marble, and put it into 

 a test-tube fitted with a cork Fig. 73. 



and bent tube, as represented 

 in the margin. .Add a little 

 dilute hydrochloric acid ; and 

 having fitted in the tube, allow 

 the gas which is given off to 

 bubble through some lime- 

 water. If no precipitate be 

 produced, then the gas is not 

 carbonic acid. If, however, a precipitate be thrown 

 down, then carbonic acid is certainly present, which will 

 be at once evident by the formation of chalk or car- 

 bonate of lime. 



It will thus be shown that the marble is chiefly com- 

 posed of two substances lime and carbonic acid : and 

 another step may be taken to verify the investigation. 



(/.) Heat a small piece of the marble in an open cru- 

 cible, or in an iron ladle, at a red heat for some time. 

 It will be found that the colour and appearance of the 

 remainder will have entirely changed a yellowish- 

 coloured substance will remain. If this be placed in a 

 few drops of water it will heat and fall to powder ; and, 

 in fact, will present every characteristic of the earth, 

 lime. 



The marble having thus afforded carbonic acid and 



