ORGANIC ANALYSIS. 



815 



within three-quarters of an inch of the large 

 end, the chloride broken into fragments about 

 the size of peas, is put, and a loose piece of 

 cotton-wool, occupying another quarter of an 

 inch, thrust in ; the opening is then closed by 

 a coik, through which passes a bit of straight 

 tube, rather larger in the bore than that at- 

 tached to the bulb, and projecting about an 

 inch outside the whole; the cork is trimmed 

 close to the large tube and covered neatly with 

 melted sealing-wax; and, lastly, air is drawn 

 through the apparatus by the mouth to ascer- 

 tain that no obstruction exists. It is now ready 

 for use : after two or three experiments the 

 chloride-should be renewed, or there will be a 

 danger of the gases being imperfectly dried. 

 The tube when not in use must always be 

 placed in a rack with the bulb end uppermost 

 to prevent the loss of any small pieces of chlo- 

 ride. Concentrated sulphuric acid may be 

 advantageously substituted for chloride of cal- 

 cium in the drying tube. In this case the 

 tube is filled with fragments of pumice-stone; 

 these are moistened with the oil of vitriol, and 

 the apparatus is fitted up as usual, excepting 

 that the employment of the cotton-wool is dis- 

 pensed with. 



The combustion tube is then prepared by 

 selecting a sound elastic cork, which is made 

 accurately to close the mouth of the tube ; it is 

 pierced with a round file, and fitted firmly 

 upon the fine tube proceeding from the bulb 

 of the drying tube ; the cork is then well 

 dried on the sand-bath, and forms the medium 

 of connection between the retort and drying 

 tube. 



The potash apparatus is one invented by 

 Liebig, represented in Jig. 430, e. It consists 

 of a fine but stout tube, upon which is a series 

 of bulbs, three in the middle horizontal part 

 of the instrument, and one on each of the 

 ascending limbs; one of the latter bulbs is 

 made considerably larger than the other; the 

 apparatus is filled by adjusting a suction tube 

 to one of the openings, and exhausting by the 

 cheeks until a certain measured quantity of 

 solution of potash has entered ; when the 

 liquid fills each of the three lower bulbs rather 

 more than three-quarters of their capacity, suf- 

 ficient has been introduced. 



The solution of potash employed has a 

 sp. gr. of from 1.25 to 1.27, and must be 

 renewed for every experiment ; the portions 

 that have been used may be put aside, and 

 afterwards, when sufficient has been collected, 

 may again be rendered caustic in the usual 

 way by quicklime. 



The compound commonly used for supply- 

 ing oxygen to the substance burned is oxide 

 of copper, which readily imparts oxygen to 

 combustible matter in contact with it, but 

 bears a very high temperature per se without 

 decomposition. It is best procured by dis- 

 solving copper in pure nitric acid, evaporating 

 to dryness and decomposing the nitrate by 

 heating it strongly in an earthen crucible. Ig- 

 nition is kept up till red fumes cease to appear; 

 if the heat be too great the oxide becomes 

 agglutinated, and requires strong pounding in 



an iron mortar to pulverize it ; it however, in 

 this dense state, is much less hygroscopic, and 

 therefore belter adapted for the purposes of 

 analysis. The powdered oxide is sifted through 

 a fine copper sieve, and secured in stoppered 

 glass bottles. 



Immediately before each analytical ope- 

 ration a sufficient quantity of the oxide is 

 ignited, and while still hot transferred to a dry 

 tube, by plunging the mouth of the tube into 

 the oxide in the crucible and then shaking it 

 in piecemeal. The tube with the oxide is 

 immediately closed with a dry cork, and al- 

 lowed to cool. Meantime the interior of the 

 retort is completely dried by heating each por- 

 tion of it in succession in the flame of a spirit- 

 lamp, beginning at the closed end, and draw- 

 ing air through the heated tube, by means of 

 a narrower tube passed down just beyond the 

 heated part and exhausting by the mouth ; 

 when every part has thus been dried, the 

 retort is corked and allowed to cool. Five 

 or six grains of the powdered and dried sub- 

 stance are put into a perfectly dry test tube, 

 and the whole is very accurately weighed ; its 

 contents are then added to the oxide of copper 

 in the mortar and the empty tube again 

 weighed ; the difference gives the weight of the 

 substance employed. 



The best kind of mortar is one of Wedge- 

 wood ware or Berlin porcelain, capable of con- 

 taining about half a pint, with a pestle com- 

 posed of a single piece of the same material ; 

 it should be thoroughly dried and well warmed. 

 Much caution is requisite in charging the re- 

 tort. The warm dry mortar is placed on a sheet 

 of glazed paper and first cleared out with a 

 little of the dried oxide of copper, which is 

 put aside. Oxide to the depth of an inch is 

 poured into the combustion tube ; a small 

 quantity of oxide is put into the mortar, then 

 the substance to be analysed, then more oxide ; 

 the mixture must be made quickly and care- 

 fully, adding so much oxide as shall be suf- 

 ficient to fill a little more than half the retort; 

 the mortar is then taken in the palm of the left 

 hand and the mixture introduced, carefully 

 picking it up piecemeal by the retort tube it- 

 self; fresh portions of oxide are rubbed in the 

 mortar to clear out the last traces, and the 

 retort is then filled up with pure oxide of cop- 

 per to within two inches of the extremity. 



The proportions of the mixture are repre- 

 sented in Jig. 430 : the portion from the tail of 

 the tube to the letter a consists of pure oxide 

 of copper, from to b of the mixture, from 

 b to c of the rinsings of the mortar, and from 

 c to within an inch of the cork is again pure 

 oxide. 



If the process of mixing has occupied much 

 time, it may be advisable to subject the tube 

 and its contents to a further operation to re- 

 move any traces of moisture that may have 

 been absorbed. The tube is struck smartly in 

 a horizontal position on the table, to clear the 

 tail-like prolongation, and to make an air-way 

 above the oxide from end to end ; an exhaust- 

 ing syringe made fast to the table by a screw- 

 vice or other convenient means, is attached to a 



