816 



ORGANIC ANALYSIS. 



long tube filled with chloride of calcium ; and 

 this drying apparatus is fitted by a sound cork 

 to the retort tube. This latter is laid in a shal- 

 low trough open at one end, which is slightly 

 elevated ; the trough is then filled with sand 

 heated to about 212 F., and cautious exhaus- 

 tion is performed by the syringe, taking care 

 that none of the charge is carried out of the 

 tube by the current of air; on gradually open- 

 ing the stop-cock air is slowly re-admitted, 

 being dried in its passage over the chloride of 

 calcium ; it is allowed to remain in the appa- 

 ratus a few seconds, and the exhaustion re- 

 peated ; these operations are performed in suc- 

 cession ten or twelve times. It is, however, 

 rarely necessary to resort to this process of 

 desiccation, and it is objectionable from the 

 ease with which many compounds rich in hy- 

 drogen decompose the oxide of copper at com- 

 paratively low temperatures. 



The drying tube having been accurately 

 weighed is next fitted to the dried perforated 

 cork, and connected by it air-tight to the retort 

 tube; this is now placed in the furnace, which 

 has been disposed in a convenient place rest- 

 ing on bricks ; to the drying tube the potash 

 apparatus, also previously weighed, is attached 

 by a connecting piece of caoutchouc, taking 

 care that the largest bulb is on the arm con- 

 nected with the drying tube ; the potash ap- 

 paratus should be slightly inclined by placing 

 a cork under the end of the horizontal portion 

 nearest the open extremity. Matters being 

 thus arranged, we proceed to ascertain if the 

 whole be tight, and for this purpose expand 

 the air in the large bulb by heat, so as to ex- 

 pel a few bubbles ; if, on cooling, the liquid 

 rise in the limb and maintain its elevation 

 steadily for a few minutes, the combustion may 

 safely be begun. Charcoal broken into pieces 

 about the size of a walnut is ignited in a cru- 

 cible furnace or by any other convenient 

 means, and when red-hot applied to the por- 

 tion of the tube nearest the cork where the 

 pure oxide of copper lies ; the action of the 

 heat is limited by a double sheet-iron screen 

 which fits into the furnace, and has a central 

 slit which allows it to bestride the tube; this 

 screen can by degrees be moved further and 

 further down the furnace until the whole tube 

 is heated. An additional screen of single iron 

 plate is hung over the closed end of the fur- 

 nace to protect the cork, which usually should 

 reach to within an inch of the fire, care being 

 taken that the heat never rises so high as to 

 scorch it, or falls so low as to allow of the 

 condensation of moisture in the portion of the 

 retort which projects from the furnace. 



When the first part of the retort is red-hot 

 and the escape of air from expansion has 

 ceased, about an inch more of it may be 

 heated, and so the fire gradually carried 

 down; about three bubbles of air may pass in 

 two seconds, it is better not to attempt a more 

 rapid disengagement. At first but a small 

 portion of the gas is absorbed, but when the 

 substance is fairly undergoing decomposition, 

 and the atmospheric air in the apparatus has 

 been expelled, it is almost entirely taken up by 



the potash-ley. When the whole tube is ig- 

 nited the heat must be continued till bubbles 

 are no longer disengaged ; the potash-ley will 

 now gradually recede into the large bulb ; 

 when this is observed to commence, the char- 

 coal must be removed from the tail of the tube; 

 and as soon as the potash has risen to fill half 

 the large bulb, the tip of the tail must be 

 nipped off, and over the opened extremity a 

 tube about eighteen inches long, and one quarter 

 in diameter, should be supported ; gentle suc- 

 tion is then effected by a sucking tube (Jig. 430, 

 D) fitted to the free extremity of the potash 

 apparatus, drawing air through the combustion 

 tube to displace the carbonic acid and-aqueous 

 vapour it contains. The use of the long tube 

 over the end of the retort is to supply pure air, 

 and to prevent that from the furnace charged 

 with carbonic acid from passing freely into 

 the apparatus : the actual process of combus- 

 tion performed in the manner above described 

 usually occupies from an hour to an hour and 

 a half. 



The plan of drawing air through the tube is 

 that practised by Liebig, and it admits of con- 

 siderable accuracy. Dumas, however, con- 

 nects the extremity of the retort with a drying 

 tube, and this again with a receiver containing 

 oxygen, which gas is carefully driven over the 

 contents of the tube. This renders the ope- 

 ration somewhat more complicated, but it is 

 unquestionably more exact, especially in com- 

 pounds where the proportion of carbon is great. 

 The tube for supplying oxygen is easily ad- 

 justed to the retort by drawing out the tail 

 horizontally instead of obliquely, and fitting it 

 on by a caoutchouc connector, care being taken 

 to screen the junction from the influence of 

 heat. 



The apparatus is now dismounted, and the 

 whole allowed to cool ; in about an hour the 

 drying tube may be weighed, and the increase 

 of weight carefully noted ; one-ninth of the 

 gain indicates the quantity of hydrogen the 

 substance contained ; the potash apparatus is 

 also weighed, and three-elevenths of what it 

 has gained shews the quantity of carbon. The 

 deficiency is oxygen. 



The oxide of copper used in these expe- 

 riments may again be rendered serviceable by 

 moistening it with nitric acid, and igniting as 

 before. 



2. Analysis of a liquid not containing ni- 

 trogen. 



If the fluid be volatile, we take a piece of 

 tube rather less than a quarter of an inch dia- 

 meter, heat it in the blowpipe flame, and draw 

 on it a capillary portion about four inches 

 long; about a quarter of an inch below this 

 the tube is sealed ; the little piece of tube thus 

 left connected with the capillary part is heated 

 and blown into a small bulb about as big as a 

 good-sized pea; this is cut off, leaving a ca- 

 pillary neck of about two inches long. Having 

 made a sufficient number of these little bulbs, 

 we take two of them, which we have ascer- 

 tained will freely enter the combustion-tube, 

 and weigh them accurately ; a little of the liquid 

 to be analysed is put into a small tube, and the 



