794 



ORGANIC ANALYSIS. 



which combines the application of moderate 

 heat with Leslie's method, already described, 

 may be advantageously employed, though the 

 manipulation is ralher complicated and trouble- 

 some. In a counterpoised retort which will 

 sustain exhaustion, a given weight of the sub- 

 stance to be dried is placed and connected with 

 a tubulated receiver containing oil of vitriol, by 

 a sound cork secured on the exterior with several 

 folds of bladder, well soaked before applying it. 

 Through the tubulure of the receiver passes a 

 small glass tube. This junction likewise is ren- 

 dered air-tight, with a cork and bladder. The 

 tube, about an inch from thetubulure, is, previous 

 to its insertion, drawn out and narrowed to a 

 capillary bore, so that at pleasure it may be easily 

 drawn off and sealed by a jet of flame from the 

 blowpipe. Matters being thus arranged, the 

 tube from the receiver is united by a connecter 

 of caoutchouc with another tube, and this again 

 with the air-pump, and exhaustion is performed. 

 When a sufficient vacuum has been produced, 

 the whole is allowed to stand for an hour. If, 

 at the end of that time, the mercury in the 

 gauge retain its level, the apparatus is air-tight 

 and may be detached from the pump by seal- 

 ing the tube proceeding from the receiver in 

 its capillary portion. We may now apply a 

 gentle heat to the bulb of the retort by means 

 of a water bath, or otherwise, and can cool the 

 receiver. Great caution is of course requisite 

 in handling the exhausted vessels, as the 

 slightest abrasion of the surface might cause 

 fracture. 



In some cases, as in drying blood, a tempe- 

 rature of 230 may be safely used by employing 

 a boiling solution of Rochelle salt as the exte- 

 rior bath ; and in the analysis of the bile a heat 

 of even 260 is recommended by Berzelius. 

 Where sugar or urea is present, even a heat of 

 200 is injurious, and must therefore be avoided. 

 The operation we are now considering appears 

 one of the simplest that the chemist can have 

 to perform, but I have been induced to dwell 

 the longer upon it as it is one from which, 

 without great care, more mistakes arise than 

 from any other, owing to the pertinacity with 

 which water adheres to most organic substances. 

 The temperature attainable in an open basin 

 over the water bath is much lower than we 

 should, a priori, have been led to imagine. 

 I found, for example, that the temperature of 

 some wheat flour thus drying in an open basin 

 was only 144 F., whilst the water in the bath 

 continued steadily at 196 F. When the basin 

 was covered with a piece of paper, a tempe- 

 rature of 161 F. was the highest attained, 

 while a thermometer placed in the water of 

 the bath stood at 194 F. With liquids eva- 

 porating it rises somewhat higher ; when plain 

 water was evaporated it stood at 164, the water 

 in the bath being 208 ; and in the case of a 

 viscid fluid like yeast, it varied between 176 

 and 180, while the bath raised a thermometer 

 inserted in it to 210. 



It is therefore desirable to have an appa- 

 ratus in which we can ensure any given tempe- 

 rature from 212 upwards. For this purpose 

 Liebig has contrived a kind of hot-water oven, 



consisting of a double box of copper ; in the 

 interval between the outer and inner walls, 

 water, saline solution?, or oil, may be poured 

 and heated in the usual way. In one side of 

 this box is a door which may be closed when 

 necessary. The interior chamber and its con- 

 tents can thus be maintained with certainty at 

 the same temperature as that of the fluid around 

 them. The best plan of proceeding consists in 

 evaporating liquids to apparent dryness upon 

 the open water bath ; and afterwards subjecting 

 the solid residue, when a temperature of 212 

 is not injurious, to complete desiccation in 

 Liebig's oven. So long as the material under 

 examination loses weight, the application of 

 heat must be continued. 



Capsules of Wedgewood ware or Berlin 

 porcelain are indispensable, and one or two 

 small platinum dishes will be found most valu- 

 able, especially in the evaporation of albu- 

 minous fluids, as the dry residue adheres so 

 strongly to the glaze of earthen vessels that a 

 portion of the basin is invariably removed 

 along with the animal matter, which thus ac- 

 quires an undue increase of weight, and the 

 surface of the vessel becomes rough and unfit 

 for use from the difficulty of cleaning it. The 

 Berlin porcelain crucibles are excellent vessels 

 for evaporation, as, being fitted with covers, the 

 dry residue may be preserved from absorbing 

 moisture during the operation of weighing, by 

 exposure to air. It may be worthy of no- 

 tice that adhering organic substances may be 

 removed from the surface of vessels in which 

 they have been kept, by digestion in concen- 

 trated nitric or sulphuric acid, or else by strong 

 solution of potash. 



Incineration of the dry residue is accom- 

 plished by taking a counterpoised porcelain or 

 platinum capsule with a determinate quantity, 

 say 10 or 12 grains, of the material to be burned, 

 and heating it over a circular wicked spirit-lamp 

 until the ash completely loses its black colour. 

 The capsule should at first be covered to pre- 

 vent loss by dispersion on the first application 

 of heat; when visible fumes cease to arise, the 

 cover may be removed to allow freer access of 

 air; as, however, the temperature rises higher 

 when the vessel is covered, it will often be 

 found advantageous to leave it loosely covered 

 and maintain a steady heat ; sufficient air finds 

 access to consume the carbonaceous matter. 

 Sometimes the ash may be stirred carefully with 

 a platinum wire in order to expose it more fully 

 to the air. When the ash contains alkaline 

 phosphates, the last traces of carbon are burned 

 off with difficulty, as the phosphates fuse and 

 protect the unburned particles from the further 

 action of the air. This inconvenience may be 

 overcome by moistening the residue (after the 

 capsule has been allowed to cool) with a few 

 drops of nitric acid, and again igniting, repeat- 

 ing this manoeuvre as often as may be neces- 

 sary. A new difficulty, however, arises when 

 chlorides are present, as is almost always the 

 case ; for at a high temperature these salts are 

 decomposed by nitric acid, and the residue 

 therefore appears to contain less chlorine than 

 is really combined with it. 



