DISTILLATION 



49 



is preferable to employ a tubulated retort; and if a powdered solid is to be mixed with 

 a fluid, it is preferable to introduce the fluid first. 



Heat may be applied to the retort either by the argand gas flame, as in jig. 609, or 

 a water-, oil-, or sand-bath may bo employed. 



In distilling various substances, e. g., sulphuric acid, great inconvenience is ex- 

 perienced, and even danger incurred, by the phenomenon termed ' bumping.' This 

 consists in the accumulation of large bubbles of vapour at the bottom of the liquid, 

 which bursting cause a forcible expulsion of the liquid from the retort. It is pre- 

 vented by the introduction of a few angular fragments of solid matter of such a nature 

 as not to be acted upon by the liquid which is to be distilled. Nothing answers this 

 purpose better than a piece of platinum foil cut into a fringe, or even a coil of plati- 

 num wire introduced into the cold liquid before the distillation is commenced. Even 

 with this precaution the distillation of sulphuric acid, which it is often desirable to 

 perform for the purpose of its purification, is not unattended with difficulty and 

 danger. 



Dr. Mohr suggests the following method : A glass retort of about two pounds 

 capacity, is placed on a cylinder of sheet iron in the centre of a small iron furnace, 

 while its neck protrudes through an opening in the side of the furnace (fig. 611). 

 Ignited charcoal is placed round the cylinder, without being allowed to come in contact 

 with the glass, and a current of hot air is thus made to play on all parts of the retort, 

 excepting the bottom, which is protected by its support. There is a valve in the flue 



612 



of the furnace for regulating the draught, and three small doors in the cupola or head, 

 for supplying fresh fuel on every side, and for observing the progress of the distil- 

 lation. 



Instead of the sheet-iron cylinder a Hessian crucible may be employed, and this, if 

 requisite, elevated by placing it on a brick. If the vapour be very readily condensed, 

 nothing more is necessary than to insert the extremity of the retort into a glass receiver 

 as in fig. 609. 



If a more efficient condensing arrangement be requisite, nothing is more convenient 

 for use on the small scale than a Liebig's condenser, shown in fig. 612. It consists 

 simply ^of a long glass tube into which the neck of the retort is fitted, and the opposite 

 extremity of which passes into the mouth of the receiver ; round this tube is fitted 

 another either of glass or metal, and between the two a current of water is made to 

 flow, entering at a and passing out at b. The temperature of this water may bo 

 lowered to any required degree by putting ice into the reservoir c, or by dissolving 

 salt in it. (See FREEZING.) 



Even on the small scale it is sometimes necessary to employ distillatory apparatus 

 constructed of other materials besides glass. 



Earthenware retorts are now constructed of very convenient sizes and shapes. 

 There is one kind which is very useful when it is required to pass a gas into the 

 retort at the same time that the distillation is going on, as in the preparation of 

 chloride of aluminium, &c. which has a tube passing down into it also made of 

 earthenware, as in fig. 613. The closest are of Wedgwood ware, but a common clay 

 retort may be made impermeable to gases, by washing the surface with a solution of 

 borax, then carofully drying and heating it. 



VOL. H. 



1 Mohr and Redwood's 'Practical Phannacj.' 

 E 



