DISTILLATION, DESTRUCTIVE 57 



ture more dense than atmospheric air, and might be performed with any body forming 

 a dense vapour, such as mercury, iodine, zinc, &c. 



It has, however, practically been confined to the English process of refining zinc. 

 See ZINC. 



The two most remarkable cases in which the process of destructive distillation is 

 carried out on a manufacturing scale, are the dry distillation of wood, for the manu- 

 facture of wood charcoal, acetic acid, and pyroxilic spirit; and of coal, for the purpose 

 of obtaining coal-gas, and coke. This process will be found fully described in the 

 article on GAS, COAX. 



Distillation of Essential Oils or Essences. The separation of volatile flavouring oils 

 from plants, &c., by distillation with water, will be fully treated under another head. 

 See PERFUMERY ; ESSENCES. 



Fractional Distillation. A process for the separation of volatile organic substances 

 (such as oils) is very extensively employed in our naphtha works under this name. 



If we have two volatile bodies together, but differing appreciably in their boiling 

 points, we find, on submitting them to distillation in a retort, through the tubulature 

 of which a thermometer is fixed, so that its bulb dips into the liquid, that the tem- 

 perature remains constant (or nearly so) at the point at which the more volatile con- 

 stituent of the mixture boils, and the distillate consists chiefly of this more volatile 

 ingredient ; and only after nearly the whole of it has passed over, the temperature 

 rises to the point at which the less volatile body boils. Before this point has been 

 reached, the receiver is changed, and the second distillate collected apart. By sub- 

 mitting the first product to repeated redistillation, as long as its boiling-point remains 

 constant, the more volatile constituent of the mixture is ultimately obtained in a state 

 of absolute purity. See NAPHTHA. 



This method may in fact be adopted when the mixture contains several bodies ; 

 and by changing the receiver with each distinct rise of temperature, and repeating 

 the process several times, a fractional separation of the constituents of the mixture 

 may be effected. H. M. W. 



DXSTXXiIiATXOXr, DESTRUCTIVE. Organic matters may be divided into 

 two groups, founded on their capability of withstanding high temperatures without 

 undergoing molecular changes. Bodies that distil unchanged form the one, and those 

 which break up into new and simpler forms, the other. The manner in which heat 

 acts upon organic substances differs not only with the nature of the matters operated 

 upon, but also with the temperature employed. We shall study the subject under the 

 following heads : 



1. Apparatus for destructive distillation. 



2. Destructive distillation of vegetable matters. 



3. Destructive distillation of animal matters. 



4. Destructive distillation of acids. 



5. Destructive distillation of bases. 



6. General remarks. 



1. Apparatus for destructive distillation. Destructive distillation on a large scale is 

 most conveniently performed in the cast-iron retorts used in gas-works. Where quan- 

 tities of materials not exceeding fifteen or twenty pounds are to be operated on, for 

 the purpose of research, a more handy apparatus can be made from one of the stout 

 cast-iron pots sold at the iron wharves. They are semi-cylindrical, and have a broad 

 flange round the edge. The cover should be made to fit in the manner of a saucepan- 

 lid. The aperture by which the products of distillation are to be carried away should 

 be of good size, and the exit pipe must not rise too high above the top of the pot before 

 it turns down again. This is very essential in order to prevent the less volatile portion 

 of the distillate from condensing and falling back. The exit tube should conduct the 

 products to a receiver of considerable capacity, and of such a form as to enable the 

 solid and fluid portions of the distillate to be easily got at for the purpose of examin- 

 ation. From the last vessel another tube should conduct the more volatile products 

 to a good worm supplied with an ample stream of cold water. If it be intended to 

 examine the gaseous substances yielded by the substances under examination, the exit 

 pipe of the worm must be connected with another apparatus, the nature of which 

 must depend on the class of bodies which are expected to come over. If the most 

 volatile portions are expected to be basic, it will be proper to allow them to stream 

 through one or more Woulfe's bottles half filled with dilute hydrochloric acid. Any 

 very volatile hydrocarbons of the OH n family which escape may be arrested by 

 means of bromine water contained in another Woulfe's bottle. The pressure in the 

 Woulfe's bottles must be prevented from becoming too great, or the leakage between 

 the flange of the pot and its cover will be very considerable. The luting may consist 

 of finely-sifted Stourbridge clay, worked up with a little horse-dung. A few heavy 



