DISTILLATION, DESTRUCTIVE 59 



and alkalis, sometimes yields products, the relation of which to the parent substance 

 can be clearly made out. This holds more especially in the case of organic acids ; the 

 buses too often yield such complex results, that the decomposition cannot be expressed 

 by an equation giving an account of all the products. We shall study a few cases 

 separately. 



4. Destructive distillation of acids. The destructive distillation of acids takes place 

 in a totally different manner, according as we have a base present, or the operation is 

 carried on without any addition. Many if distilled per se undergo a very simple re- 

 action, consisting in the elimination of carbonic acid, and the formation of a pyroacid. 

 But if an excess of base be present, the decomposition often results in the formation 

 of a ketone. We shall offer a few examples of these decompositions. Gallic acid, 

 heated to about 419 Fahr.,is decomposed into pyrogallic and carbonic acids, thus : 



C U H 6 10 = C 12 H 6 G + 2C0 3 



Gallic acid. Pyrogallic acid. 

 C 7 H 6 O 5 = C 6 H 6 O 3 + CO 2 . 



There are cases in which the action of heat upon organic acids results in the for- 

 mation of two substances, not produced simultaneously, but in two epochs or stages. 

 In reactions like this, the first, effect is the removal of two equivalents of carbonic 

 acid, and by submitting the resulting acid to heat again, two more are separated. 

 Under these circumstances, it is the second which is generally called the pyroacid. As 

 an example, we will take meconic acid, which breaks up in the manner seen in the 

 annexed equations : 



C 14 H 4 U = C 12 H 4 10 + 2C0 2 C 12 H 4 10 = C 10 H 4 8 + 2C0 2 



Meconic acid. Comenic acid. Comcnic acid. Pyroineconic acid. 



C 7 H 4 O 7 = C 6 H 4 O 5 + CO 2 C*H 4 O 5 = C 5 H 4 O 3 + CO 2 . 



It will be seen that the hydrogen remains unaffected. Perhaps the name pyroco- 

 menic acid would be preferable to pyromeconic acid, inasmuch as it is derived from 

 comenic acid in the same manner as pyrogallic from gallic acid. 



But pyroacids are not always derived from the parent acid by the mere elimination 

 of carbonic acid ; thus mucic acid, in passing into pyromucic acid, loses two equivalents 

 of carbonic acid, and six equivalents of water, thus : 



2C0 2 + 6HO 



Mucic acid. Pyromucic acid. 



C 6 H io O 8 _, C s H 4 O 3 + CO 2 + 3H 2 O. 



It does not invariably happen that the destructive distillation of acids per se results 

 in the formation of a pyroacid ; the disruption is sometimes more profound, the products 

 being numerous and somewhat complex. Let us take, as an illustration, a case where all 

 the results can ba reduced to an equation. Oxalic acid, when heated in a retort with- 

 out addition, yields water, oxide of carbon, carbonic and formic acids, in accordance 

 with the annexed equation : 



4(C 2 3 ,HO) = 4 CO 2 + 2 CO 2 + 2 HO + C 2 H0 3 ,HO 



Oxalic acid. Formic acid. 



2 i C-H-0 4 ) = 2 CO 2 + CO + H-0 + CH-O-. 



The admixture of sand, pulverised pumice-stone, or any other inert substance in a 

 state of fine division, often remarkably assists in rendering the decomposition more 

 easy and definite. Thus, if pure sand be mixed with oxalic acid, the quantity of formic 

 acid is so increased, that the process is sometimes employed in the laboratory as a 

 means of affording a pure and tolerably strong acid. 



We have said that the destructive distillation of acids proceeds in a very different 

 manner according as we operate upon the acid itself, or a salt of the acid. The dis- 

 tillation of the pure salt yields different products to those which are obtained when 

 the salt or dry acid is mixed with a large excess of a dry base (such as quicklime), 

 before the application of heat. If, in the former mode of proceeding, two atoms of 

 the acid are decomposed, yielding a body containing for four volumes of vapour, 

 the elements of two atoms of carbonic acid and two of water less than the parent acid, 

 such body is called a ketone. Thus when two atoms of acetate of lime are distilled, 

 the products are one atom of acetone and two of carbonic acid. Of course the carbonic 

 acid combines with the lime, thus : 



