DISTILLATION (Lat. destillatia, from de stillare, to distil. from de, down + stillarc, to drop, from stilla. drop). A process consisting in the evaporation of liquids by boiling and the subsequent liquefaction of their vapors by cool ing. The purpose of distillation is to separate different snbstances from one another more or less completely. The process has been in use since ancient times, and in the first centuries of our era employed in much the same man ner as at the present time. In the third cen tury A was by distillation rendered fit for drinking, and it was known that prac tically pure warier may be isolated from wine and other liqui I mixtures by distillation. The al ( liemists obtained the mineral acids and other valliabh substances by processes of and at the present day distillation is one of the most potent proe(•sses in the hands of the scien tific and industrial chemist.
What usually takes place (luring the distilla tion of a mixture is very simple: The given liquid becomes divided into portions of unequal volatility, every portion that distils over being more volatile than the portion that still remains behind. And as less volatile liquids boil at higher temperatures than more volatile ones, it is obvious that the boiling-point of a distilling mixture must continually rise. If the mixture were allowed to evaporate at constant tempera ture its vapor-tension must continually decrease. A liquid whose vapo•-tension, during evapora tion, might increase, is an impossibility for al most obvious reasons. Such it liquid and its vapor would constitute a system whose volume would increase, and not if some com pressin• power was brought to act upon it: which is absurd. Further, by nexus of such a liquid, it would be easy to obtain a form of what physicists call 'perpetual motion of the second kind.' ivhich the science of therincidynainics reeo;_mizes as no less impossible than the •rea tion of niechanieal work out of nothing. Titt:axtonyx.‘xlics.) But \dine the vapor-ten sion of an evaporating mixture cannot liossildy it may, and in certain eases does, re main constant. Thus a mixture of three parts of water and 07 parts of absolute alcohol will evaporate without change of vapor-tension. The question therefore remains, what mixtures are subject to decrease of vapor-tension, and what mixtures evaporate Nvillimit much change. The answer is: If the ingredients are present in such i(roportions that the vapor-tension of the mix ture is either the highest or the lowest that can be obtained with the given substames, then the vaportension will rennin C011-42111t (Inning evapo ration; in (-very other ease it will decrease. If for a certain proportion of the ingredients the Napor tension of the mixture is lower than for proportion. the composition of the boiling liquid will approaeli nearer and nearer to that proportion as a limit onee that limit reached, the vapor-tension eaunot decrease any further, and thenceforward the composition of the distilling liquid must remain unehanged: the vapor passing off and the liquid remaining be hind will have precisely the same relative com positi•m. Again, if at a certain temperature a
giN en mixture has the highest vapor-tension that .can possibly be obtained with its ingredients, then a separation into more and less volatile por tions is evidently impossible, and hence the dis tillation can produce no change of composition. A mixture of 3 parts of water and 97 parts of absolute alcohol has, at its boiling-temperature, it higher vapor-tension than any other mixture of water alai alcohol at the same temperature; and this is why that mixture distils over with out ehange of composition.
From the above it may he seen that the pos sible changes of the total vapor-tension of a mixture determine in a general manner the di rection in which the ellallgeN Of composition will proceed in course (if a distillation. To recapitu late, distillation can effect a change in the composition of a given mixture only if both of the following conditions are ttihMleds ( I) If there exist mixtures of the same whose vapor-tensions are hss than that of the given mixture: (•) if there also exist mixtures of the saute substances, whose vapor-tensions are greater than that of the given mixture. Distilla tion will then divide the given mixture into. say, two portions, one of which will be more, the other less. volatile than the given mixture. But the exact course of a distillation is far from being determined by these laws. For example, no is thus furnished to the question: being given a mixture of a pound of water and a pound of alcohol. and supposing that half the mixture has distilled over, what is the composition of the distillate? The total vapor-tension of a mixture, it must be remembered. is the sum of the 'partial' xapor-tensions of its ingredients. The relative amounts of the ingredients passing over at any moment are proportional to their partial vapor tensions at that moment. The partial vapor tensions in a mixture are known to he lower than the vapor-tensions of the ingredients ill an isolated state. But what they are exactly is hut seldom known. To determine them experi mentally is a matter of considerable difficulty; and no theory is known which would permit of caleulating them in all ordinary cases. Hence our knowledge of the process of distillation is as yet extremely meagre. An empirical rule capable of yielding some practical results has been pro posed by Brown. and has been somewhat de veloped by Thomas. Barrell• and Young. But it is by no means sufficiently •staldkhed to require explanation here. The modern theory of solutions, too, has been brought to bear on the problem of distillation: but that theory permits of ealeulating the partial vapor-tensions only in mixtures containing a very large excess of one of the ingredients ('dilute solutions'), and Renee cannot furnish a sullieiently general solution of the problem of distillation.