Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/281

DISTILLATION 203 oily, or resinous portion ("tar ); and 3d, a gaseous por tion. The " tar-water " is the one, of all the four products, of which the qualitative composition most directly depends on the nature of the material distilled. In the case of wood it has an acid reaction, from the presence in it of acetic acid, which id associated (amongst many other things) with acetone and methyl alcohol. In the case of coal it is alkaline, from ammonia, present as carbonate, sulphide, sulphocyanide, and in other forms. Alcohols and oxygenated acids are absent. The " tar " is a complex mixture of carbon com pounds, all combustible, but, although all directly derived from a vapour, not by any means all of them volatile. (.Regarding the components, see TAR.) The quantity and quality of the tar naturally depend on the kind of material used, but perhaps yet more on the mode in which the dis tillation is conducted. Thus, for instance, a coal tar pro duced at low temperature contains a considerable per centage of paraffins. If, on the other hand, the dis tillation is conducted at a high temperature, the paraffins are almost absent, while the proportion of benzols con siderably increases. A similar remark applies to the gaseous portion, as will readily be understood when we say that all volatile tar constituents, when passed through red hot tubes, are decomposed with formation of hydrogen and gaseous hydrocarbons, which hitter again, when submitted to the same operation, are all liable to undergo dissociation into simpler compounds and associa tion into more complex. DISTILLATION OF WATER. The continual interchange and circulation cf water, between oceans and other great reservoirs of water on the one hand and dry land on the other, may be regarded as a process of distillation. Rain is thus a form of distilled water ; and when it falls through a pure atmosphere it is found to possess the softness and freedom from dissolved salts characteristic of water artificially distilled. Rain water, however, absorbs a considerable proportion of air and some carbonic acid from the air, and also frequently contains ammonia, salts, and free acids. Water of that purity which can be secured only by dis tillation is of indispensable value in many operations both of scientific and industrial chemistry. The apparatus and process for distilling ordinary water are very simple. The body of the still is made of copper, with a head and worm, or condensing apparatus, either of copper or tin. The first portion of the distillate brings over the gases dissolved in the water, ammonia, and other volatile impurities, and is consequently rejected, and scarcely two-fifths of the entire quantity of water can be with safety used as pure distilled water. Among the innumerable schemes which have been pro posed for the production of a potable fresh water from the salt water of the ocean, two or three dependent on simul taneous distillation and aeration have been found, in practice, to produce most satisfactory results. Of course the simple distillation of sea water, and the production thereby of a certain proportion of chemically fresh water, is a very simple problem ; but it is found that water which is merely evaporated and recondensed has a very disagree able empyreumatic odour, and a most repulsive fiat taste, and it is only after long exposure to pure atmospheric air, with continued agitation, or repeated pouring from one vessel to another, that it becomes sufficiently aerated to lose its unpleasant taste and smell and become drinkable. The water, moreover, till it is saturated with gases, readily absorbs noxious vapours to which it may be exposed. For the successful preparation of potable water from sea water, therefore, the following conditions, are essential : 1st, aeration of the distilled product so that it may be immedi ately available for drinking purposes ; 2d, economy of coal to obtain the maximum of water with the minimum expenditure of fuel ; and 3d, simplicity of working parts, to secure the apparatus from breaking down, and enable unskilled attendants to work it with safety. Among the forms of apparatus which have most fully satisfied these conditions are the inventions of Dr Normandy and oi Chaplin of Glasgow. While these have met with most acceptance in the United Kingdom, the apparatus of Rocher of Nantes, and that patented by Gall6 and Mazelirie of Havre, have been highly appreciated by French maritime authorities. Normandy s apparatus, while leaving nothing to be desired in point of economy of fuel and quality of water produced, is very complex in its structure, consisting of very numer ous working parts, with elaborate arrangements of pipes, cocks, and other fittings. It is consequently expensive, and requires for its working the careful attention of an ex perienced workman. It consists of three essential parts, in addition to any convenient form of boiler from which steam under a certain amount of pressure may be obtained. These parts are called respectively the evaporator, the con denser, and the refrigerator. These are all closed cylindrical vessels, permeated internally with sheaves of pipes, through which pipes the steam generated percolates, condenses, and is aerated as explained below. The refrigerator is a horizontal vessel above which the condenser and the evaporator are placed in a vertical position. When the apparatus is in operation the refrigerator and condenser are filled with sea water, and a constant current is main tained which enters by the refrigerator, passes upwards through the condenser, and is discharged by an overflow pipe at a level a little above the top of the condenser. The evaporator is filled only to about two-thirds of its height with water fiom the condenser, and the admission and regulation of its contents are governed by a stop-cock on the pipe communicating between the two vessels. The vessels being so prepared, superheated steam is admitted by a pipe leading from the boiler into the top of the evaporator, and, passing through the sheaf of pipes immersed in water, is there condensed. The condensed water passes direct from the evaporator into the pipes of the refrigerator, in which it is cooled to the temperature of the surrounding sea water. Here then is produced pure distilled but non-aerated water ; and the means by which it is aerated and rendered fit for immediate use may be now traced. The superheated steam in permeating the pipes in the evaporator heats and vaporizes a portion of the water around them. The steam so generated passes into the sheaf of pipes in the condenser, in which, as already explained, a current of water is constantly rising and pass ing away by the overflow pipe. The condensation of the steam within the pipes, again, communicates a high temperature to the upper stratum of water in the condenser. As water at a temperature of 54 5 C. parts with its dis solved air and carbonic acid gas, a stream of water is con tinually rising to the upper part of the condenser at a temperature more than sufficient to liberate these gases, and by means of a pipe these pass over into the upper part of the evaporator, and there mingle with and supersaturate the steam generated in that vessel. Instead, therefore, of it being simply steam which passes from the evaporator to the tubes of the condenser, it is a mixture of steam and gases, the latter being in sufficient quantity not only to supersaturate the steam with which they are mixed, but also fully to aerate the condensed steam which passed direct from the evaporator into the refrigerator. The super-aerated condensed steam passes from the pipes in the

condenser into those in the refrigerator, where it meets the