Vol. XXIII. No. ii.] 



POPULAR SCIENCE NEW8. 



163 



Practical Cljonjistry and tlie ^Irts. 



DISTILLATION. 



If we boil water and allow the steam to 

 strike against some cold body, or jjass through 

 a pipe surrounded by cold water, the steam is 

 condensed back again into water, and all the 

 solid impurities are left behind. Distilled 

 water, as this product is called, is largely 

 used in chemical laboratories, and is one of 

 the purest forms of water known, being abso- 

 lutely free from solid impurities, although 

 still .containing traces of any volatile sub- 

 stances that may have been present in the 

 original water. Every shower of rain is but 

 a repetition of this process of distillation in 

 Nature's laboratory. The heat of the sun 

 causes the water from the land and ocean to 

 rise as aqueous vapor into the higher and 

 colder regions of the air, where the vapor is 

 once more condensed into water, and falls 

 again to the surface of the earth, separated 

 from its impurities. It is on account of its 

 great purity that this naturally distilled water 

 is so wholesome to use, when stored in clean 

 cisterns and reservoirs. 



Not only water, but any volatile substance 

 can be distilled if heated to the temperature of 

 vaporization, and the process of distillation 

 thus becomes a most important one, both in 

 practical and technical chemistry. Sulphur, 

 mercury, and zinc are readily purified by 

 distillation, and it is only the excessively high 

 boiling-point of many other metals which 

 prevents the application of the same process 

 to them. Iron, for instance, can only be 

 volatilized in small quantities by the heat of 

 the electric arc, but in the atmosphere of the 

 sun the vapor of iron is certainly present in 

 immense quantities, and is probably being 

 continually condensed and re-volatilized, like 

 the aqueous vapor in the atmosphere of the 

 earth. If there are any inhabitants on the 

 sun, they must be so constituted as to be 

 proof against constant showers of molten iron. 



When two liquids of different boiling- 

 points — such as water and alcohol — are 

 mixed together, their separation by distilla- 

 tion is by no means as simple a matter as at 

 first appears. Water boils at 312° and alco- 

 hol at 141°, but if a mixture of alcohol and 

 water is heated, say to 145°, the vapors 

 formed do not consist entirely of alcohol, luit 

 of a mixture of alcohol and water, the pro- 

 portions depending upon the proportions in 

 the original liquid. It is evident, however, 

 that at the beginning of the distillation the 

 vapor that passes over will contain a greater 

 proportion of alcohol, while towards the last 

 the alcohol will nearly all be vaporized, and 

 only vapor of water will be formed. There- 

 fore, the first portions that are distilled over 

 are saved and re-distilled, until finally we can 

 obtain a liquid containing about 95 per cent, 

 of pure alcohol. If we desire a perfectly 



pure — or absolute alcohol, as it is called — we 

 must use chemical reagents having a strong 

 affinity for water, such as caustic lime, or 

 chloride of calcium. Absolute alcohol is but 

 verj' little used, principally in chemical labo- 

 ratories, and the ordinary alcohol of the arts 

 always consists of the weaker liquid. 



This process of separating two or more 

 liquids of different boiling-points is called 

 fractional distillation. One of its most iin- 

 portant practical uses is in the preparation of 

 alcohol from fermented liquors, known as 

 rectification, and many very ingenious stills 

 have been devised which automatically sepa- 

 rate the stronger from the weaker mixtures, 

 and give as the product of the final condensa- 

 tion, commercial alcohol. They all depend 

 upon the varying boiling-point of the different 

 mixtures, which is lower in proportion to the 

 percentage of pure alcohol present. 



Fractional distillation is often carried on in 

 the laboratory, especially in connection with 

 the study of organic chemistry. It is often 

 the onl}' available means oi separating two or 

 more liquids of different boiling-points, and, 

 if properly conducted, the mixture can be 

 divided into different liquids, or fractions, 

 the boiling-point of which will only vary 

 within a part of a degree from the beginning 

 to the end of their final distillation. It is, 

 however, a most tedious operation, as every 

 student of chemistry knows, and calls for the 

 exercise of considerable patience. 



The manufacture of kerosene from crude pe- 

 troleum is, in part, a process of fractional distil- 

 lation. Kerosene oil is not a simple body, but 

 a mixture of various hydrocai-bons, and the 

 quality and safety is indicated to a very great 

 degree by its boiling-point and the tempera- 

 ture at which it gives off" inflammable vapors. 



Destructive distillation is that where the 

 original substance is decomposed in the pro- 

 cess, and broken up into other forms. The 

 most common example is found in the manu- 

 facture of illuminating gas, where coal, upon 

 the application of heat, is changed into nearly 

 one hundred different volatile substances, 

 while a large proportion of the carbon 

 remains behind as the inorganic, non-volatile 

 coke. 



Distillation was one of the favorite pro- 

 cesses of the alchemists, and in their day the 

 retort, the still, and the condenser held a 

 much more important place in the laboratory 

 than they do at present. The volatility of 

 mercury was discovered at an early date, and 

 many were the experiments tried by the early 

 investigators to transform the peculiar liquid 

 metal into true silver or gold. One alchemist 

 is said to have distilled the same quantity of 

 mercury several himdred times, and it is a 

 pity that such perseverance should have 

 brought forth nothing but a few pounds of 

 very pure mercury ; but olten there was a 

 minute quantity of silver or even gold present 



as an impurity, and it is probable that they 

 sometimes succeeded in separating it, and 

 were thus encouraged to continue their re- 

 searches. But even at the present time we 

 do not know enough about the true nature of 

 the elements to say just why mercury differs 

 from silver. We can only say that so far we 

 have no knowledge which would lead us to 

 believe that one form of matter can ever be 

 transformed into the other. More than this 

 we must wait for future investigators to 

 determine. 



A NEW APPARATUS FOR THE 

 RECOMPOSrnON OF LIGHT. 



Every student of optics is familiar with 

 Newton's disk, which is painted with the dif- 

 ferent colors of the spectrum, in radial 

 sectors, and which, when quickly rotated, so 

 as to confuse the colors in the eve, appears of 

 a white color, — thus proving that white light 

 is really a mixture of all the different colors 

 of the spectrum. It is very difficult, how- 

 ever, to obtain the different colors in their 

 true purity and proportion, so that tlie revolv- 

 ing disk usually appears of "a dirty gray 

 instead of a pure white color. This difficulty 

 has led a French scientist, M. Poitevin, to 

 devise an apparatus by which the actual 

 spectral colors tliemselves are used, instead of 

 :ntificial paints or dyes. His apparatus (Fig. 

 2) is described and illustrated in I. a Nature 

 :is follows : 



It consists of a disk, A (Fig. i), which can 

 be made to rotate rapidly. Tw(5 narrow 

 slits, a b, are cut in tliis disk, before which 

 two direct vision prisms, C D and C' D', 

 are placed. The apparatus is placed before a 

 screen, and a beam of white light thrown 

 upon it, so as to pass through the prisms, 

 which decompose it in such a way that two 



Fig. I. 



spectral images appear upon the screen, 

 showing all the colors in their natural shades 

 and brilliancy, ;-, v, etc. Now if the 

 disk and prisms are rotated, the colored 

 sectors will move over the screen so rapidly 

 that the eye cannot follow them, the image in 

 one position remaining persistent in the eye 

 till another one takes its place. In this way, 

 all the colors of the spectrum are recombined 



