Vol. XXII. No. 7.] 



POPULAR SCIENCE NEWS. 



101 



Prattttal C&emtetrp ant) tl}e SlttK. 



GAS -MAKING. 



The methods of artificial illumination have 

 kept pace with the modern improvements in 

 many other directions. The early settlers of 

 this country were obliged to carry on their 

 work or amusements during the long winter 

 evenings by the light of the resinous pine knot 

 or home-made tallow dip, both excellent de- 

 vices for producing a minimum of light and a 

 maximum of bad-smelling vapors. The intro- 

 duction of lard and whale-oil lamps was but a 

 slight improvement ; and not until the discovery 

 of petroleum, was it possible for the dweller in 

 the country to have his home made bright and 

 cheerful during the hours of darkness. About 

 the beginning of the present century it was 

 discovered that a brilliant illuminating gas 

 could be distilled from bituminous coal, and 

 the manufacture of this gas rapidly spread, 

 until at present there is hardly a city or large 

 town in the civilized world which is not pro- 

 vided with gas-works, which furnish this safe, 

 brilliant, and convenient light at a cost within 

 the reach of ever}' one. 



The first inventor of practical gas-lighting 

 was a Coruishman named William Murdock, 

 who in 1792 lighted his workshop with gas 

 obtained from coal,. although the principle was 

 suggested as earl^' as 1727. In 1812 the 

 streets of London were first lighted with gas ; 

 and since that time it has come into universal 

 use, although during the past few years it has 

 to a limited extent been superseded by the 

 more costly but in some respects more desir- 

 able electric light. It will be a long time, 

 however, before electricitj' can replace gas as 

 an illurainant for houses and detached build- 

 ings where expense is a consideration. 



The principle of gas-manufacture is very sim- 

 ple, but there are man}' practical details which 

 complicate the actual process. It depends 

 upon the fact, that, when bituminous coal con- 

 taining carbon and hydrogen is subjected to 

 heat in a retort, it is partiallj' decomposed, giv- 

 ing off gaseous hydrocarbons and other prod- 

 ucts, and leaving behind a mass of carbon 

 commonly known as colce. But many other 

 compounds are formed at the same time, which 

 must be separated from the gas before it is fit 

 to be sent into the pipes, and delivered to the 

 consumer. 



The products of the distillation of coal may 

 be divided roughl}' into four classes, — coke, 

 coal-tar, ammoniacal liquor, and illuminating 

 gas. Coke contains from ninety to ninety-five 

 per cent of pure carbon, the remainder being 

 ash and sulphide of iron. The ammoniacal 

 liquor contains at least five different salts of 

 ammonia. The coal-tar Contains about fort}' 

 diflferent liquid and solid hydrocarbons, and the 

 illuminating gas twenty-five or more gaseous 

 hydrocarbons, — a total of over seventy distinct 

 chemical substances, all produced by the dis- 

 tillation of a piece of soft coal. It must be 

 remembered that few or none of these exist by 

 themselves in the coal, but are all formed by 



the chemical reactions which take place during 

 the distillation. 



The retorts in which the ooal is distilled are 

 made either of cast-iron or fire-clay ; they are 

 about nine feet long and one and a half feet in 

 diameter. After being filled with coal, they 

 are raised to a red heat, or even higher. The 

 impure gas thus formed passes off through an 

 upright pipe, which connects with a horizontal 

 one partly filled with water, known as the 

 hydraulic main. The pipe from the retort 

 dips under the water, so that all the gas must 

 pass through it. The coarser and less volatile 

 impurities, including a large proportion of the 

 tar, are condensed in this main. The gas, 

 however, is still at a high temperature, and 

 very impure : so it is sent through a set of ver- 

 tical pipes connected with each other like a 

 steam-radiator, which are either exposed to 

 the air, or cooled artificially. Many of the 

 impurities are condensed and separated from 

 the gas in the cooler, but it is sometimes still 

 further purified "by being passed through large 

 iron vessels filled with lumps of coke over 

 which a stream of cold water is constantly 

 flowing. This still further purifies the gas, 

 especially by absorbing the ammoniacal com- 

 pounds ; but the offensive and dangerous sul- 

 phur compounds remain, and to remove these 

 the gas is sent through large iron boxes con- 

 taining movable trays covered with moist, 

 freshly slaked lime. The lime absorbs the 

 greater part of the compounds of sulphur, and 

 the gas is then ready to be stored in the large 

 holders, improperly called gasometers, and 

 from thence delivered to the customer. After 

 absorbing the impurities, the gas-lime, as it is 

 called, possesses a most horrible odor, and in 

 thickly settled localities its disposal without 

 creating a nuisance is an important and some- 

 times a difficult matter. A more effective and 

 simpler means of purifying gas is much to be 

 desired. 



What is known as water-gas has been sub- 

 stituted to some extent for that distilled from 

 coal. It is essentially a mixture of hydrogen 

 and carbonic oxide, and is formed by a curious 

 reaction which takes place when steam is 

 passed over red-hot coals. The steam is de- 

 composed, and hydrogen is set free, while the 

 oxygen unites with the carbon of the coal to 

 form carbonic oxide (CO). As this gas 

 burns with a non-luminous flame, it must be 

 "enriched" with light-giving hydrocarbons, 

 which are usually obtained from the distilla- 

 tion of crude petroleum. It is claimed that 

 this gas is much cheaper than that produced 

 by the old process ; but practical trials have 

 not as yet confirmed this, and the presence of 

 the highly poisonous carbonic oxide renders it 

 a much more dangerous agent to introduce into 

 dwellings. Numerous fatalities have occurred 

 from the accidental escape of this gas into 

 sleeping-rooms, and it has been clearly proved 

 that its poisonous properties greatly exceed 

 those of the common coal-gas. It has also a 

 less characteristic odor, thus preventing the 

 early discovery of an accidental escape or leak, 

 to say nothing of the increased danger to ' ' the 



man who blows out the gas," who is still to be 

 found in this country, as an eminent member of 

 Congress has recently exemplified. 



THE PINHOLE CAMERA. 



Having recently had occasion to take several 

 photographs, using a pinhole as a substitute for a 

 lens, I was much surprised at the distinctness of 

 the images. The resulting negative is, of course, 

 not as sharp, and a silver print from it would not 

 look quite as well, as if taken with a lens; but for 

 bromides, where the detail required is not so great 

 as for silver prints, very satisfactory results may be 

 obtained. I have recently taken a number of 

 photographs with pinholes of various sizes, and the 

 following facts have been deduced: (1) The dis- 

 tance from the hole to the plate may be as short as 

 desired, but should not exceed twelve inches. (2) 

 The shorter the distance, the better the definition. 

 (3) The size of the hole is regulated by the distance. 

 For a distance of twelve inches the best results are 

 obtained with a hole measuring three-hundredths 

 of an inch in diameter. If smaller than this, 

 the image is blurred by diffraction; if larger, the 

 image is likewise blurred. But for most purposes 

 where shorter foci are used, we may lay it down as 

 a rule that the aperture should in no case exceed 

 one-fiftieth of an inch, nor be much less than a 

 hundredth of an inch. As regards exposure with 

 a hundredth of an inch aperture and a focus of 

 three inches, on a sunny day with a rapid plate, 

 one should give about ten seconds. So that, al- 

 though the exposures are longer than with a lens, 

 it will be seen that they are by no means excessive. 



The pinhole may be made in a piece of black 

 paper, or in a piece of thin sheet metal, which 

 should afterward be thoroughly blackened. In 

 either case the burr nmst be carefully removed. A 

 simple method of avoiding the burr is to burn tlie 

 hole in paper with a red-hot needle. 



The advantages of the pinhole camera are: (1) 

 that, doing without the lens, one saves weiglit and 

 expense; (2) that one can take as wide an angle 

 as the camera will admit of — say, 120° on the 

 horizon, against 80° to 85° with a wide-angle lens; 

 (3) that all objects, near and far, will be in equally 

 good focus; (4) that one may suit the size of the 

 image to fit the plate, without changing one's point 

 of view ; (5) that one may take a view, if neces- 

 sary, directly toward the sun, as there is no trouble 

 from fogging caused by the sun illuminating the 

 surfaces of the lenses. Indeed, very satisfactory 

 photographs may be secured, showing the sun in 

 the picture. In this case, however, the sun takes 

 black instead of white, owing to the reversal caused 

 by over-exposure. Finally, while not advising pho- 

 tographers to throw away their lenses, and substi- 

 tute pinholes, I wish to call their attention to tlie 

 fact that the pinhole picture is not a thing to be 

 wholly despised, and that there may occur circum- 

 stances under which the pinhole may prove a very 

 useful auxiliary. 



The pinhole principle may also be used for 

 another purpose, more amusing, perhaps, than 

 artistic, which was first suggested to nie by Mr. J. 

 R. Edmands. Let us substitute for the lens a nar- 

 row vertical slit, about three inches long by one- 

 fiftieth of an inch wide, made by pasting two strips 

 of black paper side by side. About two inches be- 

 hind this arrange a horizontal slit of the same 

 dimensions; two inches behind this, place the sen- 

 sitive plate. The apparatus is analogous to two 

 cylindrical lenses of different foci placed at right 

 angles, but is more readily adjusted. If an exposure 

 is now made, we shall find every thing distorted to 

 twice the size horizontally that it is vertically. By 



