Another step was taken when Cross of London discovered that when the mercerized cotton was treated with carbon disulfide it was dissolved to a yellow liquid. This liquid contains the cellulose in solution as a cellulose xanthate and on acidifying or heating the cellulose is recovered in a hydrated form. If this yellow solution of cellulose is squirted out of tubes through extremely minute holes into acidulated water, each tiny stream becomes instantly solidified into a silky thread which may be spun and woven like that ejected from the spinneret of the silkworm. The origin of natural silk, if we think about it, rather detracts from the pleasure of wearing it, and if "he who needlessly sets foot upon a worm" is to be avoided as a friend we must hope that the advance of the artificial silk industry will be rapid enough to relieve us of the necessity of boiling thousands of baby worms in their cradles whenever we want silk stockings.
On a plain rush hurdle a silkworm lay
When a proud young princess came that way.
The haughty daughter of a lordly king
Threw a sidelong glance at the humble thing,
Little thinking she walked in pride
In the winding sheet where the silkworm died.
But so far we have not reached a stage where we can altogether dispense with the services of the silkworm. The viscose threads made by the process look as well as silk, but they are not so strong, especially when wet.
Besides the viscose method there are several other methods of getting cellulose into solution so that artificial fibers may be made from it. A strong solution of zinc chloride will serve and this process used to be employed for making the threads to be charred into carbon filaments for incandescent bulbs. Cellulose is also soluble in an ammoniacal solution of copper hydroxide. The liquid thus formed is squirted through a fine nozzle into a precipitating solution of caustic soda and glucose, which brings back the cellulose to its original form.
In the chapter on explosives I explained how cellulose treated with nitric acid in the presence of sulfuric acid was nitrated. The cellulose molecule having three hydroxyl (—OH) groups, can take up one, two or three nitrate groups (—ONO2). The higher nitrates are known as guncotton and form the basis of modern dynamite and smokeless powder. The lower nitrates, known as pyroxylin, are less explosive, although still very inflammable. All these nitrates are, like the original cellulose, insoluble in water, but unlike the original cellulose, soluble in a mixture of ether and alcohol. The solution is called collodion and is now in common use to spread a new skin over a wound. The great war might be traced back to Nobel's cut finger. Alfred Nobel was a Swedish chemist—and a pacifist. One day while working in the laboratory he cut his finger, as chemists are apt to do, and, again as chemists are apt to do, he dissolved some guncotton in ether-alcohol and swabbed it on the wound. At this point, however, his conduct diverges from the ordinary, for instead of standing idle, impatiently waving his hand in the air to dry the film as most people, including chemists, are apt to do, he put his mind on it and it occurred to him that this sticky stuff, slowly hardening to an elastic mass, might be just the thing he was hunting as an absorbent and solidifier of nitroglycerin. So instead of throwing away the extra collodion that he had made he mixed it with nitroglycerin and found that it set to a jelly. The "blasting gelatin" thus discovered proved to be so insensitive to shock that it could be safely transported or fired from a cannon. This was the first of the high explosives that have been the chief factor in modern warfare.
But on the whole, collodion has healed more wounds than it has caused besides being of infinite service to mankind otherwise. It has made modern photography possible, for the film we use in the camera and moving picture projector consists of a gelatin coating on a pyroxylin backing. If collodion is forced through fine glass tubes instead of through a slit, it comes out a thread instead of a film. If the collodion jet is run into a vat of cold water the ether and alcohol dissolve; if it is run into a chamber of warm air they evaporate. The thread of nitrated cellulose may be rendered less inflammable by taking out the nitrate groups by treatment with ammonium or calcium sulfide. This restores the original cellulose, but now it is an endless thread of any desired thickness, whereas the native fiber was in size and length adapted to the needs of the cottonseed instead of the needs of man. The old motto, "If you want a thing done the way you want it you must do it yourself," explains why the chemist has been called in to supplement the work of nature in catering to human wants.
Instead of nitric acid we may use strong acetic acid to dissolve the cotton. The resulting cellulose acetates are less inflammable than the nitrates, but they are more brittle and more expensive. Motion picture films made from them can be used in any hall without the necessity of imprisoning the operator in a fire-proof box where if anything happens he can burn up all by himself without disturbing the audience. The cellulose acetates are being used for auto goggles and gas masks as well as for windows in leather curtains and transparent coverings for index cards. A new use that has lately become important is the varnishing of aeroplane wings, as it does not readily absorb water or catch fire and makes the cloth taut and air-tight. Aeroplane wings can be made of cellulose acetate sheets as transparent as those of a dragon-fly and not easy to see against the sky.
The nitrates, sulfates and acetates are the salts or esters of the respective acids, but recently true ethers or oxides of cellulose have been prepared that may prove still better since they contain no acid radicle and are neutral and stable.
These are in brief the chief processes for making what is commonly but quite improperly called "artificial silk." They are not the same substance as silkworm silk and ought not to be—though they sometimes are—sold as such. They are none of them as strong as the silk fiber when wet, although if I should venture to say which of the various makes weakens the most on wetting I should get myself into trouble. I will only say that if you have a grudge against some fisherman give him a fly line of artificial silk, 'most any kind.
The nitrate process was discovered by Count Hilaire de Chardonnet while he was at the Polytechnic School of Paris, and he devoted his life and his fortune trying to perfect it. Samples of the artificial silk were exhibited at the Paris Exposition in 1889 and two years later he started a factory at Basançon. In 1892, Cross and Bevan, English chemists, discovered the viscose or xanthate process, and later the acetate process. But although all four of these processes were invented in France and England, Germany reaped most benefit from the new industry, which was bringing into that country $6,000,000 a year before the war. The largest producer in the world was the Vereinigte Glanzstoff-Fabriken of Elberfeld, which was paying annual dividends of 34 per cent. in 1914.