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.