The absence of the knowledge how to obtain a really gas-proof envelope is, no doubt, one of the greatest difficulties of airship construction. As has already been pointed out, the gas-holding quality of gold-beaters’ skin is remarkable. Its cost, however, is fairly prohibitive in the case of large airships. A material which is a combination of india-rubber and cotton surfaces is now generally used for large airships, but this has undoubted disadvantages. India-rubber is a substance which time, low temperature, and certain climatic conditions deteriorate. All those who have worked with india-rubber experimental _ballon-sondes_ (sounding balloons) can testify to its perishing qualities. Very much can be accomplished with a brand-new airship. Turned out of a factory it will retain its gas-holding qualities for a short time excellently. The lapse of time reveals deterioration and leakiness. Considering the extreme importance of a varnish that will retain pure hydrogen for a reasonable time, it is a matter of surprise that chemists should have almost entirely neglected its production. Mr. W. F. Reid alone of British chemists seems to have given any serious thought to the question. In a paper which Mr. Reid read before the Aëronautical Society of Great Britain, he made some exceedingly important suggestions in the way of obtaining balloon and airship varnishes. In case this little volume should fall into the hands of any chemists who may like to devote their powers of original research to the production of one missing link in airship construction, the following quotation from Mr. Reid’s remarks are appended below.