Aircraft in war

by Eric Stuart Bruce

It has, however, been pointed out by the writer—and the suggestion elicited the keen interest of the late Professor Langley—that if electricity could be used as the motive power in an airship the Severo system could be reasonably revived. Then the electric motors could be inside the gas-bag. There, electric sparks and electric heating could do no harm. For it is only the borderland that is the place of danger, where there are oxygen atoms to combine with the hydrogen atoms. In the case of a balloon filled with gas it is surprising to what short distance the danger zone extends. In the case of the writer’s electric signalling balloons, on one occasion the ladder framework which supported the incandescent lamps was being hauled up into the balloon. Through some fault in the connections there was sparking at the framework just as it had passed over the dangerous borderland. The sparks went on with safety. An inch or two lower and there would have been an explosion!

But on account of the weight of the battery the practical application of electricity for propelling navigable balloons seems to be as far off as it was in the days of “La France,” and in airships we have to continue placing the screws in the wrong place.

5. Difficulties of making gas envelope gas-proof.

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.

Varnishes may be divided into two classes—those in which the film solidifies or “dries” by absorption of oxygen from the air, and those in which the varnish “sets” by the evaporation of a volatile solvent in which the solid ingredients have been dissolved. To the first class belong the drying oils, chiefly linseed oil, for, although there are a number of “drying” oils, but two or three of them are used commercially in the manufacture of varnishes. When exposed to the air, especially in warm weather, linseed oil absorbs oxygen and forms an elastic translucent mass termed by Mulder “linoxyn.” This linoxyn has completely lost its oily nature, does not soil the fingers, and is, next to india-rubber, one of the most elastic substances known. It possesses but little tensile strength, however, and can be crumbled between the fingers. It forms the basis of all linseed oil paint films, and is largely used in the manufacture of linoleum. Linoxyn, however, is not, as Mulder supposed, the final product of the oxidation of linseed oil. When exposed to the air it is still further oxidised, and then forms a sticky, viscid mass, of the consistency of treacle and of an acid reaction. This latter property is of importance because it is due to it that fabrics impregnated with linseed oil so soon become rotten. In order to hasten the oxidation of linseed oil it is usually heated with a small quantity of a lead or manganese compound, and is then ready for use. No method of preparation can prevent the super-oxidation of linseed oil, but experience has indicated two ways of diminishing the evil effects so far as paints and varnishes are concerned. The first is to mix the oil with substances of a basic character or with which the acid product of oxidation can combine. In the case of paints, white lead or zinc oxide are chiefly used for this purpose. The other method consists in mixing with the oil a gum resin which renders the film harder and prevents liquefaction. Such a mixture of linseed oil and Kauri gum forms an elastic, tough mass, which is much more durable than the linoxyn alone, and also possesses greater tensile strength. During oxidation the linseed oil absorbs about 12 per cent. of its weight of oxygen, and when the area exposed is very large in proportion to the weight of the oil the temperature may rise until the mass catches fire. At a high temperature the super-oxidation of the oil takes place more rapidly than in the winter, and I have seen fabrics that had only been impregnated with an oil varnish for a month cemented together in one sticky mass, and, of course, completely ruined. When the linseed oil is thickened by the addition of a gum resin, it is too thick for direct application, and is thinned down with a solvent, usually turpentine or a mixture of this with light petroleum. Many resins and gum resins are used in the manufacture of varnishes in conjunction with linseed oil, but none of them can deprive the oil of the defect referred to, and if used in too large a proportion they become too brittle for balloon purposes. Both scientific investigation and practical experience show that any varnish containing linseed oil must be looked upon with suspicion by the aëronaut, in spite of the glowing testimonials some manufacturers are always ready to give their own goods.

When we consider those varnishes which are solutions and which do not depend upon oxidation for their drying properties we enter upon a very wide field.

Practically any substance that is soluble in a neutral solvent and leaves an impermeable film on drying is included in this class. One of the simplest examples is gelatine in its various forms, with water as a solvent. Until recently glue or gelatine would have been useless for our purpose on account of its ready solubility in water, but now that we are able to render it insoluble by means of chromic acid or formaldehyde it comes within the limits of practical applicability. A fabric may be rendered almost impermeable to gas when coated on the inside with insoluble gelatine, and on the outside with a waterproof varnish. Animal membranes are far less permeable to gases than fabrics coated with varnishes of the usual kinds. A balloon of gold-beaters’ skin, if carefully constructed, will retain hydrogen gas for a long time, and if treated with gelatine that is afterwards rendered insoluble it becomes practically impermeable. Fabrics treated with linseed oil varnish, on the other hand, allow gas to pass with comparative ease. This is not a question of porosity or “pinholes,” as is sometimes imagined, but a property inherent to the material. Hydrogen or coal gas is absorbed on the one side of the film and given off on the other in the same way as carbonic oxide will pass through cast iron. An inert gas, such as nitrogen, does not appear to diffuse in this way, even when there is a considerable difference in pressure between the two sides of the film. Such a varnished fabric transmits hydrogen readily, but retains nitrogen, and is perfectly watertight. In filling up the interstices of a fabric composed of cellulose the most obvious substance to use would be cellulose itself, but until recently solutions of this kind were difficult to obtain. Toy balloons have long been made of collodion, and are fairly satisfactory, but a cotton fabric impregnated with pure collodion becomes hard and even brittle. Celluloid solution, which is collodion with camphor and a small quantity of castor oil, is more flexible, but, probably on account of the camphor, is more permeable to hydrogen than collodion. A variety of collodion known as flexile collodion is a solution of collodion cotton with a slight addition of castor oil, and is much to be preferred to any of the preceding forms. In using it great care must be taken to exclude moisture, as the presence of this renders the film opaque, in which case it is always more or less porous. A substance allied to collodion is velvril material, composed of collodion cotton and nitrated castor oil. It is tough and flexible, even in thick films, and gives a good coating to paper or cotton fabric. Unless very carefully prepared, however, acid products may be generated from the decomposition of the nitro-compounds present, in which case the strength of the fabric would suffer. Another form of cellulose in solution is viscous, which forms a good coating when applied in a very thin layer, but makes the fabric harsh and brittle if used in excess. The solutions of this substance do not keep well and are liable to spontaneous decomposition.

The difference in flexibility between thin and thick films of the same materials is very considerable.

Given an elastic, supple cement, such as is afforded by concentrated solutions of some of the above-mentioned substances, it is quite possible to cement a tough, close-grained paper to a cotton fabric of open mesh, and the compound material thus produced is much more easily rendered impermeable than the fine cotton fabric now used. An extremely tough paper made from silk, a recent invention of T. Oishi, a Japanese manufacturer, would be specially useful for such a purpose....