out the ideal distribution of floating effect so as to prevent submergence of the mouth of the wearer. The properties of the fibre causing this are in the main structural. The fibre canal holds a relatively large volume of air: the smooth contour of the fibre and the resilience of the air-filled tube give a large interstitial (air) volume of the mass even under considerable compression.
In a general survey of the fibre industries there is no disturb- ance of their fundamental perspectives nor any radical changes in their many-sided technology to record during 1910-21. The developments of the artificial (celluloses) fibres was rather im- peded under the stress of war conditions, though without preju- dice to the financial prosperity of the leading manufacturing corporations. There is a new manufacture of an artificial silk from the cellulose acetate of the British Cellulose Co. (Dreyfus processes). This product, as a cellulose ester, has certain prop- erties e.g. lower specific gravity, with a water-resistant qual- ity which are points of superiority in relation to the cellu- lose " artificials." On the other hand, it is of lower tensile strength, of inferior dyeing capacity, and its cost of production is higher. Its production therefore is limited in scale.
In raw fibrous materials for the paper-making industry, there have been developments in the production of cotton-seed lint, and the prep- aration of pulp from the bamboo, and of a concentrated fibre (quar-- ter-stuff) from the papyrus (Cyperus). (See E. de Segundo, " Re- sidual Fibres from Cotton Seed," Jour. R. Soc. Arts, Feb. 1919; C. F. Cross, "Cellulose Industries," ibid. 1920 [Cantor Lectures]; W. Raitt, " Paper Supplies from India," ibid. May 1921.)
Note should be made of an investigation by W. L. Balls of the ultimate structure of the cotton fibre. By a chemical reaction which induces a controlled distention (by hydration) of the cell wall, without structural distention, the dimensions of the structure are exaggerated to a large multiple which brings into evidence a series of concentric rings which are the daily growth rings of the hair or fibre (Proc. R. S., B., vol. xc. 1919). (C. F. C.)
FILON, (PIERRE MARIE) AUGUSTIN (1841-1916), French man of letters (see 10.345), died at Croydon May 13 1916. In 1910 he published a short biography of Marie Stuart, and in 1911 L'Angleterre d'Edouard VII. and a dramatic poem Shakespeare amoureux. His Souvenirs et documents, relating to his former pupil, the Prince Imperial, appeared in 1914.
FILTER-PASSING GERMS. The discovery by Pasteur of the significance of microbic life in the phenomena of fermentation, putrefaction and disease and the development by Koch of an appropriate technique for the new science of bacteriology had already led in a comparatively few years to the determination of the causation of many infectious diseases of man and animals. There remained, however, a number of diseases of man and animals and amongst these some common maladies such as typhus, measles, smallpox, foot and mouth disease, swine fever, rabies and cattle plague in which the cause had not been dis- coverable by the methods of microscopical examination and cultivation which proved successful in so many cases. It was suggested by Pasteur, who searched in vain for the infective agent of rabies, that some microbes were too small to be visible with the optical apparatus at disposal. There is no reason a priori to suppose that the lower limit of size of microbic organisms should be of a dimension at present discernible, and the question raised by Pasteur was answered ten years later by the discovery made by Loeffler in 1898 that the virus of foot and mouth disease was invisible. The limit of visibility of a particle is in the last instance conditioned by the wave length of light. With the best modern microscopes employing white light of which the average wave length is 0-55/1 (n T^ira mm.) this limit is rather less than o-2/i. If ultra-violet light of half this wave length is used, photo- graphs of objects of about o-i/x in diameter can be made. The existence of particles of much smaller size can be demonstrated by the method of dark-ground illumination (Tyndall phenome- non), when they appear as bright points. The limit appears to depend upon the intensity of illumination and with direct sun- light is 0-004/1..
Invisibility of a microbe commonly handicaps every effort at its isolation, propagation and identification, but does not render them impossible of achievement, for the existence of a living virus in an optically clear liquid may still be demonstrated by
its power to infect an animal or plant, or produce recognizable chemical changes in a medium. The existence of these ultra- microscopic viruses was brought to light unexpectedly through the instrumentality of so-called " bacterial filters." These are constructed of fine-grained unglazed porcelain, clay or infusorial earth. They are commonly moulded in the shape of hollow candles and fired at a high temperature. Liquid is made to pass through the walls of the filters, which vary from a quarter to half an inch in thickness, by hydrostatic pressure or by suction with some form of exhaust pump. Use of such filters to separate bacteria from the products of their activity was first made in 1871. Since that time they have become a usual part of the equipment of a bacteriological laboratory and have been ex- tensively employed to free water from microbes for domestic use. In structure the wall of a filter resembles a bed of sand on a diminutive scale, with crevices of variable size between the particles and a good many splits and holes of larger dimensions throughout the matrix. The different types of filter vary in their permeability according to the fineness of the pores and thickness of the wall. The smallest passages are of the order of 0-2 to 0-5 in the case of the porcelain filters and 0-2 to 0-8 in those made of infusorial earth. The size of the smaller pores is of the same order of magnitude as that of the smallest bacteria, and the power of a filter to hold back these microbes depends upon the walls being of sufficient thickness to ensure that a bacterium will become impacted in one of the smaller passages through which the liquid in which it is suspended has to pass. If a filter through which a liquid containing bacteria in suspension has been filtered be allowed to remain for a few days immersed in a nutrient fluid, the bacteria caught in the interstices divide and multiply and generally manage to grow through the walls of the filter, for, during growth, the cells can adapt themselves to the size and shape of the crevices. On this account filters cannot be relied upon to render drinking-water secure unless removed and steri- lized by heat at least every second day. The similarity in magni- tude of filter-pore and bacterium is not a coincidence, but due to selection of material for the manufacture of the filters of such size of grain as to afford a bacteria-free filtrate and at the same time the maximal flow of liquid. In other words, bacterial filters have been made to fit the known microbes.
The first discovery of an ultra-microscopic or filterable virus was made by Loeffler in 1898 in the course of some experiments upon foot and mouth disease in which a filter of infusorial earth was being used to remove ordinary recognizable bacteria from the diluted contents of the superficial vesicles which are charac- teristic of this disease. The filtrate was free from any particles visible by the microscope and no bacteria developed in it on cultivation. Nevertheless, injection of this filtrate into animals caused the disease. Material removed from the vesicles of the animal so infected and filtered again reproduced the disease in a fresh animal. Similar experiments were carried out through a number of generations of experimental animals, so that there was no doubt that a virus capable of propagation was contained in the filtrates. In the next few years the filterability of the virus was established in the case of infectious pleuro-pneumonia of cattle, South African horse-sickness, fowl plague and mosaic disease of the tobacco plant, in which patches of discoloration occur in the leaves and, spreading rapidly, destroy them. With the exception of the virus of pleuro-pneumonia, which is just on the margin of visibility, all are invisible.
The first virus of a human disease which was found to be small enough to pass a bacterial filter was that of yellow fever. The observation was made by the American commission to study yellow fever in Havana in 1901. The cause of yellow fever has recently been shown to be an organism which, owing to its thin- ness and motility, can pass through a bacterial filter.
Up to the year 1921 the virus of 38 diseases of man, animals or plants had been found to pass through a bacterial filter by some reliable observer. The more important of these are the following: foot and mouth disease, contagious bovine pleuro- pneumonia, mosaic or spotted disease of the tobacco plant, African horse-sickness, fowl plague, yellow fever, cattle plague,
