LUMINOSITY OF FLAMES 275 



to the general reader, and would be useless to the technical 

 reader unless furnished with minute particulars. 



For those who desire to pursue the subject there exist several 

 recent works which contain full information. Of these Bohm's 

 Fabrication der Gluhkorper fur Gasgluhlicht (1910) gives a fairly 

 complete account of the extraction of the earths from monazite, 

 and the application of these substances to the manufacture of 

 " incandescent mantles." Mr. S. I. Levy's recent work on the 

 Rare Earths contains a very full account of the minerals, followed 

 by a condensed description of the mantle industry (1915). 



For the accompanying views of the chief operations by which 

 incandescent mantles are manufactured in this country the 

 author is indebted to Mr. C. S. Garland, Managing Director of 

 the Volker Lighting Corporation of Wandsworth, London. 



The process of mantle making consists in first knitting from a 

 fine ramie, cotton, or artificial silk thread, a continuous cylin- 

 drical hose on a rotary knitting machine, the length of the stitch 

 and the tension of the thread being regulated according to the 

 width and depth of mantle required. The hose is then made up 

 into loose bundles for washing. The yarn contains about 1 per 

 cent of non-cellulose matter, chiefly fatty material introduced in 

 spinning, and a certain amount of siliceous and calcareous matter 

 from the original bast fibres of the plant " Ehea Elastica," from 

 which the ramie fibre is prepared. (See Fig. 69 facing page 276.) 



The earlier mantles were very liable to shrink and suffer con- 

 tortion whereby they were often withdrawn from the hot part 

 of the flame and so produced less light. The luminosity was also 

 reduced in course of use by the presence of the small quantities 

 of mineral impurities left after the burning of the vegetable 

 fibre. Ramie appears to be very superior to cotton as a basis 

 for the mantle-maker, and artificial silk is still better, being 

 practically free from mineral matter, and therefore leaves no 

 ash. But the use of artificial silk has not become general owing 

 chiefly to the cost. 



The very careful and elaborate washing of the fabric before 

 impregnation is, therefore, a part of the manufacturing process, 

 and is of great importance. 



The washing process varies in different factories, but its object 

 is, first of all, the removal of the grease and silica by digestion of 

 the stocking with a weak solution of caustic alkali, followed by 

 thorough rinsing in distilled water. The excess of alkali and the 



