27 



when subjected to severe dyeing processes, particularly 

 the bleaching one of recent date, thus giving a substantial 

 and important reason why its coloured cements should be 

 removed, if possible, by gentle action. The natural colour 

 of Tusser silk is a darkish shade of fawn, much unlike the 

 golden and white colours of mulberry silk. 



The fibrets have a distinct structure, upwards of 20 in 

 number, composing each fibre, and seem compactly laid 

 together, showing longitudinal striae under the microscope 

 (Plate XXVIIL, Fig. 4). I conceive it is this fibrous com- 

 pound structure, absent in the mulberry fibre (Plate 

 XXVIIL, Fig. 2), which is an element in its dye-resisting 

 power. I found permanganate of potash to be the best 

 agent to separate these fibrets. 



The diameter, from edge to edge, of a single fiat fibre of 

 Tusser from the outer part of the cocoon averages yi-^ part 

 of an inch, and from deeper in the substance of the cocoon 

 t^-q- of an inch, but the 'external fibres are much more 

 variable than the internal ones. The thickness from side 

 to side is y^g-Q of an inch. The outside fibres are capable 

 of supporting without breaking an average weight of 

 seven drams, and the inner eight drams, whilst the usual 

 amount of tension before breaking in all the fibres is one 

 inch to the foot (see table, page 68). The fibres, like all 

 other silk fibres, are laid in the cocoon by the silkworm in 

 pairs, and are, like all flat silk fibres, united by their edges, 

 and not by their flat surfaces. 



All the Satumiidw fibres I have examined are more or 

 less striated and composed of fibrets, and are flattish, except 

 the English species, Satumia carpini, or Emperor moth 

 (Plate XXI., Figs. 1 and 2), which in North Staffordshire 

 spins a beautiful cocoon in the heather of our moorlands. 

 Plate XXIX., Fig. 6, is an enlarged microscopic appearance 

 of this silk, which shows its transparent and fibreless 

 nature ; and also, what is very curious, that the fibres are 

 round except when they come in contact with each 

 other, when they become flat ; no doubt, from pressure 

 whilst the fibres were in their fresh viscous state. This 

 apparent exception to the Saturniidcn depositing flat 

 fibres, and the Bombycidw round ones, which I had esta- 

 blished, possibly points to the secretion of the sericine in 

 the different species, varying in viscidity, that of the 

 SatwrniidoB being secreted in a more fluid state than in 

 that of the BombycidcB. There may also be a difference in 

 the structure of their seripositors ; but this I have not had 

 at present an opportunity of investigating. 



