1024 



TRASS 



out to the circular form with a tool something like a carpenter's centre-bit, or -with a 

 crown saw in the lathe. The disks so cut out were used for inlaying in the tops of 

 boxes, &c. This required a piece of shell as large as the front of the spectacle ; but 

 a piece one-third of the size will now suffice, as the eyes are strained or pulled. A. 

 long narrow piece is cut out, and two slits are made in it with a saw. The shell is 

 then warmed, the apertures are pulled open, and fastened upon a taper triblet of the 



2010 



appropriate shape ; as illustrated by./fys. 2013, 2014, and 2015. The groove for the 

 edge of the glass is cut with a small circular cutter, or sharp-edged saw, about three- 

 eighths or half an inch in diameter ; and the glass is sprung in when the frame is 

 expanded by heat. 



In making tortoise-shell boxes, the round plate of shell is first placed centrally over 

 the edge of the ring, as in fig. 2010 ; it is slightly squeezed with the small round 

 edgeblock g, and the whole press is then lowered into the boiling water ; after immer- 

 sion for about half an hour, it is transferred to the bench, and g is pressed entirely 

 down, so as to bend the shell into the shape of a saucer, as at Jiff. 2018, without 

 cutting or injuring the material ; and the press is then cooled in a water-trough. The 

 same processes are repeated with the die d, which has a rebate turned away to the 

 thickness of the shell, and completes the angle of the box to the section, ./iy. 2017, 

 ready for finishing in the lathe. It is always safer to perform each of these processes 

 at two successive boilings and coolings. Two thin pieces are cemented together by 

 pressure with the die e, and a device may be given by the engraved dieffjig. 2016). 



TOSSING or TOZING. A process in dressing ores, by which they are kept 

 suspended in water by agitation. See DRESSING OF OKES. 



TOUCH-NEEDLES and TOUCH-STONE are means of ascertaining the 

 quality of gold trinkets. The touch-needles are bars of known composition, and the 

 touch-stone is black basalt ; according to the streak made by the article to be tested, 

 as compared with that made by the needles, its quality is inferred. 



TOURMAXXNE. A silico-borate of alumina and several other bases, usually 

 with fluorine. This mineral is used in the construction of polariscopes. The black 

 varieties are known as Schorl, the red as Eubellite, and the blue as Indicolite. 



TOUS-IiES-lWOIS. A name given to a kind of starch obtained from the (.anna, 

 edtdis, one of the Marantacecc, or Arrowroot order. 



TOW. See FLAX. 



TRAG.ac.aNTH, GUM, (Gomme adracante, Fr. ; Traganth, Ger.) See GUM. 



TRASS or TARSIAS. A German term for a tertiary earth, probably volcanic, 

 which occupies wide areas in the Eifel district of the Rhine. Its basis appears to be 

 pumice-stone, mixed with fragments of basalt and calcined slate. When powdered it 

 is used, like the pozzolano of Italy, as an hydraulic cement. 



