>ARATION OF ADHESION-PLATES. 157 



ae same time round its own centre and round the centre of the fixed 

 late, so that it always projects somewhat over the edge of the 

 ',tter as shown in fig. 110, where the two pairs of arrows indicate 

 le two rotatory motions of the plate. The grinding is continued, 

 ith occasional additions of emery and water, until both plates pre- 

 >nt everywhere a dull surface, They are then washed and dried, 

 id the fixed plate rubbed over with a drop of oil and a little red 

 ad (minium), until it appears uniformly coloured red. If the 

 cond plate is now placed upon the other, and moved just the 

 lallest distance, it should appear also uniformly coloured red 

 I over, if the plates are already evenly ground. If only scattered 

 aces appear coloured, the grinding must be continued ; if the 

 ites touch only in the middle, it is necessary to describe a 

 mller circle with the moveable plate, while the grinding is con- . 

 med ; but if the plates touch along the edges, and the central parts 

 ; 3 thus hollow, it may arise from a small flexure of the thin glass 

 consequence of the pressure upon the handle, and in continuing 

 1 3 work the plate should not be moved by the handle, but by 

 ] icing the points of the fingers along the edge and describing now 

 ' arger circle. When the plate takes the colour uniformly, but not 



I ore, a finer kind of emery is used for grinding ; the surfaces will 

 t >reby lose a certain amount of roughness and become more plane. 

 ( adually repeating from time to time the test with oil and 

 r lead finer and finer kinds of emery are employed, altogether 

 a >ut four sorts of various degrees of fineness. Finally when the 

 p tea already manifest a certain amount of adhesion, and after being 

 w died and dried they exhibit a small degree of polish, so as to 

 nect slightly the image of a window sash or a lamp flame they 

 a polished with 'jeweller's rouge ' (an oxide of iron), which is 



II d in the same manner as the emery ; but in using the finer sorts 

 oihe latter and the rouge, very little water must be added, and the 

 U r iding always continued until the mass is almost dry. 



I'he operation of grinding requires about two hours; if not suc- 

 <( ful with the first pair, attempt another. Good plates will adhere 

 "t only in a horizontal but also in a vertical position, one plate 

 Ixag held by the handle of sealing-wax. 



i order to show the adhesion between different substances, pre- 



a plate of plaster of Paris. This substance mixed with water 



lie poured upon one of the polished glass plates. After a 



1 <le of hours the plate of plaster may be removed from the glass, 



'>n in a day it will be perfectly dry. The adhesion manifested by 



' not very considerable, but sufficient "to maintain it horizontal, if 



; mtact with the glass plate, for about a second. 



