Off MICROSCOPIC OBJECTS. 19 



engrave names, numbers, &c., neatly upon slide?, for classifi- 

 cation in the cabinet. The glass to be engraved must be 

 coated with an etching ground of oily varnish or wax, and 

 the necessary writing effected upon it by a point, which 

 must pierce through the protective material. A shallow 

 basin, made by bending up the edges of a piece of sheet-lead, 

 is then prepared, a little powdered fluor spar placed in it, 

 and enough sulphuric acid added to form a thin paste. The 

 glass is then placed in any convenient way over the basin, 

 the waxed side downwards. A gentle heat is next applied, 

 whereby the vaptour of hydrofluoric acid is disengaged. This 

 acts upon the glass exposed by. the point in a very few 

 minutes, removing a portion of its surface. The wax must 

 then be removed by turpentine. If the lines which result 

 are then rubbed over with any coloured varnish, and the 

 varnish gently wiped off by a soft piece of rag, a sufficient 

 portion will most probably remain in the etched marks to 

 render them easily visible and legible. Of course it will be 

 as well to prepare many slides in this way at once. It is not 

 necessary to coat the whole surface of the slides with the 

 protective varnish, if the leaden basin be covered with a 

 thin piece of wood or sheet-lead perforated with holes 

 slightly larger than the surface to be etched, over which 

 holes the slides must be inverted for a sufficient time. This 

 latter hint applies more particularly to finished slides requir- 

 ing to be labelled. 



STH DIVISION. 



The proper solvents of the 'fixed oily and fatty matters 

 are ether, benzole (or benzine), turpentine, and the essen- 

 tial oils generally. Castor oil is nearly the only one which 

 is soluble in alcohol, the rest being only slightly so. They 

 are all capable of saponification with caustic alkalis, and so 

 become indirectly soluble in soft or distilled water, other- 

 wise they are wholly insoluble in it. 



The volatile or essential oils mix in all proportions with 

 fatty oilsj and are wholly soluble in ether and alcohol. 



