OF MICROSCOPIC OBJECTS. 19 
engrave names, numbers, &c., neatly upon slides, 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 vapour 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 atonce. 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. 
8tH 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 oils, and are wholly soluble in ether and alcohol. 
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