84 
THE YOUNG SCIENTIST. 
strength of the acid, the hardness of the 
copper, and the degree of pressure which 
has been laid on the point when etching. 
The following figures illustrate the ap- 
pearance of the plate at different stages of 
the biting-in process. Fig. 9 represents 
the etching as it would appear after the 
acid had been applied five minutes; 
Fig. 10, the etching with the lightest tints 
Fig. 10. ■ 
stopped up with varnish. Fig. 11 repre- 
jsents the etching as it would appear after 
the acid has been applied ten minutes; 
and Fig. 12 after it has been applied fif- 
teen minutes. 
Generally the nitrous acid sold bydrug- 
gists may be diluted with a little more 
than twice the quantity of water; but 
until the student has by experience ac- 
quired a knowledge of the action of the 
acid, it will be advisable to make frequent 
Fig. 12. 
examinations of the etching, lest the tints 
are bit too dark. In order to effect this, 
the acid must be poured off, and then the 
plate carefully washed with water, and 
dried either by blowing with a pair of 
bellows or by dabbing with a very soft 
handkerchief; a portion of the etching- 
aground can then be removed by the 
scraper (see page 72). If the line is not 
dark enough the plate can be stopped up 
with varnish, and when dry the acid can 
be again applied. It is, perhaps, '\ffell to 
mention that the common Brunswick 
black, used for blacking chimney orna- 
ments, is a very good varnish for stop- 
ping-out. This varnish may be had at 
any oil shop. A plate, if covered with 
this varnish, and permitted to dry, will 
as effectually resist the action of acid 
as if covered by the etching-ground; 
but it is not so proper for the purpose 
of etching, as it cannot be so neatly 
removed by the point. Care must be 
taken in all cases not to put the acid 
on the plate until the varnish is dry. If 
this is not attended to, the varnish, in- 
stead of protecting the plate, will rise to 
the surface of the acid, and the plate will 
bite into holes in such portions as the 
varnish has been removed from. It may 
be easily known when the varnish is suf- 
ficiently dry, by breathing on it ; if the 
Fig. 11. 
