t 169 ] 



wax and plate, then pinch it up to an edge, and 

 fo make it into a kind of wall of half an inch 

 high ; after which pour on your Aqua fortis^ a 

 proper fort of which is to be had at moft of the 

 chemifts fhops in London, and is generally too 

 ftrong to be ufed without a mixture of water. 

 You will know when it is too ftrong, by its al- 

 moft boiling on the plate where it touches the 

 copper ; therefore you are to lower it with wa- 

 ter, till you fee the bubbles rife very moderate- 

 ly ; for too great a ftrength oî Aquafortis will 

 break up your Ground, and fpoil your plate. 



When the Aquafortis has been on the plate 

 a little while (about half an hour) if you per- 

 ceive by the equal bubblings that it has bit kind- 

 ly, it may be taken off, and the plate clean 

 wafhed with fair water, and well dried -, then 

 you may try the depth of your ftrokes by rub- 

 bing off a little fpot of the Ground, and, if you 

 find it too faint, put the Aqua fortis on again ; 

 but if you think it has bit enough for the firft 

 time, you may ftop (with the before- mentioned 

 varnifti) all thofe parts that you would preferve 

 Jight and tender : let yqur varnifti dry a little, 



t:hen 



