ENGRAVING. 227 



nitric acid, is then poured on, which "bites" or dissolves the 

 metal where thus exposed ; when the etching has gone far enough 

 at any particular spot, further action is stopped there by applying 

 a little " resist " to the lines etched. Finally, the resist composi- 

 tion is cleaned off the plate, and those lines, &c., that require 

 touching up with a sharp tool are then cut further until the plate 

 is in a condition to receive the ink and print off impressions. 

 This process can be easily illustrated by obtaining a polished 

 copper plate (such as those used for printing visiting-cards), 

 covering the polished face with wax, drawing the design, or 

 writing with a sharp needle, so as to cut through the wax and lay 

 bare the copper, and then cautiously pouring on the aqua fortis ; 

 but considerable practice is necessary before etched plates fit to 

 print from can be obtained. 



Copper rollers with patterns thus engraved on them are largely 

 employed in cotton-dyeing, calico-printing, and similar industries, 

 in order to apply the colouring materials, &c., to the fibre in some 

 parts and not in others (vide Expt. 291). 



Expt. 273. To produce a Liquid from two Solids. Rub 

 together in a mortar some sugar of lead (acetate of lead) and 

 Epsom salts (sulphate of magnesium) ; the two solids will react on 

 one another and form a wet white mass. The chemical change 

 here occurring is that acetate of lead and sulphate of magnesium 

 form acetate of magnesium and sulphate of lead. The wetness 

 arises from the fact that crystals of Epsom salts, like many other 

 crystallised substances, contain not only true sulphate of magnesium, 

 but also water combined therewith, and so to speak rendered solid 

 in the crystal; in consequence such water is termed water of 

 crystallisation. As neither the lead sulphate nor the acetate of 

 magnesium combine with the water thus present in the Epsom 

 salts, it is set free, rendering the whole mass wet. 



This experiment is only apparently the converse of No. 128, 

 where two solutions mixed together formed a nearly solid mass ; 

 there double decomposition also took place, with the result of 

 forming a large quantity of insoluble matter which physically 

 absorbed the water present (as sand would do), but did not com- 

 bine with it ; in the present case a substance insoluble in water 

 (sulphate of lead} is also formed by double decomposition, but this 

 is not bulky enough to absorb completely all the watery solution 

 of acetate of magnesium also formed, without showing visible signs 

 of the presence of a fluid. 



When plaster of Paris is mixed with water for taking casts 

 (Expt. 3), the setting or hardening is largely due to the combina- 

 tion of the dry plaster (sulphate of calcium} with some of the 



