If the cellulose be in excess, e.g. when the solution is evaporated on the surface of paper, calico, &c., merely dipped in the solution, the copper oxide is often not formed at all, but a green varnish-like mass of cellulose combined with copper oxide, which coats the surface of each filament, welding and cementing them together. This cement-like _cupro-cellulose_, as it may be termed, being insoluble in water, communicates water-resisting properties to the material so treated: moreover, the presence of copper renders the fabric less prone than before to be attacked by insects and mould, so that animal and vegetable life of a parasitic nature and fungoid growths are rarely, if ever, to be observed, even when the material is kept under conditions where boring worms, ants, rot, and mould, would be likely to attack it. As has been already indicated (p. 5), the solution of cuprammonium hydroxide is preferable to one containing cuprammonium salts; not only is the action on cellulose more energetic, but various other advantages are obtained. _Preparation of the Copper Solution._—The cuprammonium solution is prepared according to the patent of Dr. C. R. Alder Wright (No. 737, 1883). {218} A series of cast-iron towers, two to three feet in diameter and ten to twelve feet high, is so arranged that a current of air can be blown by a powerful engine successively through the whole series. The towers are then nearly filled with fragments of metallic copper (crumbled up sheet, cuttings, &c.), and solution of ammonia: the air-current being turned on, oxidation of the copper and solution of the oxide so produced are rapidly effected. The ammonia solution employed is previously impregnated with a considerable quantity of copper by passing a stream of water over scrap copper in similar towers, a current of ammonia gas mixed with a suitable proportion of air being at the same time forced in. The liquor passing from one tower is used over again to supply a second instead of water, and so on through the series: finally the liquor is brought up to full copper strength in the series of towers described above. The spent air issuing from the towers carries with it a notable quantity of ammonia; this is intercepted by means of an “exhaust” scrubber containing copper, and well supplied with water, whereby a comparatively weak solution of ammonia and copper is obtained, which can be used instead of water in the first series of towers. In order to produce the maximum effect on the cellulose, the solution should contain from 100 to 150 lbs. of ammonia, and from 20 to 25 lbs. of copper per 100 gallons.