1 87 1.] Technology. 411 



applied and well-known means now in use for this purpose, the author states 

 that the best plan to treat these solutions is the following: — They are boiled 

 either in a porcelain basin or glass flask, and while boiling there is added to 

 them recently precipitated, well washed, and moist oxide of silver, the boiling 

 being continued for sometime. The liquid is next filtered, and then evaporated 

 to dryness, the heat being increased to fusion, so as to destroy ammoniacal 

 salts; the residue is pure nitrate of silver. The sediment on the filter contains 

 some oxide of silver, which must be added in excess ; and, therefore, in order 

 not to lose that, the filter is preserved, and the contents worked up at a subse- 

 quent operation. The nitrate of silver thus obtained is, by practical photo- 

 graphers, pronounced to be of excellent quality. 



For coating brass with copper, Dr. C. Puscher dissolves 10 parts, by weight, 

 of sulphate of copper, and 5 of sal-ammoniac, in 150 parts, by weight, of water. 

 Place the brass, well cleaned and free from grease, into this mixture, leave it 

 in it for a minute, let the excess of liquid drain off first, and heat the metal 

 next over a charcoal fire, until the evolution of ammoniacal vapours ceases and 

 the coppery film appears perfect. Wash with cold water, and dry. The coat- 

 ing of copper adheres firmly. 



The best varnish to protect polished metals from rusting is, according to Dr. 

 C. Puscher, a solution of paraffin (i part, by weight, in 3 parts of petroleum). 

 This varnish may be usefully applied to polished metals, especially as, after 

 having brushed the liquid over the surface of the metals, they may be gently 

 wiped clean with a soft piece of flannel, so as to leave only a very thin film of 

 the varnish, yet sufficient for the protection of the polish. 



The protection of iron water-pipes from oxidation is very necessary, not 

 only to provide against the premature giving way of the metal, but to secure 

 the scarcely less important desiderata of avoiding any undue roughening of 

 the internal surface or the diminution of the bore by the accumulation of rust. 

 The most successful means of preventing the results just referred to consists 

 in coating the surfaces with a tenacious varnish. That used on the pipes of 

 the Cochituate Water-works in Boston has withstood a ten years' test, and at 

 the end of that time the pipes "were found to be almost free from rust or 

 ochreous deposits." It was composed of coal-tar, distilled until the naphtha 

 was wholly eliminated, and the tar or pitch brought down to the consistency of 

 wax. The tar reduced to this condition was heated to 300 , a tem- 

 perature at which it was kept during the dipping operation, and about 

 8 per cent of linseed oil added to it. The pipes of cast-iron were heated 

 to a degree determined by the judgment of the engineer in charge, and im- 

 mersed in the prepared tar for from thirty to forty-five minutes, at the expira- 

 tion of which they were placed in such position to drip that the remaining 

 layer or coating of the material should be of uniform thickness. This method 

 of protecting the pipes is now exclusively adopted in the water-works of most 

 of the cities in the United States, and its advantages have been found to be far 

 in excess of its cost. 



For the purpose of obtaining decorative colours on metals, Dr. Puscher em- 

 ploys a solution of hyposulphite of lead in hyposulphite of soda, in the pro- 

 portion of 3 parts, by weight, of hyposulphite to 1 part of acetate of lead. 

 The clear solution of the salts deposits, when heated to about ioo°, a thin layer 

 of sulphide of lead upon any metal — brass, zinc, copper — placed in the solu- 

 tion, and thereby produces a beautiful display of various colours on these 

 objects. 



The native Indians of Mexico employ a rapid method of tanning hides and 

 skins, which may prove useful as a basis for experiment in more civilised 

 countries. It appears that they form, with the brains of the animal and cer- 

 tain vegetable produces locally called cascalote and huizache (substances con- 

 taining a large proportion of tannic acid), an emulsion, which is forced into the 

 skins by means of squeezing between the hands, or, more rapidly, by stamping 

 with the bare feet. In some parts of Mexico, 4000 or 6000 goats are killed at 



