When sal-ammoniac leaves a non-volatile residue, it may contain _sulphate of soda_. This is the principal cause of failures with the bleaching solution, as the sulphate of soda has a tendency to blacken rather than bleach Daguerreian impressions. The sulphate of soda as well as the chloride of sodium, is often found in unpurified sal-ammoniac to the amount of ten per cent.
When sal-ammoniac contains much sulphate of ammonia, it fuses and sputters before it sublimes; whereas it otherwise sublimes without fusing. When sal-ammoniac, sublimes without residue, but gives a precipitate when its solution is tested with a solution of chloride of barium, it contains sulphate of ammonia; but when it leaves a non-volatile residue, the precipitate indicates sulphate of soda or sulphate of magnesia.
M. Berthier says, that in the district of Plateros, seventeen leagues from Zacatecas in Mexico, silver ore is found in two different states; first, native silver; and secondly and principally in a state of combination in small olive-green or yellowish crystals, supposed to be chloride, but which he found to be bromide of silver. According to M. Duport, from whom M. Berthier received these specimens, this substance is not rare in Mexico, but occurs frequently in fine cubic and octahedral crystals.
The specimen examined by M. Berthier was from San Onofe. It was compact, of a slightly reddish gray color; fracture uneven; splendent; penetrated with small cavities, some of which were partially filled with a substance of a dull pale yellow color, and which the miners call oxide of lead; other cavities contain very small imperfect crystals; which are brilliant, and of a pale olive green-color, and have the appearance of chloride of silver. This specimen was very rich, for it yielded 0.0688 of silver, and contained 0.45 of carbonate of lead, which, intimately mixed with quartz and a little oxide of iron, formed the principal portion of the mass.
M. Berthier has also found this mineral among the silver ores of Huelgoeth, department of Finistère in France. Two specimens were obtained by him: the first of these is described as being porous or scoriform, containing white quartz imbedded in foliated hydrate of iron. On the edges of the foliated iron ore the naked eye could distinguish small cubic grains of a pearl-white color, which had all the characters of chloride of silver.