Chemical Science. 385 



of iron, oxide of zinc, and water : when analyzed, the following pro- 

 portions were obtained : — 



Water of crystallization 30.90 



Sulphate of iron 3.1fJ 



Sulphate of zinc 39.00 



Sulphate of ammonia 26'.9-t 



100.00 



The water of crystallization surpasses the quantity which would be 

 required by the sulphates separately, and this fact is adduced by M. 

 Tassaert, as an argument in favour of the whole being the result of 

 chemical combination, and not a mere mixture.— Ann. de Chim. xxiv. 

 100. 



20. Pyrophorus from Tartrate of Lead. — Dr. Gobel, whilst work- 

 ing with the tartrate of lead, remarked that when heated in a glass 

 tube, a very perfect and beautiful pyrophorus was produced. When 

 some of the dark-brown mass thus formed was shaken out into the air 

 it immediately inflamed, and brilliant globules of lead covered the 

 ignited surface ; some of these changing by degrees into litharge, 

 offered a very beautiful appearance. The ignition continues much 

 longer than with other pyrophori, which circumstance, with the faci- 

 lity of preparation, may make it a convenient means of obtaining fire. 



The inflammation of these substances, as Dr. Gobel remarks, has 

 been attributed principally to the presence of potassium, but this sub- 

 stance affords a new proof that other metallic compounds are sus- 

 ceptible of spontaneous inflammation on the accession of air. 



21. On a Green Pigment. — The preparation of a beautiful green 

 colour is described in our last volume, at p. 309 ; but an easier pro- 

 cess for the production of the same colour having been given by Dr. 

 Liebig, we insert it beneath. A given weight of verdigris is to be 

 dissolved by heat in a copper vessel in a sufficient quantity of pure 

 vinegar, and then an aqueous solution of an equal quantity of white 

 arsenic added ; generally a dull green precipitate falls, which must be 

 redissolved by adding more vinegar. The mixture is then to be 

 boiled, and alter some time a crystalline precipitate appears, of the 

 finest green colour, which, separated, washed, and dried, is the sub- 

 stance in question. If the liquor still contains copper, arsenic is 

 again to be added ; or if it contains an excess of arsenic, the prepara- 

 tion of copper must be added, and the process carried on as before. 

 Sometimes the liquor contains an excess of acetic acid, and may then 

 be employed to dissolve verdigris, as at first. 



Thus prepared, the colour has a bluish tint, but it may be ob- 

 tained of a deeper and more yellow tint, yet with the same bril- 

 liancy and beauty ; for this purpose a pound of common pearlash is 



Vol. XVI. 2 C 



