136 Paftes in Imitation of Sapphire. [Book VI, 



out the fined blue. The larger particles fall more 

 fpeedily, and the azure brought out by the water 

 of the three cocks forms the different degrees of 

 finenefs, known by the names of azure of the firft, 

 fecond, and third fires. Powder and flone blue, ufed 

 by laundrefies, is a preparation made by the Dutch 

 from the coarfe fmalt. 



A fmall quantity of the calx of cobalt tinges glafs 

 of different fhades, according to the quantity ufed. 

 One grain of cobalt to feven thoufand of the glafs, 

 renders it very blue : hence the paftes in imitation of 

 fapphire are produced. Stained flint glafs is made by 

 fixteen parts of fand, eight or ten of pearl aft, and 

 metals in different proportions. 



Cobalt diflblves readily in aqua-fortis, both in its 

 metallic (late and in that of a calx. If we take a folu- 

 tion of its calx in that fluid, and add a quantity of water 

 v to it, and then let it ftand a while, it turns red, and 

 on evaporation forms cryftals of the fame colour, and 

 by this it may be diftinguifhed from all other me- 

 tals. This folution forms a red fympathetic ink, which 

 appears on the paper by heat, and again difappears by 

 cold. 



Aqua regia difTolves cobalt more eafily than the 

 muriatic, but not fo eafiiy as the nitrous, acid. This 

 folution is a celebrated fympathetic ink. If it is 

 diluted with a fufficienr quantity of water to prevent 

 its action on paper, and then ufed to write with, the 

 letters are invifible as foon as the clear folution be- 

 . comes dry ; but if the paper is held to the fire for a 

 fhort time, they appear of a fine green colour ; which 

 again difappears by removing it, and furTering it to 

 cool. If held to the fire too long, the green colour 

 becomes permanent. From thefe two inks a veiy 

 curious changeable lar,dfcape may be made. The 



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