CHR 



CHR 



shining*, and intermediate between ada- 

 mantine and resinous ; the crystals are 

 translucid; the fracture is uneven; the 

 specific gravity 6.0269. It depreciates 

 before the blow-pipe, and melts into a 

 blackish scoria. It colours borax green 

 by fusion. According to \ 7 auquelin, it is 

 composed of 57.10 of lead, 6.86 of oxy- 

 gen, and 36.04 of chromic acid. There 

 is found with the chromate of lead, a mi- 

 neral of a green colour, in minute crys- 

 tals, which Vauquelin found to be com- 

 posed of the oxides of chrome and lead, 

 and which, as he conjectures, has proba- 

 bly originated in the decomposition of the 

 perfect chromate, from some process by 

 which part of its oxygen has been ab- 

 stracted. 



Native chromate of iron has more late- 

 ly been found in the department of Var 

 in France, and likewise in Siberia. This 

 mineral is massive, of a blackish brown 

 colour, with no great lustre, and opaque; 

 its fracture is uneven,* and it is hard and 

 difficult to break ; its specific gravity is 

 4.0. It is scarcely fusible before the 

 blow-pipe, but with borax it melts into a 

 glass of a fine green colour. According 

 to an accurate analysis of it, it consists of 

 63.6 of chromic acid, or perhaps rather 

 oxide of chrome, and 36 of oxide of iron. 

 Chrome has been also found in smaller 

 quantities in other minerals, 'particularly 

 in some gems, of which it appears to be 

 the colouring principle. It exists in the 

 emerald, in the state of green oxide, 

 and in the spinal ruby, in the state of 

 acid. 



Vauquelin extracted the metal from 

 the red lead ore, by adding to it muriatic 

 acid, which combines with the oxide of 

 lead, and forms a compound that is pre- 

 cipitated, the chromic acid remaining in 

 solution. To abstract a little muriatic 

 acid combined with it, oxide of silver is 

 cautiously added, and the pure chromic 

 acid being decanted from the precipitate 

 of muriate of silver, and evaporated, is 

 exposed to a very strong heat, excited 

 by a forge, in a crucible of charcoal, 

 placed within another of porcelain. It 

 is thus reduced to the metallic state. It 

 is to this chemist that we are indebted, 

 principally, for a knowledge of its pro- 

 perties. 



Chrome is of a white colour inclining 

 to grey : it is very brittle ; its fracture 

 presents a radiated appearance, needles 

 crossing in different directions, with in- 

 terstices between them Its other physi- 

 cal qualities have not been determined. 

 This metal is difficult of fusion. Expos- 

 ed to the heat nf tho blo\v-pipc, it dors 



not melt. When fused, by having been 

 exposed to the intense heat necessary to 

 its reduction, it presents crystalline fila- 

 ments, which rise above the metallic 

 mass. Chrome is not easily acted on by 

 the acids. Even when reduced to a fine 

 powder, and treated with concentrated 

 boiling, nitric acid, it is oxydized with 

 much difficulty, and communicates to the 

 acid only a green tinge. 



Chrome, in the state of acid, appears 

 to be more susceptible of combination, 

 and this acid being obtained without dif- 

 ficulty from its native combinations, its 

 chemical relations have been more ex- 

 amined. Chromic acid is very soluble in 

 water; the taste of the solution is sharp 

 and metallic; it is of an orange-red co- 

 lour ; by evaporation, either spontaneous 

 or with a gentle heat, it affords crystals 

 in long slender prisms, of a ruby-red 

 colour. This acid combines with the 

 alkalies, earths, and metallic oxides, 

 forming neutral salts, which are named 

 chromates. 



The combinations of this acid with me- 

 tallic oxides are in general possessed of 

 very beautiful colours, jmd are well 

 adapted to form the finest paints. That 

 with oxide of lead has an orange yellow, 

 of various shades; that with mercury, a 

 vermilion red ; with silver, a carmine 

 red; with zinc and bismuth, the colours 

 are yellow ; with copper, cobalt, and an- 

 timony, they are dull. 



CHRONOLOGY, is that science which 

 relates to time; treats of the division of it 

 into certain portions, as days, months, 

 years, centuries ; and the application of 

 these portions, under various forms and 

 combinations, as cycles, aeras, &c. to the 

 elucidation of history. What is proposed 

 in the present article is, to point out the 

 chief methods by which the several por- 

 tions of time have been computed, and 

 in which they have been employed in as- 

 certaining the connection, and determin- 

 ing the dates, of past transactions. 



The divisions of time which most pro- 

 bably first attracted the notice of man- . 

 kind, as most obvious to their senses, 

 were those marked by the revolutions of 

 the heavenly bodies, days, lunar months, 

 and years: and if these had correspond- 

 ed so exactly to each other, that every 

 lunation had consisted uniformly of the 

 same number of clays, and each year of a 

 regular number of complete lunations, 

 the business of chronology would have 

 been attended with comparatively little 

 difficulty. In consequence, however, of 

 variations in the revolutions of the earth, 

 which if is not requisite here to explain*. 



