different Earths for Carlon. £^97 



in the second an equal quantity, less a portion that was 

 found untaken up ; :uid in the last two grains, or equal to 

 4 per cent, of carbon. 



The general results with the deacificd carbonates may be 

 thus briefly suninied up : — Calcined refracting spar p.-r^e 

 gives a class of a lis^ht blueish watery cast, somewhat allied 

 to pure Time, Experiment IV; calcined chalk, a dark eme- 

 rald green. Experiment X ; and calcined marble affords a 

 rich dark brown amber, Experiment 3vV 11. The roasted 

 chalk becomes completely saturated with 1 -33d part the 

 weight of the earth of carbon, minus about 1 -Gth of a grain; 

 so That about S^ds per cent, of carbon disappears. The 

 refracting spar yields a result similar; but the disappear- 

 ance of carbon with the Kilkenny roasted marble is only 

 equal to l-30th, or 2 per cent. When the linie was thus 

 used in a caustic state, a greater proportion of iron was at 

 the same tim^e revived by all that would have been con- 

 tained in a weight of lime equal to the measure of the car- 

 bonic acid dispelled. It would appear from this fact, that 

 the extra doses of carbon necessary to produce the fmal 

 change of colour and the disengagement of the iron, used 

 in the experiments with the carbonates, were not dissipated 

 by uniting to the iron contained in them respectively, see- 

 ing that in the experiments with the deacified compounds 

 nol above half the carbon was requisite to revive fiom 30 to 

 40 per cent, of more iron. This may be accounted for in 

 two ways ; either by supposing a quantity of moisture in 

 the carbonates, which by evaporating at a high heat dissolves 

 a portion of the carbon, and escapes with it ; or by sup- 

 posing that a peculiar affinity is exerted upon the carbon 

 bv the carbonic acid, and a portion of the former by that 

 means fixed in the calcareous matter by fusion. One thing 

 however is certain, that the fusion per se of caustic and 

 carbonate lime of the same nature forms glasses of very op- 

 posite colours. (See Experiments No. I and IV, No. VI 

 and X, and No. XIV and XVII.) As the only difference be- 

 twixt these states of lime arises from the presence or absence 

 of the llxed air and what water they contain, it can only 

 be attributed to them, unless we can suppose that a small 

 portion of oxide of iron contained in the lime, not exceed- 

 ing 3 per cent., can, by undergoing an attenuated process- 

 of roasting along with the calcining lime, occasion a change 

 ct colour by superoxygenation. 



Finally, upon this' head of experiment, it seems obvious 



that the colouring principle in these glasses, from whatever 



they are obtained, i^ iron. Their colour in general is green 



5 and 



