VOL. LIX.]| PHILOSOPHICAL TRANSACTIONS. 637 



to that elastic vapour, so often mentioned by writers on these subjects; which 

 cannot be wholly retained by the closest corking, but, gradually escaping, suf- 

 fers the ochrous matter to subside. And that fixed air has greater affinity with 

 alkalies than with iron, because the addition of alkaline substances, not saturated 

 with fixed air, will disengage the metal, while such as are charged with this 

 principle produce no alteration. 



These conclusions seemed to account for many particulars relating to medi- 

 cated springs; but as all Mr. L.'s trials had been made with iron in its metallic 

 state, which is rarely found in nature, it was necessary to repeat them on this 

 mineral in the state of ore. Mr. L. proceeded therefore to different experiments 

 on various ores; but did not find any of them to answer his expectation, except j 



what is called iron sand ore, which seems to contain a perfect iron. This at first 

 offered a material objection to his former inference. But on a little considera- 

 tion, it occurred, that waters, being first charged with pyritical matter, might 

 afterwards have their acid neutralized with alkaline or calcarious substances, and 

 the iron yet remain suspended by air generated in the saturation. And he was 

 the more ready to adopt this opinion, as it would explain, very naturally, the 

 application of fixed air to this business of solution; which he confessed had 

 hitherto been to him somewhat difficult to account for. It was necessary how- 

 ever to examine the truth of this theory, by the test of experiment, which he did 

 in the following manner : 



Exp. 6. To a pint of distilled water, mixed with 1 gr. of iron dissolved, as in 

 exp. 5, were added 40 grs. of soap-leys. And to 2 oz. of lime-water, diluted 

 with 14 oz. of distilled water, was added 1 gr. of iron in solution. In both 

 cases the point of saturation was intended. The two mixtures instantly turned 

 green, became turbid, and let fall sediments of the same colour. The liquors, 

 being filtered, gave no tinge with tincture of galls. 



Exp. 7. A quart of water was mixed with 2 grs. of iron, as before. To one 

 moiety, 3 grs. of salt of tartar in solution was instilled. It first appeared green, 

 soon changed yellow, and gave an orange-coloured precipitate. To another 

 portion 2 grs. of powdered chalk being added, it presently became straw-coloured, 

 and after continuing Q hours in a well-corked phial, was yet turbid, with a yellow 

 sediment. The waters being both filtered, part of each changed to a purple, 

 with tincture of galls. The remainder being boiled, a further precipitation en- 

 sued, and the clear liquor no longer produced any alteration with galls. 



This experiment being repeated with magnesia, and with the earth of alum, 

 showed no sensible difference. The quantity of iron, left dissolved in the liquor, 

 was found in proportion to the volume of air generated during the saturation. 

 If the quantity of alkali or absorbent earth was insufficient to saturate the acid, 



