VOL. LXXII.] PHILOSOPHICAL TRANSACTIONS. 237 



1. In computing the quantity of acid taken up by 10.5 gr. of mild vegetable 

 fixed alkali, I made no allowance for the small quantity of earth it contains, viz. 

 0.7035 of a grain; but in large quantities of alkali, this proportion is consider- 

 able, and it occasioned a small but sensible error in my subsequent calculations 

 of the proportion of ingredients in neutral salts, the quantity of alkali being, by 

 that fraction, less than I supposed it in 10.5 gr. This correction being made, it 

 will be found, that 1 00 gr. of perfectly dry vegetable fixed alkali, abstracted from 

 the quantity of earth, generally contain 22.457 gr. of fixed air, instead of 21, 

 as before determined: though the former determination is right, where the earth 

 is not separated, yet may well be supposed to exist, as in the alkali of pearl-ash, 

 purified by 3 repeated calcinations and solutions. Hence also 100 gr. of such 

 alkali, free from earth, water, and fixed air, take up 46.77 gr. of the mineral 

 acids, that is, of the mere acid part; and 100 gr. of common mild vegetable 

 alkali take up about 36.23 of real acid. 



Now, 100 gr. of perfectly dry tartar vitriolate contain 30.21 of real acid, 64. 61 

 of fixed alkali, and 5.18 of water. Crystallized tartar vitriolate loses only 1 per 

 cent, of water in a heat in which its acid also is not separated in any degree, and 

 therefore contains 6.18 of water. 



Again, 100 gr. of nitre, perfectly dried, contain 30.86 of acid, 66 of alkali, 

 and 3.14 of water; but in crystallized nitre the proportion of water is somewhat 

 greater; for 100 gr. of these crystals, being exposed to a heat of 180° for 2 hours, 

 lost 3 gr. of their weight, without exhaling any acid smell; but when exposed to 

 a heat of 200°, the smell of the nitrous acid is distinctly perceived. Hence 100 

 gr. of crystallized nitre contain 29.89 of mere acid, 63.97 of alkali, and 6.14 



of water. 



And 100 gr. of digestive salt perfectly dry contain 29.68 of marine acid, 63.47 

 of alkali, and 6.85 of water. 100 gr. of crystallized digestive salt lost but 1 gr. 

 of their weight before the smell of the marine acid is perceived; and hence they 

 contain 7-85 gr. of water. 



But the mistake which cost me most time and pains to correct, was that fallen 

 into when I imagined, that the mixtures of oil of vitriol and water, and spirit of 

 nitre and water, had attained their maximum of density when they had cooled to 

 the temperature of the atmosphere, which at the time of the experiments stood 

 between 50 and 60° of Fahrenheit. The former I had even suffered to stand 6 

 hours which was much longer than was necessary for its cooling; but when the 

 acid was so much diluted as to cause little or no heat, I allowed it to stand but 

 for a very little time before I examined its density: yet several months after I 

 found many of these mixtures much denser than when I first examined them, 

 and that at least 12 hours rest was requisite before concentrated oil of vitriol, to 

 which even twice its weight of water is added, attains its utmost density, and still 

 more when a less proportion of water is used: thus, when I made the mixture 



