548 PHILOSOPHICAL TRANSACTIONS. [ANKO 1784. 



terra ponderosa caustica ought to be placed below the alkalis, except in the 

 column appropriated to the vitriolic acid ; and it may be separated even from that 

 acid, by the vegetable fixed alkali, if the alkali be applied via sicca, as will ap- 

 pear in the next section . 



7thly. The necessity for using pure acids on many occasions, and the difficulty 

 of obtaining them pure, are sufficiently obvious. The vitriolic acid, as made in 

 the large way, is generally pure enough for most purposes. It is apt to get 

 coloured by inflammable matter ; but this is seldom an inconvenience ; and, 

 when it would be so, it is easy to drive it off by boiling the acid in a Florence 

 flask over a common fire. But there is another cause of impurity in this acid, 

 which appears on diluting it with water ; for then it becomes milky, and in a 

 short time a powder subsides.* The acid may be freed from this powder either 

 by distillation in glass vessels, which is a tedious and dangerous process, or by 

 the affusion of water ; and after the powder has subsided, a gentle evaporation 

 will drive off most of the superfluous water. 



Nitrous acid may be freed from vitriolic and marine acids, by solution of silver 

 in the acid of nitre, as is daily practised ; but the marine acid has not, to my 

 knowledge, been purified by any other method than the laborious one of re- 

 distilling it from common salt. It is generally mixed with vitriolic acid, and 

 often in large proportion. There is no temptation, and scarcely an opportunity, 

 for it to be contaminated by nitrous acid. From the vitriolic acid then it may 

 be readily purified by the addition of terra ponderosa caustica dissolved in water, 

 or by the terra ponderosa salita. If the latter be used, a small portion of the 

 acid must first be tried in a diluted state, whence we must judge how much of 

 the terra ponderosa salita the whole will require ; or else the whole of the acid 

 must be diluted with water. Whether we use the terra ponderosa dissolved in 

 water or in marine acid ; in either case the acid of vitriol immediately seizes on 

 it, and subsides with it in form of an insoluble powder. 



Sect. 2. — Terra Ponderosa Vitriolata.-\- Bergman's Sciagraphia, ^ 58, 89. 



* About 2 years ago I examined this powdery matter ; both that which was thrown down by di- 

 lution with water, and also some which Dr. Priestley gave me, being the residuum of vitriolic acid dis- 

 tilled to dryness in a flint-glass retort. 1st, Repeated boiling in water, reduced 6\ grs. to 2 grs. 2dly, 

 This solution, by gentle evaporation, afforded 5 grs. of crystals, as hard and as tasteless as selenite. 

 3dly, To these crystals, re-dissolved in water, mild fossil alkali was added, and a white powder pre- 

 cipitated. 4thly, This powder, after exposure to a pretty sharp heat, was thrown into water ; part 

 of it dissolved, and the water got the taste and other properties of lime-water, othly, The insolu- 

 ble part (1) suffered no change by boiling in nitrous acid ; one-half of it mixed with borax, and ex- 

 posed to the blow-pipe on charcoal, vitrified ; the other half, mixed widi borax and charcoal-dust, 

 likewise vitrified. It appears then, that the greater part of this substance was calxvitriolata, or selenite; 

 the remainder a verifiable earth. I had before found, that the terra ponderosa vitriolata, or heavy 

 gypsum, would dissolve in concentrated vitriolic acid ; but always separated in a powdery form on 

 the affusion of water ; and now it appears that calx vitriolata, or selenite, does the same. — Orig. 



+ Sulphate of barytes. 



