VOL.LXXIII.] 



PHILOSOPHICAL TRANSACTIONS. 



337 



sides that which escapes in an aerial form, much more of it, though separated 

 from the metallic earth, is yet retained in the solution by the compound of acid 

 and calx. It is this calx, thus differently dephlogisticated by the different acids, 

 whose proportion I endeavoured to ascertain. 



The great difficulty that occurred in this inquiry was, that of finding the ex- 

 act quantity of acid necessary to saturate the metallic substances; for all metallic 

 solutions turn solution of litmus red, and consequently contain an excess of acid. 

 And the reason is, because the salts, formed by a due proportion of metallic calx 

 and acid, are nearly insoluble in liquids that do not contain a further quantity of 

 acid; and in some cases this quantity, and even its proportion to the aqueous 

 part of the liquor, must be very considerable, as in solutions of bismuth. Hence 

 I in vain endeavoured, by caustic alkalis and lime-water, to deprive these solu- 

 tions of this excess; for when deprived even of only part of it, many of the 

 metals precipitated, and all would, if deprived of the whole of it. On this ac- 

 count I was obliged to use different methods. The vitriolic solutions of tin, bis- 

 muth, regulus of antimony, nickel, and regulus of arsenic, containing a large 

 excess of acid, I saturated part of it with caustic volatile alkali before I tried 

 them with the infusion of litmus, and I used the same expedient with the nitrous 

 solution of iron, lead, tin, and regulus of antimony, and all the marine solu- 

 tions. The proportion of vitriolic and marine acid taken up by lead, silver, and 

 mercury, I determined by computing the quantity of real acid necessary to pre- 

 cipitate these metals from their solutions in the nitrous acid; and of all the deter- 

 minations these appeared to be the most exact. However, as all the vitriols of 

 these metals are, though in a slight degree, soluble in the nitrous acid, I was 

 obliged to rectify the result from other considerations, and the same necessity 

 occurred with regard to the marine salts of lead and mercury. 



The result of these experiments was, that 100 grs. of each of these acids take 

 up, at the point of saturation of each metallic substance, dephlogisticated to 

 such a degree as is necessary for its solution in each acid, the quantities expressed 

 in the following table, which denote their degree of affinity to each metal. 



Table of the affinity of the three mineral acids to metallic substances. 



100 grs. 



Vitriolic acid 

 Nitrous acid 

 Marine acid 



Reg.ofars. 



260 



220 

 290 



Of the precipitation of metals by each other from the mineral acids. — I am 

 now come to the last point of my inquiry, and the most difficult to be set forth 

 with that degree of precision which I have been enabled to attain in ttie former 



VOL. xv. X x 



