jj.4» ■Comh'tmtknt efthe Acid ?/"Chvomei 



•Jn boiling upon one part of red lead reduced into fine powder, two parts of carbonate of 

 nlkali with forty parts of water. By this means a double aflinity is made to operate, 

 ■by virtue of which carbonate of lead is formed, and falls to the bottom, while a combina- 

 tion of thii acid of red lead with the alkali made ufe of remains fufpended in the water, 

 by virtue of its folubility. 



'I he proportion of alkaline carbonate proper to be ufed for the decompofitioii of the 

 ■native red lead muft vary according to the proportions which fubfift between tlie bafes, 

 the aciiLs, and the water of cryftallization. The proportions dated in the lad paragraph 

 are fuitcd to the carbonate of pot-afli ; but the proper quantises in the other cafes are 

 eafily afcertained, by ufing a fmall quantity of the carbonate at firft, and afterwards 

 gradually increafing it, till the greateft part of the red lead is decompofed. On the 

 whole, it is better to ufe a fmaller thaft too large a portion of the carbonate, jn order 

 that its bafe may be mote completely faturated vv-ith the acid of the red lead. 



There is no reafon to fear a fimilar inconvenience with the carbonate of ammoniac. It 

 is proper, on the contrary, to add more than is fufiicient to faturate the acid of the red 

 lead; becaufe a portion of this fait will always be driven off during the ebullition which 

 is indifpenfably nece(T\try to produce the intended decompofition. And even in cafe there 

 fliould remain an excefs after the complete decompofition of the red lead, there would 

 be no danger, becaufe this excefs of alkali is driven off by the evaporation of the fait 

 newly formed, and there is always a certainty of obtaining the combination perfedly 

 pure. 



The colour of the combinations of the acid of red lead with the alkalis is an orange 

 yellow *. Their folutions afford cryftals nearly of the fame tinge, the differences being 

 only flight variations of fliade. Thefe falts are decompofed by barytes, lime and (tron- 

 tian. The mineral acids likewife decompofe them by an inverfe operation. 



Thefe falts afford oxygen gas by the adion of heat, and tbeir refidue has the appear- 

 ance of a green mafs. The ammoniacal compound muft however be excepted, as its bafe 

 is partly decompofed by the oxygen of the acid, and leaves in the retort a pure green 

 oxide, the undecompofed portion of ammoniac being alfo driven off by the heat. 

 - Thefe falts decompofe by double affinity the calcareous, barytic, magnefian, aluminous 

 falts, &c. Mod of the metallic falts are likewife decompofed by thefe fubdances, and 

 afford new combinations fcarcely or not at all foluble in water; and for the mod part 

 red, yellow, orange, or lemon colour. 



I have not hitherto been able to determine the figure of thefe falts with accuracy, nor 

 their degree of folubility in water, for the reafons 1 have already explained ; neither have 

 I made all the combinations of this acid with the metallic oxides, nor determined its mode 

 of a6tion on the metals. Such experiments would no doubt have afforded ir^tereding 

 "phenomena for the completion of the hidory of this metallic acid. 



But I indulge the hope that thefe experiments are only retarded, and that fome fortunate 

 opportunity will hereafter enable us to perforni them : and, at all events, the experiments 

 already made will be fufficient to afcertain the prefence of the acid of red lead wherever 

 it may be met with, and didinguifli it from every other natural fubdance. 



* That of the ammoniacal fait, which has tlie form of plated cryftals, is yellow, and exhibits the metallic 

 briUiancy of gold. V. 



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