60 MATERIA MEDIC A AND THERAPEUTICS. 



2. PlumM Carbouas. Carbonate of Lead. " White 

 Lead." PbC0 3 . 



Characters. A heavy white powder. Insoluble in water. 

 Impurities. Lime ; sulphate of lead. 



Preparation. 

 TJnguentum Plumbi Carbonatis. 1 in 8. 



3. PlumM Nitras. Nitrate of Lead.^ Pb(N0 3 ) 2 . 

 Source. Made by dissolving lead in boiling nitric acid 



slightly diluted, and crystallising out. 



Characters. Colourless octahedral nearly opaque crystals. 



from Plumbi Nitras is made : 



Plumbi lodidum. Iodide of Lead. PbI 2 . 



Source. Made by mixing solutions of Nitrate of 

 Lead and Iodide of Potassium. Pb2(N0 3 ) + 2KI = 

 PbI 2 -f 2KNO 3 . 



Characters. A bright yellow powder or crystalline 

 scales. 



Preparations. 



a. Emplastrum Plumbi lodidi. 1 in 9. 



b. Unguentum Plumbi lodidi. 1 in 8. 



ACTION AND USES. 

 1. IMMEDIATE LOCAL ACTION AND USES. 



Externally. Lead is said not to be absorbed by the un- 

 broken skin ; yet the dilute solution of the subacetate (Goulard 

 water) is of unquestionable value in the treatment of contu- 

 sions and superficial inflammations, such as erysipelas, and is 

 extensively used in these conditions. In the same form, or 

 as the ointment of the subacetate, it relieves itching ; a 

 symptom, the cause and pathological nature of which are still 

 obscure. Lead may be absorbed when applied in the form of 

 ointment, probably by decomposition ; and the specific effects 

 to be presently described may arise in this way. Lead salts 

 act readily upon wounds, ulcers, and exposed mucous mem- 

 branes : (1) precipitating the albuminous fluids which cover 

 their surface, or are flowing from them as a discharge ; (2) co- 

 agulating the protoplasm of the young cells of the superficial 

 layers ; and (3) contracting the small arteries and veins of the 

 part, thus diminishing or even arresting the circulation within 

 them, and preventing the escape of the plasma and blood-cells 

 through their walls ; whilst (4) the nerves are probably also 



