Naltiral Ilhtoty. 359 



one was aftected by the third rubbing, and the fourth put her 

 under ptyalism. Another, after rubbing three times, was so 

 salivated, that she spit two quarts in twenty-four hours, and 

 this, in a less degree, was kept up for ten days. The third was 

 not affected until she rubbed six times, and then not consider- 

 ably. They were all at length recovered. 



Finding the ointment so very active, Mr. Donovan conceived, 

 that by forming a chemical union between fat and oxide of 

 mercury, in very small quantity, the same result might be ob- 

 tained ; lard and black oxide of mercury were, therefore, kept at 

 the temperature of about 350° for two hours, continually stirring 

 tliem. At the end of the process, it appeared that every ounce 

 of lard had dissolved, and united with twenty-one grains of 

 oxide. 



This ointment was tried on many persons, and found to be 

 as active as the common mercurial, containing twelve times the 

 mercury. One drachm could be rubbed in completely, in from 

 six to ten or fifteen minutes, whilst common ointment required 

 thirty or forty minutes, and rarely was any eruption produced 

 on the part rubbed. The use of it is extremely cleanly, and 

 its expense is very much below that of the common ointment. 



For the preparation of this ointment, it is essential that the 

 lard be entirely free from salt, or else calomel will be formed. 

 The oxide may be prepared by decomposing calomel by pure 

 potash, or by pouring solution of nitrate of mercury into caustic 

 alkaline solution. 



The fat only dissolves three grains of oxide for each drachm, 

 but the quantity in the ointment may easily be increased. The 

 oxide should be first triturated with a little cold lard, to make 

 the penetration complete. 



The degree of heat applied is important. At 212° the oxide and 

 lard will not combine. At 600° the oxide will be decomposed and 

 mercury volatilized. At 500° and 400° the oxide is partially 

 decomposed, some red oxide being formed, and mercury re- 

 duced. The best heat is between 300° and 320° ; it should be 

 maintained at least an hour, and the ointment should be stirred 

 till cold. 



