Nitric J. ul and the Nitrates of Mercury. 56 



through ihe mass. After it is prepared, the acid thus diffused 

 probably suffers no change, but the pronitrate present suffers 

 decomposition, its acid being; attracted slowly by the ointment, 

 the protoxide is separated, which being blue, and the ointment 

 itself being yellow, makes the mass appear green. This change 

 is accelerated by heat, and therefore, if the ointment is kept 

 melted the ointment soon acquires a green colour, after which, 

 if it is still kept melted, the blue oxide subsides, when the 

 ointment recovers its yellow colour. When the ointment is 

 prepared so as lo contain no pronitrate, it remains [permanently 

 of a yellow colour. 



The colour of the ointment varies according to the propor- 

 tion of pernitrnte of mercury which it contains, a small quan- 

 tity forming a light coloured ointment, the colour increasing in 

 intensity in proportion to the quantity of pernitrate which a 

 given weight contains. The colour also depends in some 

 measure upon the degree to which the lard is oxygenized, for 

 when the operation is continued some time, during which the 

 acid continues to suffer decomposition, the lard attracting 

 oxygen, and nitric oxide with nitrogen being disengaged, this 

 compound forms with nitrate of mtrcury a darker compound 

 than is formed from the same materials when the operation has 

 not been carried on for any length of lime. 



The ointment, as usually prepared, therefore containing in- 

 gredients which are riot in a st;ite of chemical combination with 

 it, it may be a proper subject of inquiry whether those ex- 

 traneous substances tend to impair its medical properties, and, 

 therefore, whether a preparation consisting only of materials in 

 intimate union with each other may not be more eligible. I 

 therefore have used the ointment when washed, and though the 

 number of the trials I have made docs not enable me to say 

 confidently that it will be found always superior in efficacy to 

 the other, it has fully answered my wishes. But, as by washing 

 i be ointment a part of the materials is lost, it appears to me 

 desirable to prepare it in such a way, as that it shall be similar 

 in compositon to the ointment when washed, and so effect this 

 without the loss of the materials, or at least of the mercury. 

 This I think may be effected in the following manner: — First 

 prepare an ointment of nitrated lard, wash it so as to free it 

 from uncombined acid, and afterwards combine it with perni- 

 trate of mercury. My this means, though a portion of acid is 

 lost, there is no loss of the mercurial salt, and in making an 

 ointment of the same strength as that usually prepared, we 

 have only to calculate the additional quantity of acid required, 

 in order that there may be left the same quantity of acid as 

 ~d its it, Itate of - 



