206 VETERINARY HYGIENE 



was as high as eightfold, using B. paratyphosus as test organisms 

 in each case."* 



PERCHLORIDE OF MERCURY (Mercuric Chloride, Corrosive Sub- 

 limate, HgCl 2 ). Perchloride of mercury for a long time held first 

 place among disinfectants both for surgical and general use. It is 

 now generally recognised, however, that though it is a very power- 

 ful bactericide, that is to say, it can destroy micro-organisms in a 

 short space of time even in very dilute solution, its practical use is 

 far more limited than was formerly thought. 



Corrosive sublimate is soluble in 18 parts of water, but for 

 general disinfection purposes is rarely used in greater strength 

 than 1-500, while 1-1000 to 1-2000 are strengths more commonly 

 used. When the salt is dissolved in water the molecules of HgCl 2 

 do not remain as such but undergo ionisation to separate atoms of 

 mercury and chlorine, one molecule of the salt splitting into one 

 atom of mercury and two atoms of chlorine. 



The mercury ions are positively charged and the chlorine ions 

 negatively. The ionisation is, however, never complete, and a 

 certain proportion of the perchloride molecules remain in solution 

 as such. The disinfectant power of this metallic salt is chiefly due 

 to the free mercury ions which, coming into contact with the proto- 

 plasm of the bacteria, unite with it to form the insoluble albuminate 

 of mercury to the destruction of the organism. 



The free chlorine ions also possess a disinfectant action, but 

 their power is less than that of the mercury. Since it is the free 

 mercury ions that actually destroy the bacteria, the potency of any 

 solution of corrosive sublimate depends upon the ionisation that has 

 taken place. Any substances added to the solution, therefore, that 

 tend to hinder ionisation reduce the utility of the salt. The ionisa- 

 tion, however, may be increased by the addition of a small amount 

 of an acid. The potency of the Seymour- Jones Formic-Mercury 

 process of disinfecting hides is largely due to the action of 

 formic acid. Tartaric acid is often combined with the corrosive 

 sublimate for the purpose of increasing its action when it is made 

 up in the tablets of commerce that are so frequently used. Hydro- 

 chloric acid is often used to acidify the solution, and the Local 

 Government Board recommends the following : corrosive sub- 

 limate, % oz. ; hydrochloric acid, 1 oz. ; aniline blue, 5 grains ; 

 water, 3 gallons. This gives a strength of about one of the sub- 

 limate to 980 of water. 



Common salt, though it has its uses when combined with the 

 mercury salt, as will be explained later, actually hinders free ionisa- 

 * Handbook of Antiseptics, Dakin and Dunham. 



