LABORATORY STERILIZATION 275 
bacteria form spores. If tetanus, anthrax or gas bacillus cultures 
are to be destroyed, the autoclave is necessary. 
Chemical Solutions.^— Chemical disinfectants are most efficient in 
aqueous solutions, and they must therefore be soluble in water. 
Moisture is also essential for gaseous disinfectants. 
The theory of the germicidal action of disinfectants is not avcU 
understood; apparently the efficiency of salts of heavy metals is 
associated with their noteworthy affinity for proteins, with which 
they fo'rm firm combinations. It must be remembered that these 
salts react more quickly with animal proteins than bacterial proteins, 
therefore greater concentrations of metallic salts are required to kill 
bacteria suspended in protein solutions than to destroy the same 
organisms in aqueous suspension. Thus, typhoid bacilli may be 
killed by a 1 to 500,000 solution of bichloride of mercury if they are sus- 
pended in water, but a concentration of at least 1 to 1500 is required to 
sterihze the same organism in blood serum. Absolute alcohol does not 
appear to be a very powerful germicide; possibly its rather limited 
germicidal value is associated with its dehydrating properties. Dilute 
solutions of alcohol, 20 to 30 per cent, are practically as destructive 
of bacteria as absolute alcohol is. Phenols are excellent germicides 
in aqueous solutions, but their tendency to go into solution in oils 
(which do not readily penetrate the ectoplasm of cellular structures), 
makes them unreliable germicides in oily menstrua. 
Salts of Heavy Metals. — 1. Mercuric Chloride. HgCl 2.— Mercuric 
chloride or bichloride of mercury is a powerful germicide, very soluble 
in hot water, less soluble in cold water.- It is usually dispensed in 
tablet form mixed with NaCl, which increases its solubility and also 
prevents somewhat its marked tendency to unite with proteins. This 
is of importance in the treatment of wounds and secretions of wounds 
with this germicide. A 1 to 1000 solution of bichloride in water is 
the dilution commonly used for practical purposes. This strength 
of solution will kill all pathogenic bacteria in a very short time; a 
solution of 1 to 500 strength will even kill anthrax spores within a few 
hours. 
The advantage of bichloride of mercury as a germicide resides in its 
great bactericidal powers. Its disadvantages are: its marked affinity 
for protein which, in the case of wounds, may lead to local necrosis of 
tissue, or in greater concentrations, by absorption, to toxic action of 
the kidneys, intestinal tract, and even the central nervous system. It 
is unreliable for the disinfection of sputum, feces, urine, purulent dis- 
charges and other excreta, and it should never be employed in the ster- 
ilization of instruments or eating utensils. Linen soiled with blood or 
stained in any way should not be immersed in bichloride, for it acts as 
a mordant and "sets" the stain. 
1 See Rideal, S. and E.: Chemical Sterilization, London, 1921. 
- One part of bichloride will dissolve in 3 to 4 parts of boiling, distilled water; upon 
cooling, much of the bichloride becomes insoluble; 1 part of the salt will dissolve in 
16 to 18 parts of water at room temperature. 
