74 BACTERIOLOGY. 
taking tetanus toxin as an example, it may be said that 
in aqueous solution it is not coagulated by heat, but is 
in time deprived of its poisonous qualities. The addi- 
tion of small quantities of acids or alkalies to the solu- 
tion, and the continued passage through it of carbon 
dioxide or sulphuretted hydrogen, distinctly reduce its 
toxicity. When exposed to light and air, either in a 
dry state or in solution, the toxin deteriorates rather 
rapidly. It withstands a temperature of 70° C. for 
some time without being wholly destroyed ; higher 
temperatures decompose it rapidly. When protected 
from the light and air it is slowly converted into an in- 
active substance; it is better preserved under absolute 
alcohol, pure ether, and the like. The toxicity of the 
purest tetanus toxin now obtainable is almost incred- 
ible: 0.00005 milligramme of it kills a mouse of 15 
grammes; a man of 150 pounds weight, if he were equally 
susceptible, would be killed with 0.23 milligrammes. 
It requires 30 to 100 milligrammes of strychnine to 
kill a man under ordinary circumstances. The most 
virulent diphtheria bacilli produce a specific poison 
which does not fall far behind that of tetanus in power. 
Sulphuretted Hydrogen. Sulphuretted hydrogen is a 
very common bacterial product. Its presence is deter- 
mined by pasting a piece of paper moistened with lead 
acetate inside the neck of the flask containing the cul- 
_ ture, closing the mouth with a cotton-wool stopper, and 
over this again an India-rubber cap (black rubber free 
from sulphur). The paper is colored at first brownish 
and later black; repeated observation is necessary, as 
the color sometimes disappears toward the end of the 
reaction. Apparently negative results should not be 
rashly accepted as conclusive. 
