In order to test the action of disinfectants on the development and growth of bacteria, Koch put into a number of small watch-glasses, or rather crystallisation-glasses with flat bottoms, a few drops of blood-serum, or a solution of extract of meat and peptone, mixed with varying quantities of the disinfectant. Into each of these a silk thread, which had been dipped in the fluid containing bacteria and dried, was placed. In one glass serum alone, without any disinfectant, was placed, in order to ascertain, by comparison with the growth which takes place in it, how the disinfectant in the other glasses had interfered with the growth of the bacilli.

In experiments of this sort a difference was found between anthrax bacilli and other microzymes. A dilution of carbolic acid, 1 in 1,250 and 1 in 850, sufficed to prevent the growth of anthrax bacilli, while a strength of 1 in 500 was required to prevent the growth of others.

Other species are therefore more resistant than anthrax bacilli to the action of carbolic acid. The following table shows the strength of various disinfectants required to hinder or entirely prevent the development of anthrax bacilli: -

Solution

Hinders

Prevents

Iodine ....

1 to 5,000 . .

-

Bromine ....

1 to 1,500 . .

-

Chlorine ....

1 to 1,500 . .

-

Osmic acid . . .

1 to 1,500 . .

-

Permanganate of potassium

1 to 3,000 . .

-

Corrosive sublimate . . .

1 to 1,000,000 . .

1 to 300,000

Allyl alcohol . . . .

1 to 167,000 . .

-

Oil of mustard. . . .

1 to 330,000 . .

1 to 33,000

Thymol ....

1 to 80,000 . .

-

Peppermint oil. . . .

1 to 33,000 . .

-

Oil of turpentine . . .

1 to 75,000 . .

-

Oil of cloves . . .

1 to 5,000 . .

-

Arsenite of potassium . .

1 to 100,000 . .

1 to 10,000

Chromic acid . . .

1 to 10,000 . .

1 to 5,000

Picric acid . . . .

1 to 10,000 . .

-

Hydrocyanic acid . .

1 to 40,000 . .

1 to 8,000

The following are about the same strength as carbolic acid: -

Fluid

Hinders

Prevents

Boric acid . . .

1 to 1,250 . .

1 to 800

Borax ....

1 to 2,000 . .

1 to 700

Hydrochloric acid . .

1 to 2,500 . .

1 to 1,700

Salicylic acid . . .

1 to 3,300 . .

1 to 1,500

Benzoic acid . . .

1 to 2,000 . .

-

Camphor ....

1 to 2,500 . .

-

Eucalyptol . . .

1 to 2,500 . .

-

Soft soap . . . .

1 to 500 . .

1 to 5,000

Quinine . . . .

1 to 830 . .

1 to 625

Hydrate of chloral .

1 to 1,000 . .

-

Chlorate of potassium . .

1 to 250 . .

-

Acetic acid . . .

1 to 250 . .

-

Benzoate of sodium . .

1 to 200 . .

-

Alcohol . . . .

1 to 100 . .

1 to 12.5

Acetone . . . .

1 to 50 No action . .

-

Chloride of sodium . .

1 to 64 . .

-

Influence Of The Solvent

Although a 5 per cent. solution of carbolic acid in water has a well-marked destructive action on the spores, and a strong destructive action on fully-developed anthrax bacilli, a solution of the same strength in oil or alcohol has not the least disinfectant action. A similar influence with regard to iodine is observable in the previous tables.

Effect Of The Fluid With Which Disinfectants Are Mixed

This is sometimes very marked, especially in the case of free iodine, bromine, or chlorine. These in watery solutions are powerful disinfectants, but when mixed with fluids which contain alkalies, e.g. blood-serum, they are converted into bromides, iodides, and chlorides, and their action is very greatly diminished. The action of corrosive sublimate, however, and of ethereal oils is not altered.

Influence Of Temperature On The Action Of Antiseptics

The action of antiseptics is greatly increased by a high temperature. Spores of anthrax bacilli exposed to the vapour of carbolic acid at 15°-20° C. remain unchanged even after 45 days' exposure. When exposed to the vapour of carbolic acid at a temperature of 55° C. the case is very different. Half an hour's exposure does not seem to harm them at this temperature, but many are destroyed by an exposure of an hour and a half, and very few will stand 3 hours' exposure, so that probably an exposure of 5 or 6 hours would destroy the whole of them.