5. Properties of an Ideal Disinfectant for External Use. We can specify certain properties which an ideal disinfecting substance should possess, but unfortunately we possess no dis infectant in which all these properties are simultaneously pres ent. They are the following, and their importance is self-evident: (a) It should work effectively, i.e., disinfect, with or without the presence of organic matter.
(b) It should be reasonably stable from a chemical stand point.
(c) It should be soluble or miscible in water.
(d) It should be harmless to man or higher animals while highly toxic to micro-organisms.
(e) It should possess great powers of penetration.
(t) It should not corrode metal, bleach pigments, or rot fabrics.
(g) It should be commonly available at a reasonable price.
(h) It should be effective against all infective agents.
6. Conditions Influencing the Efficiency of Strength of solution employed : As a general rule the activity of disinfectants is directly proportional to their concentration, while very potent substances may be so highly diluted that they are without effect.
(b) The length of time in which they are permitted to act: No disinfectant, with the possible exception of incineration, is in stantaneous in its action on all the infective agents present in a given medium. An appreciable length of time must elapse for its effect to be achieved.
(c) Certain agencies exert what is known as a selective ac tion. That is they are lethal for some micro-organisms but without action on others. Therefore one must use a certain amount of discretion in the selection of a disinfectant for the destruction of a given parasite.
(d) The material to be disinfected may influence the results in several ways. First, organic matter may afford a mechanical protection for the infective agents and thus defeat our purpose, or there may be a chemical combination between the disin fectant and other substances present, as a result of which the effective quantity of our agent is seriously reduced. Or lastly, the material may contain resistant stages of the micro-organ isms, such as spores, which are not readily affected by the agen cies of disinfection selected.
While it is theoretically possible to estimate the necessary quantities of disinfectant to apply in any given case, yet in practice it is difficult to estimate the effect of the foregoing factors, since they are extremely variable. Hence in practice
disinfectants must be employed in excess quantities.
7. Technic of Concurrent or Concommitant Disinfection. The number of substances suitable for the purpose is not large, if we select those which most nearly possess our ideal properties. The following solutions may be considered standard.
(a) For the disinfection of feces and urine the following may be used : 1. Formally, in io per cent. solution. Mix equal volume of this solution with the excreta and allow to stand for one hour. This is very effective, but the irritating fumes prevent its use in the sick room.
7 2. Phenol in 5 per cent. solution. Mix equal volume of this solution with the excreta and allow the mixture to stand for one hour.
3. Unslaked Lime.—Add a quantity of unslaked lime of approximately equal volume to the excreta and mix well by means of a small stick. Cover with I to 2 quarts of boiling water and allow the mixture to stand for two hours. This is one of the cheapest methods.
4. Chlorinated lime, in 3 per cent. solution. One must be sure that the preparation is fresh, otherwise its strength will be greatly reduced by loss of the chlorine. Mix equal volumes of excreta and the solution and allow to stand for one hour.
5. Liquor cresolis composites in 2 per cent. solution. This is one of the cheapest and best substances for this purpose. Mix equal volumes of the excreta and disinfectant and allow to stand for one hour.
6. The foregoing will prove satisfactory for the destruction of bacterial infective agents in the feces, but will not destroy protozoan parasites or helminth ova. According to Dr. C. W. Stiles either 3 per cent. sodium hydroxide or four tenths per cent. sodium dichromate may be used to destroy protozoan parasites or helminth ova. Most species are killed relatively soon by this treatment, but the ova of Ascaris or Oxyuris may require from four to six days exposure before they are destroyed.
(b) For the disinfection of discharges from the mouth and nose.