326 PHARMACEUTICAL BACTERIOLOGY 



be brought about, as in washing, immersing or mixing. Gaseous disin- 

 fectants are effective for surface sterilization, especially useful for inac- 

 cessible rooms, buildings, ships, paintings, books, fabric, etc. Both have 

 their special advantages, however. 



The number of chemical disinfectants, variously classed as gaseous, 

 liquid, patent, proprietary, efficient, useful, useless, etc., is very great. 

 We shall mention only a* few of the more powerful kinds. No reliance 

 should be placed in any patented or proprietary disinfectant until 

 its value has been demonstrated by tests made by reliable bacteriologists, 

 giving its phenol coefficient. Nor is this all, not only must the disinfectant 

 have actual germ-destroying powers, but it must also be practically usable 

 and it must not be misrepresented as to its value and its application and 

 use in practice. 



The resistance of pathogenic germs to disinfectants is extremely 

 variable. Furthermore, the various disinfectants produce changes in the 

 tissues and substances in and upon which they act, which changes tend to 

 modify, check or inhibit the disinfecting powers. Thus a number of 

 disinfectants may have the same laboratory phenol coefficient and yet 

 their value as disinfectants in actual practice is widely different because 

 of the difference in the effects produced in and upon the substances with 

 which they are brought in Contact. 



As a rule, the action and use of disinfectants is variable according to 

 the following conditions : 



1. Disinfectants are more active when warm or hot. In aU disinfec- 

 tions hot solutions should be used, if possible and if practicable. 



2. Gaseous disinfectants act only in the presence of moisture, as will be 

 explained under formaUn and sulphur disinfection. 



3. The thoroughness of disinfection is directly proportional to the 

 time that the disinfectants are allowed to act. 



4. The activity of disinfectants is directly proportional to the degree of 

 concentration, though there are noteworthy exceptions. Absolute alcohol, 

 for example, is of very little value as a disinfectant, whereas the weaker 

 solutions (40 to 70 per cent.) are very active germ destroyers. The 

 same is true of ether, chloroform, glycerin and a number of other sub- 

 stances. Most disinfectants have a concentration of optimum or maxi- 

 mum efficiency which is the degree of concentration generally employed 

 in practice. 



5. In actual practice the cost of disinfectants is a factor of some 

 importance, as is indicated by the table giving the comparative phenol 

 coefficient and the relative cost: 



6. It is known that the disinfecting power of metallic salts is propor- 

 tionate to their electric dissociation, that is, the more strongly a salt is 



