40 LABORATORY EXERCISES IN BACTERIOLOGY. 



through the porcelain may be capable of conveying some of the bacteria through the 

 filter. Moreover, after a time, with any degree of forced filtration there is danger that 

 the smaller forms of bacteria will slowly penetrate through the porcelain. For this rea- 

 son the value of this form of sterilization is limited and to be recommended only when 

 other methods are not available for the purpose desired. In the selection of the filter 

 care should be exercised that the porcelain is of even and compact composition, and of 

 considerable thickness where the amount of fluid to be filtered is at all large. After a 

 filter has been used its further value for sterilization is often doubtful, for several reasons. 

 It may, it is true, be cleansed by reversing its position in the apparatus (Fig. 10) and 

 drawing through it a large amount of clean water, thus washing back the microbes and 

 other solid particles lodged in its interstices ; after which it is to be resterilized in the 

 usual manner in the autoclave. Such cleansing is, however, in many instances of little 

 practical efficacy, and the permeability of the filter in consequence diminished even if 

 the foreign particles are of no vitality after exposure to the heat of the autoclave. 



Exercise 14. Having prepared and set up a Kitasato or other filter as 

 above outlined, filter some tainted meat solution into the receiving flask. 

 Let each student then inoculate a fresh tube of sterile nutrient medium 

 from the filtrate, place the inoculated tube in the incubator, and note re- 

 sults as usual at the end of the first, second, and third days in order to de- 

 termine the efficiency of the process as a means of sterilization. 



Exercise 15. Repeat the above exercise, employing some large, known 

 bacterium, as the Bacillus subtilis in a hay infusion, as the material for in- 

 oculation ; and in the same way note results after inoculation of the sterile 

 nutrient medium with such material. 



(C) Sterilization by Means of Chemical Agents. Various chemicals in solution or 

 in vapor are commonly employed for the purpose of bacterial destruction or for retarda- 

 tion of their growth, the terms disinfectant and germicide being used to indicate such 

 substances as are capable of destroying microbic life, and antiseptic to indicate such as 

 are merely capable of restraining the growth or other functions of bacteria without 

 actually killing them. The confidence which was formerly reposed in many of the so- 

 called disinfectants has in recent times been considerably disturbed by the discovery that 

 in many instances the bacteria apparently killed are really living, and can by proper 

 procedures be caused to resume their vital activities. It was formerly supposed that 

 such substances as mercuric chloride, which may be taken as a type of the disinfectants, 

 were directly lethal by mere contact with the germ, the chemical substance itself not 

 being modified by such contact. Recent studies, however, indicate that true chemical 

 compounds are formed from organic materials and the metallic base of the salt, spoken of 

 collectively as mercurial albuminates, the haloid element being evolved in free state and 

 itself further combining. Such destruction of the disinfectant and combination of its 

 elements must, of course, render inert for further use in sterilization just such propor- 

 tion of the salt as has entered into the chemical change ; and the albuminate, thus 

 formed in greater or less amount, acts as a protective covering around the individual 

 germs and upon the exterior of any mass which should be penetrated by the disinfectant 

 solution. This coating may effectively protect the individual germs against further ac- 

 tivity of the disinfectant, and, although mechanically preventing in greater or less 

 measure the manifestation of their vital phenomena, may permit the maintenance of 



