42 LABORATORY EXERCISES IN BACTERIOLOGY. 



life. It has been shown that solution of such covering from the exterior of the bacteria, 

 by means of some chemical solvent, as a weak solution of ammonium sulphide, may be 

 followed by resumption of the life activities of the germs if they be transferred to a favor- 

 able surrounding. Considerable revision of opinion, particularly in connection with the 

 inorganic disinfectants, as to their relative sterilizing value has thus been en- 

 forced. At present it is believed that the organic disinfectants, as carbolic acid, and the 

 haloid elements, chlorine, bromine, and iodine, are less liable to give rise to such organic 

 combinations than the formerly esteemed metallic salts, as those of mercury, silver, 

 copper, and iron ; and the former are therefore more generally commended than formerly 

 for the purposes of sterilization. The objection mentioned concerning bichloride of 

 mercury obtains in varying degree with the other disinfectant salts of mercury and salts 

 of other metallic bases; but it should be recalled that this objection may be largely 

 obviated, if in their use in disinfection one remembers that the process is essentially a 

 quantitative one and employs a considerable excess of the disinfectant solution and 

 allows a long action period for thorough contact of the chemical with the bacteria in the 

 mass or liquid sought to be sterilized. In this connection it may be insisted upon as a 

 second and absolute requisite in the practice of disinfection by chemical means, that 

 there must be intimacy of contact between the disinfecting substance and the bacteria to be 

 destroyed, and that such contact must be maintained for at least a known minimum of 

 time. There may thus be formulated as a rule for chemical disinfection that in all such 

 usage there should be employed a free excess of the disinfectant, that it should be caused 

 to penetrate and diffuse through the mass or liquid subjected to its action so as to be 

 brought in intimate contact with the contained bacteria, and that it should be allowed 

 a sufficient period of time in which to accomplish the destruction of the bacteria. 



In the use of these chemical agents in liquid form, they are almost invariably found 

 more efficient if moderately heated (40 C., or above). They are not comparable to the 

 action of heat and are to be used only when every form of heating is unavailable for one or 

 other reason. Among the substances for the sterilization of which the chemical disinfec- 

 tants are usually selected, may be mentioned such pieces of glassware and laboratory 

 apparatus which on account of size cannot be introduced into the sterilizing ovens or 

 steam chambers, material like rubber goods, which would be injured in the heat, refuse 

 of various kinds, as the rejected contents of infected tubes (which should after collection 

 in such disinfectant solution be burned to render its destruction certain), the clothing 

 and dejecta of patients affected by infectious diseases, and a variety of similar sub- 

 stances. 



Disinfectant Solutions. Of the various disinfectant solutions, in spite of the objec- 

 tion above mentioned, that of mercuric chloride (1 : 1000 or 1 : 2000) has been and con- 

 tinues to be the most universally employed. Efforts to prevent the formation of the 

 organic compounds of mercury have been made by the addition of small proportions of 

 such materials as hydrochloric acid, sodium or ammonium chloride, or tartaric acid to 

 the solution; but it is doubtful whether any of these, save the last, serves any efficient 

 purpose or indeed does not hasten the active decomposition of the mercurial salt. If 

 used in free excess over the amount of material subjected to it for disinfection, the fol- 

 lowing stock solution offers a convenient and efficient medium for laboratory use : 



Mercuric chloride, 200 



Tartaric acid, 5 



Distilled water to 2000 



Ten cubic centimeters of the above to a liter of water make a solution of 1 : 1000 



