270 C.-E. A. WiNSLOW AND E. E. LOCHRIDGE 



in a hundred thousand or a million, may persist at much higher 

 temperatures, even 70°." Gage and Stoughton in their conclusions 

 point out that "the great majority of the bacteria in any B. coli 

 cultures are destroyed by five minutes' exposure to some tempera- 

 ture between 50° and 60° C. A few individuals, however, in each 

 culture will survive much higher temperatures, in some cases re- 

 maining alive after exposure to 90° C. The very close range (about 

 10° C.) of temperature at which the destruction of the majority of the 

 individual bacteria occurred, as compared with the considerable 

 range (about 35° C), in the temperatures at which complete sterili- 

 zation was effected would indicate that the determination of the 

 majority death-point would be of more value in species identification 

 than is the determination of the absolute thermal death-point as at 

 present employed." 



Altogether it seems clear that among what are ordinarily con- 

 sidered non-sporing bacteria there exists a small proportion of indi- 

 viduals having specially high resistant powers against unfavorable 

 conditions. The absolute death-point for these resistant forms is 

 difficult to determine accurately on account of their small numbers 

 and the consequent chances that they may be overlooked. We are 

 inclined from our experience to agree with Gage and Stoughton as 

 to the superior value for many purposes of the majority death-point 

 (99 per cent), and we shall lay special stress on this in interpreting 

 our results. 



3. THE DISINFECTANT ACTION OF HYDROCHLORIC ACID AND 

 SULPHURIC ACID UPON B. TYPHI AND B. COLI. 



Hydrochloric acid and sulphuric acid were chosen as types for 

 the study of strong mineral acids, and the experiments were carried 

 out as described above. The water used was Boston tap water, 

 containing before sterihzation about 40 parts per million of residue, 

 15 parts of hardness, 0.015 P^i"t ^^ free ammonia, and 0.144 parl of 

 albuminoid ammonia. The results are shown in Tables 1-4. 



The 99 per cent killing-point with the hydrochloric acid is reached 

 at a strength of 0.0077 normal, with 7.49 parts of dissociated hydro- 

 gen per million, and the absolute killing-point, as nearly as it can be 

 determined, with a 0.0123 normal solution containing 11.80 parts 



