16 



Scientific Proceedings (125) 



intensity as does G586A. It will be noted, however, that when 

 the latter was used rickets did not develop with long exposures, 

 whereas when G86B was interposed marked rickets developed, 

 even with exposures of 60 minutes at a distance of 9 inches. The 

 chief difference between these filters is that G86B, which is a 

 nearly neutral filter, allows far more of the longer visible rays to 

 pass than docs G586A, which is a purple filter. Further ex- 

 periments are in progress to ascertain whether the interference 

 of visible rays can account for these divergent results. 



Filters of various clothing material were also employed. It 

 was found that woolen as well as cotton goods interfered with 

 the activity of the light in proportion to their thickness, but did 

 not prevent protection if the dosage of irradiation was made 

 adequate. Black cotton material filtered out the protective rays 

 to an extent greater than white material of the same weave. 



5 (1965) 



_ The mechanism of bacteriostasis. 



By JOHN W. CHURCHMAN 



[From the Department of Hygiene, Cornell University Medical 

 College, New York City] 



The effect of bactericidal agents is often tested by adding these 

 substances to the media on which the organisms are planted ; and 

 the assumption is usually made that if the substances, when pres- 

 ent in the media, exhibit a selective hostility to bacteria they will 

 exhibit a hostility — selective in the same sense — when added 

 directly to the organisms themselves. This assumption is usually 

 justified by the facts; no single exception to such a parallelism 

 has been met with in the large number of experiments made with 

 gentian violet and allied tri-phenylmethanes. We are in the habit 

 therefore of reasoning from experiments which test bacteriostasis 

 to conclusions as to bactericidal value, at least so far as selective 

 features are concerned. 



Proof will here be presented to show that this sort of reason- 

 ing is not always justified by the facts. Suppose for example 

 that we plant B. prodigiosns and B. megatherium on acid fuchsin 

 agar and find that B. prodigiosns grows well and B. megatherium 



