62 



LAWRENCE T. FAIRHALL AND PAUL M. BATES 



THE PENETRABILITY OF OIL TO ULTRA VIOLET RAYS 



In connection with the sterilization of oils, a measure of their 

 penetrability to ultra violet rays is of some practical importance. 

 In order to determine this, agar plates were painted with a saline 

 suspension of a staphylococcus and exposed to ultra violet rays 

 filtered through layers of oil of various thicknesses. The oil was 

 contained in a cylindrical cell provided with a flat transparent 

 quartz bottom. The cell was so fixed in the cover of a light- 

 tight box that a portion of the agar plate within the box was ex- 

 posed to ultra violet rays filtered through the oil films. After 

 incubation, the area exposed, in cases where the sterilization was 

 effective, was clearly marked as a circle in which no colonies were 

 visible. The results are given in table 6 . 



In these experiments (table 6) the agar plates were at a dis- 

 tance of 6 cm. from the lamp and the quartz plate was 2 mm. 

 thick. From the results obtained it is apparent that layers of 

 oil 4 mm. in thickness may be penetrated by ultra violet rays in 

 sufficient amount to effect sterilization provided the time of ex- 

 posure is sufficient. In this connection it is interesting to note 

 that Vallet (1910) found that with aqueous salt solutions the 

 ultra violet rays were held back and sterilization did not occur 

 when the light had to penetrate a few drops of oil. In the latter 

 case the opacity was doubtless due to the oil-water emulsion at 

 the surface of contact, a condition quite different from that pre- 

 sented by an oil film itself. 



