34 



IVAN C. HALL 



seen, but never with oil nor in media without covering except 

 under the influence of Ught. It has a logical explanation, I 

 believe in the assumption that a certain limited amount of oxygen 

 and carbon dioxide are absorbed from the seal, thus accounting 

 for the band observed, but failure or reduction in the supply 

 coming through when the seal hardens permits the loss by diffu- 

 sion from the lower surface of the blue band of these gases into 

 the deeper layers of medium and their dilution thereby to a 

 concentration insufficient to recolor the dye. 



TABLE 7 



A comparison of paraffine and vaseline as seals against the recoloration of alkaline 



methylene blue agar 



Tests with the hquid solution also indicate the great superi- 

 ority of paraffine wax and especially vaseline over mineral oil 

 as a means of protecting decolorized methylene blue solutions 

 from recoloration. These compare favorably with the mechan- 

 ical seals such as the marble in a constricted tube, or the cover 

 glass in either plain or constricted tube, or sand in a Smith 

 fermentation tube, under all of which the dye may remain water 

 clear for days. 



The efficacy of these seals is not a matter of boiling points 

 since mineral oil has approximately the same boiling point as 

 paraffine. Furthermore, the lower boiling hydrocarbons such 

 as xylol (137° to 140°C.) and heptane (95° to 100°C.) are even 

 less efficacious than mineral oil. Viscosity and consistency seem 

 to be the essential elements ; perhaps the ease with which vasehne 

 clings to the glass on hardening explains its superiority over 

 paraffine. The liquid oils probably permit the return of absorbed 

 gases but more particularly operate through convection currents 



