THE VISCOSITY O F SOLS ;\IL4DEFROLI X-IRRADIATED -AGAR H. KERSTEX AND C . H. DWIGHT Department of Physics, University of Cincinnati, Cincinnati, Ohio Received February 6 , 1957 ISTRODUCTION
While investigating a problem concerned with the killing by x-rays of Bacillus subtilis spores on the surface of a n agar medium, Blank and Kersten (1) observed that, if dry agar was given a sufficient dose, a medium subsequently made from it would not jell as well as a control made from unirradiated agar. I n this paper a more detailed study of the effect has been made by comparing the viscosity of sols made from agar which had received various dosages of x-radiation. EXPERIMENTAL
The x-rays mere supplied by a copper target gas x-ray tube ( 2 ) operated at 40 peak kv. and 10 ma. The window of the x-ray tube was made of thin aluminum and Cellophane, so that the most intense part of the radiation passing through i t had a wave length of 1.54 il.U. Powdered agar was irradiated in the cavity of a drop-culture microscope slide, placed about 3 em. from the focal spot of the x-ray tube. After being irradiated, each sample was mixed and divided into several parts, each containing 0.2 g. These were placed in flasks t o which 250 cc. of distilled water n-as added. Control samples were made in exactly the same way, except that unirradiated agar was used. One flask containing the control agar, one containing agar irradiated for 15 minutes, one containing agar irradiated for 1 hour and 15 minutes, and one containing agar irradiated for 3 hours and 15 minutes were simultaneously placed in a saline bath, and held at about 105’ C. for 2 hours. They were then all removed a t the same time, and allowed to cool to room temperature. The time elapsed after a sample had reached room temperature was termed its “age.” The viscosity of each sample n-as measured a t intervals by permitting 5 cc. of it to flow through an Ostwald viscosimeter. The entire experiment n-as repeated several times. The results plotted in figure 1 show that the viscosity of all of the samples increased with their age, but that the viscosity of all of the irradiated samples remained less than that of the control. Before each mcasuremeiit the viscoqimeter n-as rinsed with distilled water, and 687
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H. KERSTEN AND C. H. DWIGHT
regarded as clean xi-hen 5 cc. of distilled water at room temperature required approximately ’76 seconds to flow. To determine whether or not the dry agar “recovered” from the effects of the irradiation, samples were irradiated for the same length of time, and allowed to stand in the laboratory for periods ranging from 13 minutes to
FIG.1. Variation of viscosiq n i t h age. T h e numbers next t o each graph show the number of minutes t h a t the agar had been irradiated.
Bot1 ”
’G k:E
1 1 I 1 1 I 1 I
’ ’ 1
IO ‘5 CF S3LS I bJ I-GLJGS
20
FIG.2. Variation of viscosity n i t h age. T h e upper giaph is that for unirradiated agar. T h e numbers next t o the lorn-er graph show the time of the “recovery” of the agar in days.
seventeen days, a i d then carried through the proccdurc iiimtioiiecl above. The curves are shovin in figure 2 to be so nearly coincident that one concludes that the effect of the irradiation endures for at least ?e-,eiiteen day-. To determine whether or not the iiiiall anicunt of heat coming through the window of the x-ray tube n-ith the x-rays caii,-ed the chaiigc, a thermo-
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couple was placed in a sample of agar being irradiated, and the temperature found t o rise only a few degrees above rooin temperature. h sample of agar heated in an oven at a higher temperature, and for a longer period, than any used in the x-ray experiments gave a viscosity curve identical with those of control samples. The effect cannot then be attributed t o heat alone. The p H of sols made from irradiated agar was found to be slightly less than that for sols made froni unirradiated agar. Measurements were made with indicators and with the quinhydrone electrode. coKcLusIo~s
1. Irradiation of dry agar decreases the viscosity of the resulting sol. 2. There is no appreciable “recovery” of the dry agar from the effects of irradiation, a t least within seventeen days. 3. The effect on the agar is not due t o heat emanating from the target of the x-ray tube. 4. Irradiation of the dry agar slightly decreases the pH of the resulting sol. REFEREKCES (1) BLANK AND KERSTEN:J. Bact. 30, No. 1 (1935). (2) KERSTEN:Radiology 23, 60-3 (1934).
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