. Radiography, X-ray therapeutics and radium therapy . w-ever, of the initial quantitydecays in both cases. An exact analogy exists in the lending of moneyat compound interest if we can imagine the capital decreased insteadof being added to in proportion to the amount at the moment. Thus,£100 lent at 10 per cent interest payable yearly, on this plan, wouldmean that at the end of the first year £10 must be deducted fromthe capital, leaving £90 to pay interest on for the next year. Atthe beginning of the third year the capital would be reduced to £81after the deduction of 10 per cent on the £90,
. Radiography, X-ray therapeutics and radium therapy . w-ever, of the initial quantitydecays in both cases. An exact analogy exists in the lending of moneyat compound interest if we can imagine the capital decreased insteadof being added to in proportion to the amount at the moment. Thus,£100 lent at 10 per cent interest payable yearly, on this plan, wouldmean that at the end of the first year £10 must be deducted fromthe capital, leaving £90 to pay interest on for the next year. Atthe beginning of the third year the capital would be reduced to £81after the deduction of 10 per cent on the £90, and so on, the amountdeducted being always proportional to the capital. If we plot a curveshowing the gradual dying away of the capital in this case, it would bea curve similar in character to that in Fig. 224, which really represents thedecay of emanation with time, the quantity disappearing being alwaysproportional to the amount present. This relationship is of fundamentalimportance in the study of radioactivity. It may be put in another TIME IN DAYS Fig. 224.—Curve showing decay of radium emanation with time. 336 RADIATION THERAPEUTICS The rate at which the emanation decays becomes less and less in the courseof time ; that is to say, the actual quantity of gas decaying per unit time isless after some hours than it was at the instant of separation from the parentradium. In Fig. 224 this fact is represented by a curve, the slope of which,though steep at first, gradually becomes natter. The rate of change of theslope must therefore represent the law governing the decay of emanationwith time. Now we know from experiment that the gas decays to halfvalue in 3-8 days, so that calling its initial quantity 100, we obtain a pointon the curve at 50. A further wait of 3-8 days gives another point on thecurve at 25, and so on. Then a line drawn through all the points forms adiagram resembling the one in Fig. 224. By taking the difference of any two consecutive ordinates rep
Size: 1545px × 1617px
Photo credit: © Reading Room 2020 / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bo, bookauthorknoxrobert18681928, bookcentury1900, bookdecade1910
