Page:Radio-active substances.djvu/92

 radio-activity of a radium chloride kept in vacuo for six days was not sensibly affected by the operation. This experiment shows that the radio-activity of the salt is principally due to the radio-active energy generated within the particles, and which is unaffected by the vacuum.

The loss of activity that radium undergoes when in solution is relatively greater for the penetrating rays than for the absorbable rays. The following are examples of this:—

A radium chloride which had reached its limit of activity, 470, was dissolved and left in solution for one hour; it was then evaporated, and its initial radio-activity determined by the electrical method. The total initial radiation was found to be equal to the fraction 0·3 of the total limiting radiation. If the determination of the intensity of radiation be made by covering the active body with an aluminium screen 0&middot;01 m.m. thick, the initial radiation which traverses this screen is found to be only the fraction 0&middot;17 of the limiting radiation traversing the same screen.

When the salt has been thirteen days in solution, the total initial radiation is found to be the fraction 0·22 of the total limiting radiation, and is 0&middot;13 of the limiting radiation after traversing 0&middot;01 m.m. of aluminium.

In the two cases the ratio of the initial radiation after solution to the limiting radiation is 1·7 times as great for the total radiation as for the radiation which has traversed 0&middot;01 m.m. of aluminium.

It must further be mentioned that on evaporating the product after solution, it is impossible to avoid a certain period of time during which the product is in an intermediate condition, neither entirely solid nor entirely liquid. Neither can one avoid warming the product to remove the water quickly.

For these two reasons it is scarcely possible to determine the true initial activity of the product passing from solution to the solid state. In the experiments just quoted, equal quantities of the active bodies were dissolved in the same quantity of water, and the solutions were then evaporated to dryness under conditions as identical as possible, and without heating above 120° or 130°.

I investigated the law according to which the activity of a solid radium salt increases, from the moment in which the salt is obtained dry after solution to the moment in which it reaches its limit of activity. In the tables which follow, the intensity of radiation, I, is represented as a function of the time, the limiting intensity being supposed equal to 100, and the time being reckoned from the moment at which the