I is the intensity at the surface before any absorbing layer is laid on, λ being the coefficient of absorption of the material considered. If the absorption is proportional to the density,

then the ratio

λ density

should be a constant.

A number of substances were tried to test this point. It was difficult to get the solids in sufficiently thin layers to give enough ionization to work with. The B rays produced too little ionization for accurate measurement. The experiments were performed in the same way as already described, the solids tested being mica, celluloid, paper, aluminium, brass, tinfoil, silver, and Dutch-metal. The value of λ for each substance could be easily calculated from the curves of absorption. The results are shown in the following table.

From an examination of these results it will be seen that

for the light substances and for aluminium the absorption is nearly proportional to density, but for the heavier metals there is a wide divergence.

. These results are similar in character to those of Rutherford and Miss Brooks, who examined the B radiation from

uranium, and found that the ratio

λ

density

was the same for

such materials as mica, ebonite, glass, and aluminium, but differed greatly for the substances of high density.

Absorption in Gases.

A series of experiments were also made on the absorption of the excited activity by air, coal-gas, carbonic-acid gas, and hydrogen. For this purpose a special apparatus was constructed, the general arrangement of which is shown in fig. 8 (Pl. XIV.), and is similar in principle to that used by Rutherford in his experiments on uranium radiation †. It consists of a cylindrical brass vessel, closed at the top by an air-tight cover, and at the bottom by a mercury trap. It is divided on the inside into two chambers by means of a horizontal partition, which has a circular hole cut in it, covered with a sheet of aluminium 00038 cm. thick. The partition was insulated from the sides of the cylinder, and connected to one pole of the battery, the other pole being earthed. Immediately below the partition was a circular table, which could be moved up and down by means of a screw passing through the bottom of the cylinder. At the top of the upper chamber was suspended an insulated disk, connected to the electrometer.

The radioactive leather was placed on top of the table immediately underneath the aluminium foil. The radiation given off by this leather penetrated through a certain layer of air or any gas with which the cylinder might be filled, and thence through the aluminium foil into the upper chamber, where it could ionize the gas and produce a movement of the needle of the electrometer. The brass cylinder was earthed and acted as a guard-ring, preventing any leak along the sides. The radiation in passing through the layer of gas before reaching the upper chamber, would be absorbed to an extent depending on the thickness of layer traversed. This thickness could be regulated by means of the screw. The volume of gas in the upper chamber remaining constant the ionization produced there would always be a measure of the strength of the radiation unabsorbed after passing through * Phil. Mag. July 1902. † Phil. Mag. Jan. 1899.

a given thickness of gas. A fixed distance, about 6 mms., between the leather and the aluminium, was always taken as a basis from which to calculate the percentage of unabsorbed

rays.

If the radius of the active surface is large compared with its distance from the aluminium foil, it can be readily shown from the ionization theory that the following equation holds : I = Ioe-^x.

Io is the intensity of the radiation after passing through a distance x of the of the gas, and λ the coefficient of absorption of the gas considered. The percentage of the radiation unabsorbed is calculated in the same way as for solids.

The results of the experiments are shown plotted in fig. 9 (Pl. XIV.), the ordinates giving the percentage of rays unabsorbed after passing through a certain distance, and the abscissæ the turns of the screw-head, each turn corresponding to 1.27 mm. These results are compared with those of aluminium in the following table.

It will be seen that the absorption by gases follows the order of their densities, and is almost proportional to density for air and carbonic-acid gas.

Increased Conductivity of Air mixed with Water Spray.

J. J. Thomson* describes some experiments in which the conductivity of air was increased by passing it through a water-pump into a large vessel, where it was tested. He also found that when a brass rod was suspended in this vessel and kept charged for a number of hours to a high negative potential, it had acquired a certain amount of excited activity †.

Phil. Mag. Sept. 1902.

Note. The effects observed by J. J. Thomson have since been shown by him to be due to a radioactive emanation present in the tap-water of Cambridge.

In view of the importance of these results, experiments were undertaken to see if the Montreal tap-water derived from the River St. Lawrence showed similar properties. For this purpose a large cylindrical zinc tank, diameter 102 cms. and height 150 cms., was used. In the centre of this was suspended a brass cylinder 5 cms. in diameter, which passed through an ebonite plate at the top of the tank, and was connected to the electrometer. The outer cylinder was connected to a battery of 300 volts. Between the two cylinders was arranged a guard-ring connected to earth. A rubber tube passed from the bottom of the tank to an ordinary water pump, from which a return tube entered the top of the tank. The natural leak of the tank filled with ordinary air was first observed, and found to vary from 4 to 5 divs. per sec. The water pump was then started, and the moist air circulated through the tank, while readings of the conductivity were taken every minute. The conductivity of the air in the tank immediately began to increase, and reached a maximum in about five minutes, reaching in one test 25 divs. per sec., or nearly six times the natural leak. When the water pump was stopped this increased conductivity at once began to decrease, and reached the natural leak in about six to eight minutes. The maximum varied from time to time, but was always from four to six times the natural leak. This modified air, when passed through pumice-stone saturated with sulphuric acid before reaching the tank, only gave 8 divs. per sec. as the maximum, but as soon as the pumice-stone was removed gave 20 divs. per sec. It was found that the quicker the air was drawn through the tank the greater was the conductivity produced. Passing the air through a cotton-wool plug destroyed a large portion of the conductivity. It was also found that when the moist air was passed through a spiral tube immersed in liquid air, or a tube heated to redness, the increase of conductivity previously observed was completely absent. The experiment was tried of allowing a quantity of liquid air to evaporate inside the tank, but no increase of conductivity could be observed.

A brass rod was suspended in the tank, and kept charged to a high negative potential for several hours, whilst the air charged with water-spray was circulating through. It was then removed and tested in another vessel, but no signs of any excited activity could be detected.

I think we may conclude from these experiments that the increased conductivity is not caused by an emanation in the water-spray, since it will not stand the tests to which an emanation may be subjected. Neither is there any

appreciable excited activity produced on a rod suspended in i It takes a far greater volume of air than the tank held produce any measurable amount of excited activity from th air, unless some radioactive substance, such as thorium radium, is present. There is certainly an increase of co ductivity produced, which dies away quickly, and which undoubtedly caused by the mixture of the water-spray wi the air. The water from the tap, when evaporated down dryness and tested, gave no signs of any radioactivity.

Conclusion.

From these results we may conclude that the excit activity from the atmosphere behaves in many respects li the radioactivity from thorium and radium. It contains, they do, an easily-absorbed a radiation, and a more penetrati B radiation. The a radiation is probably responsible for t greater part of the total energy radiated, and it is complete absorbed in about '004 cm. of aluminium and 10 cms. of a The rays are cut down to half value in 007 cm. aluminium, and completely absorbed by 06 cm. The Bra probably consist of negatively-charged particles, similar cathode rays, and projected with great velocity. The ioni tion produced by them is too small to test whether they deviable in a magnetic field.

The difference in the rates of decay of the excited activ obtained under different conditions seems to point to the f that the radioactivity of the atmosphere is of a very comp nature.

The radioactivity of snow and rain must be derived fr some radioactive matter in the air which adheres to the surf: of the snow-flake or rain-drop, and is brought down with in its descent. A possible explanation of the differe observed in the rate of decay of the radioactivity from sn and rain, and that of the excited activity on a wire, may based on the view that the radioactive matter in the air is different kinds, having different rates of decay. Snow rain may owe their activity to one kind while the negative charged wire removes all the active carriers to its surfa The rate of decay of the charged wire might thus be resultant of several different rates.

In conclusion, I wish to thank Prof. Rutherford for kindly interest in the work.