If the radiation from an X ray tube or a radio-active substance is now passed into the condensation vessel, a new series of phenomena is observed. As before, if v2/v1 < 1·25 no drops are formed, but if v2/v1 = 1·25 there is a sudden production of a cloud. The water drops of which this cloud is formed are finer and more numerous the greater the intensity of the rays. The point at which condensation begins is very marked, and a slight variation of the amount of expansion causes either a dense cloud or no cloud at all.
It now remains to be shown that the formation of a cloud by the action of the rays is due to the productions of ions in the gas. If the expansion vessel is provided with two parallel plates between which an electric field can be applied, it is seen that the number of drops, formed by the expansion with the rays acting, decreases with increase of the electric field. The stronger the field the smaller the number of drops formed. This result is to be expected if the ions are the centres of condensation; for in a strong electric field the ions are carried at once to the electrodes, and thus disappear from the gas. If no electric field is acting, a cloud can be produced some time after the rays have been cut off; but if a strong electric field is applied, under the same conditions, no cloud is formed. This is in agreement with experiments showing the time required for the ions to disappear by recombination. In addition it can be shown that each one of the fine drops carries an electric charge and can be made to move in a strong uniform electric field.
The small number of drops produced without the action of the rays when v2/v1 > 1·25 is due to a very slight natural ionization of the gas. That this ionization exists has been clearly shown by electrical methods ([section 284]).
The evidence is thus complete that the ions themselves serve as centres for the condensation of water around them. These experiments show conclusively that the passage of electricity through a gas is due to the presence of charged ions distributed throughout the volume of the gas, and verify in a remarkable way the hypothesis of the discontinuous structure of the electric charges carried by matter.
This property of the ions of acting as nuclei of condensation gives a very delicate method of detecting the presence of ions in the gas. If only an ion or two is present per c.c., their presence after expansion is at once observed by the drops formed. In this way the ionization due to a small quantity of uranium held a yard away from the condensation vessel is at once made manifest.
35. Difference between the positive and negative ions. In the course of experiments to determine the charge carried by an ion, J. J. Thomson[[67]] observed that the cloud formed under the influence of X rays increased in density when the expansion was about 1·31, and suggested in explanation that the positive and negative ions had different condensation points.
Fig. 7.
This difference in behaviour of the positive and negative ions was investigated in detail by C. T. R. Wilson[[68]] in the following way. X rays were made to pass in a narrow beam on either side of a plate AB ([Fig. 7]) dividing the condensation vessel into two equal parts. The opposite poles of a battery of cells were connected with two parallel plates C and D, placed symmetrically with regard to A. The middle point of the battery and the plate A were connected with earth. If the plate C is positively charged, the ions in the space CA at a short distance from A are all negative in sign. Those to the right are all positive. It was found that condensation occurred only for the negative ions in AC when v2/v1 = 1·25 but did not occur in AD for the positive ions until v2/v1 = 1·31.
Thus the negative acts more readily than the positive ion as a centre of condensation. The greater effect of the negative ion in causing condensation has been suggested as an explanation of the positive charge always observed in the upper atmosphere. The negative ions under certain conditions become centres for the formation of small drops of water and are removed to the earth by the action of gravity, while the positive ions remain suspended.