558 Prof. J. J. Thomson on the Masses of 



produced by the cooling is sufficient to form a cloud round 

 the ions : the problem of finding the number of ions per 

 cub. centim. is now reduced to finding the number of drops per 

 cub. centim. in this cloud. We can do this in the following 

 way : — If we know the amount of the expansion we can cal- 

 culate the amount of water deposited per cub. centim. of the 

 cloud; this water is deposited as drops, and if the drops are of 

 equal size, the number of drops per cub. centim. will be equal 

 to the volume of water per cub. centim. divided by the volume 

 of one of the drops. Hence, if we know the size of the drops, 

 we can calculate the number. The size of the drops in the 

 cloud was determined by observing v, the velocity with which 

 they fall under gravity, and then deducing a, the radius of 

 the drop, by means of the equation 



_ 2 ga 2 



where //- is the coefficient of viscosity of the gas through 

 which the drop falls. 



In this way we can determine n the number of ions per 

 cub. centim. : if e is the charge on an ion, v the velocity with 

 which it moves under a known electric force, the quantity of 

 electricity which crosses unit area in unit time under this 

 force is equal to neu. We can determine this quantity if 

 we allow the negative ions to fall on a plate connected with 

 a condenser of known capacity and measure the rate at 

 which the potential falls. We thus determine the product 

 neu, and we already know n ; u has been determined by 

 Mr. Rutherford (Proc. Camb. Phil. Soc. ix. p. 401); for air 

 at atmospheric pressure u is proportional to the potential 

 gradient, and when this is one volt per centim., u is 1*5 centim. 

 per second; for hydrogen at atmospheric pressure u is 4*5 

 centim. per second for the same potential gradient. Hence, 

 as in the known product neu we know n and u, we can deduce 

 the value of e the charge on the ion. 



There are some features in the condensation of clouds by 

 ultra-violet light which are not present in the clouds formed 

 by the Rontgen rays. In the first place, the cloud due to the 

 ultra-violet light is only formed in an electric field. When 

 there is no electric field, the ions remain close to the surface 

 of the illuminated plate, and are not diffused through the 

 region in which the cloud has to be formed ; to get the negative 

 ions into this region we must electrify the plate nega- 

 tively; when this is done, expansion produces a cloud. Again, 

 if the ultra-violet light is very strong, Mr. C. T. R. Wilson 

 has shown (Phil. Trans. 1899) that large nuclei are produced 



