254 MR JOHN AITKEN ON THE 



had recourse to large-sized particles of dust to enable me to follow the move- 

 ment of each particle. Calcined magnesia was selected for this purpose. 

 When the air in the dust-box was filled with this powder, the reason of the 

 change in the dark space at once became evident. The particles in the ascend- 

 ing current could be seen rushing towards the electrified surface and adhering 

 to it. The dark space was thus broken in upon, and its outline destroyed 

 by the attracted particles ; the air round the body was at the same time 

 deprived of a great quantity of its dust ; and over the conductor there rose a 

 thick and ill-defined band of clearer air, the particles which formerly were in it 

 having attached themselves to the electrified body. All the particles did not 

 seem to be equally attracted, but some much more than others. This gave rise 

 to the irregular movements seen all round the body. The dust particles 

 frequently deposited themselves on the conductor in small needle-like radial 

 columns, which grew by the addition of the particles till they got to a certain 

 size, when they were shot off" and flew through the air with surprising velo- 

 city. If, after the conductor had been electrified a short time, the supply of 

 electricity was cut off and the conductor connected with the electroscope, the 

 charge given to the air and the dust in the box was given back. The leaves of 

 the electroscope expanded quickly, and if discharged, rapidly became charged 

 again, the dust at the same time being attracted to and deposited on the con- 

 ductor in needle-like columns. 



After looking at this last experiment, and seeing the tendency which particles 

 in electrified air have to deposit themselves on bodies, we cannot help asking 

 the question, Does this experiment throw any light on the well-known tendency 

 to the development of certain forms of bacteria resulting in the putrefaction of 

 our foods, and in the appearance of increased quantities of certain ferments 

 during thundery weather ? Can it be that the germs of these forms of life 

 floating in our atmosphere have a far greater tendency to settle upon the 

 surface of bodies from electrified air than when there is no electrical disturb- 

 ance 1 No doubt this electrical attraction must have some effect in this 

 direction, but whether it is the principal cause or not I shall not venture 

 to say. 



If we use still higher degrees of electrification than those used in the above 

 experiments, other effects are produced, but they have no relation whatever to 

 the formation of the dark plane. From the experiments described it will be 

 seen that the effects of electricity are of quite a different kind from those of 

 heat. The electrified body, instead of repelling the particles like a hot one, 

 attracts them, and clears the air in a partial way by attracting some of the par- 

 ticles to itself, while heat acts by repelling all of them to a distance. This anta- 

 gonism between the two forces maybe illustrated by heating the conductor and 

 electrifying it slightly. At first no effect is produced by the electricity ; the 



