45 
Probably the following takes place: The rapid air-current draws 
up the water in which the substance to be examined has been dis- 
solved. This water evaporates in the air which streams past and 
which has become ionized at the vertical nozzle, and saturates the 
air with water-vapour, provided, as in our case, an excess of water 
is present. Meanwhile the over-pressure of two atmospheres being 
reduced to zero, the air suddenly expands. A considerable fall of 
temperature ensues and a cloudlike condensation appears. The spray 
splashes against the glass wall and according to GRADENIGO and 
STEFANINI, What flies past it gives a cloud, which may be stored for a 
considerable period if the precaution has been taken to dissolve in 
the water a small percentage of common salt or of a mixture of 
salts. We were able to confirm GRADENIGO’s and STEFANINI’s experience 
to the full. Even the most various non-volatile substances, electrolyte 
or not, when dissolved in water, give a beautifal, dense cloud, 
which may be stored for hours in a Tyndal! case. 
If, however, instead of salt- or sugar-solutions we take aqueous 
solutions of odorous substances, which naturally are more or less 
volatile, the cloud is less- steady and will disappear after some 
minutes. Even then we can note in the ultramicroscope fine water- 
drops close upon their formation, briskly performing Brownian 
movements. . 
It appeared at once that the odour outlasts the cloud, in other 
words that an odorous substance, aS might be expected, is noticeable 
not only and not invariably as a condensed vapour, but also in a 
purely gaseous state. A similar observation was made also by 
J. AirkeN in 1905, when he stated ®) that odorous substances, in a 
space where ions are wanting, do not form clouds from over- 
saturated air. 
However, another remarkable fact manifested itself. When a simple 
condensed water-vapour or a salt-containing cloud is driven against 
a metal plate, well-insulated, and connected to an electroscope, 
it will, under the experimental conditions, not be possible to render 
a charge visible. This is easy to understand, as according to STEFANINI 
electro-positive as well as electro-negative ions are present in the 
salt-nebula, apparently in equal numbers. When placed between 
two charged condenser dises, the vapour clarifies, though it remains, 
while the somewhat larger ions are drawn to the positive, the some- 
what smaller to the negative disc. 
If, on the contrary, we took an odour-containing condensed vapour, 
1) J. ArrkeN, Proce. Roy. Soc. of London. Vol. 25, p. 894. 1905. 
