president’s address. 
i 66 
from the sea, except in February and March, where he found 
a curious association of animal life : the tadpoles of Frogs and 
Toads, the larvae of Flies and Dragon Flies, Water Beetles, 
and Hydrachnids, together with Mysis and Palaemon, Nereid 
worms and small Medusae. 
The Caspian Sea, which no doubt was formerly connected 
with the sea, has a curious fauna, combining marine and 
limnetic species ; the percentage of salt varies here from 
1.30 to 28.5 (in the Bay of Karabugas, where both animal 
and plant life are nearly suppressed). 
As an instance of what I may call the curiosities of life, 
I may mention the Sulphur Bacteria, Thiobacteria, which is 
only found in places where free hydrogen sulphide is present 
{e.g., sulphur springs and the mud of stagnant water) ; it 
seems that this gas is actually necessary to the existence of 
the bacteria, and they are often crammed full of spherical, 
refringent masses of pure sulphur. 
Mr. Peel is at present engaged at Sutton upon a research 
into the chemical qualities of the waters of the Broad district, 
and I hope that his results will be of great value in elucidating 
the problem of the causes of the local distribution of species. 
The transparency of the water of lakes has been studied 
by means of a white disc (Forel uses one of 20 cm. diameter), 
which is let down by a line ; the depth at which the disc 
disappears from view is noted, and also the point at which 
it again reappears on being drawn up, and the mean between 
these two readings is taken to be the limit of clear vision. 
This, of course, varies with the season, according to the 
amount of solids in suspension, plankton, etc. 
On the Lake of Geneva the mean limit of visibility for the 
winter months is 12.7 m., and for summer 6.6 m. ; the 
maximum observed was 21 m. 
The absolute limit of the penetration of light has been 
studied by means of a photographic plate prepared with 
