of Gases found in Stagnant Waters. 183 
The author has endeavoured, with that attention which the subject de- 
serves, to discover what is the real cause of the abundant disengagement of 
oxygen which he has ascertained to take place in stagnant waters. He 
has satisfied himself that oxygenation was at its greatest height when the 
water, under the influence of a strong solar light, had acquired a very 
deep tint of green. He easily discovered, by means of the microscope, 
that this colour was owing to the presence of green monodarian animal- 
cules, the greater part being the species named Enchelis monadina vires- 
cens by M. Bory de St Vincent. It was sometimes accompanied by a 
larger species, also of a green colour, the Enchelis pulvisculus viridis of 
Muller. When these were sufficiently abundant in the water of the pond 
to give it a green colour, the sun at the same time shining brightly, the 
oxygenation was rapidly developed, until it contained 60 in the hundred 
of oxygen in the gas disengaged, and it sunk if the animalcules became 
less numerous or disappeared. 
We must therefore admit that these beings, which no naturalist has re- 
fused to consider as animals, possess, like plants, the property of decom- 
posing carbonic acid, and disengaging oxygen from it ; and their immense 
numbers, small size, and power of locomotion, which enables them to put 
themselves in the most favourable position for the complete action of the 
light, may explain the intensity of the effect produced. Their green co- 
lour, which becomes not so deep when they are kept for a time in the 
shade, seems to assimilate them to vegetables ; but their property, dis- 
covered by the author, of disengaging oxygen like leaves, appears to ren- 
der it impossible to draw any definite line of demarcation between ani- 
mals and plants. 
After the commencement of spring, the monadine enchelides appear 
in great numbers on the first fine day, and give a light green colour to 
the surface of still waters. Their colour by degrees becomes deeper. 
The duration of their lives is yery variable. When the weather is mild, 
the air calm, and the sun unclouded, they press to the surface of the 
water, and engage in their manifold gyratory movements ; but if the sky 
be overcast, the air agitated, or rain falling, they retire to the bottom of 
the water, apparently to shelter themselves from sudden changes of tem- 
perature, which are always fatal to them. By means of insensible and 
gradual variations, these animaleules may, however, be brought to live 
ata temperature many degrees below zero; their movements are then 
extremely sluggish. They ultimately disappear at the bottom of the 
water, and change into a mucous matter, from which, in due time, new 
generations of enchelides are produced. 
When viewed by the microscope, the monadine enchelid appears rae 
pidly to dart forth a very delicate biciliary apparatus, which enables it 
to produce a rotatory movement. It likewise makes use of this to fix 
itself to any body, for example, to the object-glass. It then commences 
a swinging movement with the cilia as a centre. 
