596 » Dr. DAvBENY on a certain Kind of 
retted hydrogen;—a gas so noxious to most other living bodies,— 
should be eminently fitted to favour the growth of certain kinds 
of Conferve, whilst thermal waters destitute of this impregna- 
tion—if I may judge from those of the Pyrenees—would seem 
not to deposit any organic matter of the same description. 
I am aware, indeed, that similar appearances are noticed as 
occurring in hot springs of other kinds; but, judging from my 
own experience, I sliould be disinclined to attribute their ex- 
istence, in this asin the former instance, to any peculiar pro- 
perty of the waters 'Thus,.1 observed on the reservoirs which 
received the-water of the hot spring of Bagneres de. Bigorre, 
departement. des Hautes Pyrenees, a red coriaceous-looking 
scum covering the surface of the water, which appeared to de- 
rive its colour from a portion of oxide of iron entangled in the 
interstices of some kind of organic matter. It is easy to under- 
stand how it happens, that the ferruginous contents of the water, 
when no longer held in solution, are found to collect in this 
instance on the surface, and not at the bottom, of the reservoir. 
Every successive portion of the water, as it issues from the 
ground, being of a higher temperature than that which has been 
for some time exposed to the cooling influence of the external 
air, will, by virtue of its inferior specific gravity, rise to the sur- 
face, where it gives out a portion of that carbonic acid, with 
which it was surcharged whilst under pressure. But this gas 
having been the solvent of the carbonate of lime and oxide of 
iron which the water contained, a portion of both these ingre- 
dients will be separated at the moment of its disengagement ; 
and, supposing any vegetable or animal matter to be at the 
time floating near, the earthy and ferruginous particles will be . 
entangled within its interstices, and thereby be prevented from 
sinking to the bottom. 
In this way I likewise account for a red scum, which I have 
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