596 Dr. Daubeny 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 Conferva, 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 should be disinclined to attribute their ex- 

 istence, in this as in the former instance, to any peculiar pro- 

 perty of the water. Thus, I 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. 

 Ever)?^ 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 



found 



