M. Thenard on Oxygenated Water. 119 



diately after, with all the precautions before given. The excess 

 of barytes may then be precipitated by a few drops of weak sul- 

 phuric acid, and it is better to leave a slight excess of the acid pre- 

 sent, rather than of the base, because the last tends to disengage 

 oxygen, whilst the first renders the combination more permanent. 



9. Finally, the fluid, which may be considered as pure oxy- 

 genated water, diluted with common water, is to be put into a 

 clean glass having a foot, and this glass placed in a large cap- 

 sule, two-thirds full of concentrated sulphuric acid. This ap- 

 paratus is to be placed in the receiver of an air-pump, and a va- 

 cuum made. Pure water having a greater tension when in 

 vapour, than oxygenated water, evaporates more rapidly, so that 

 at the end of two days the fluid will contain perhaps two hundred 

 and fifty times its volume of oxygen. The following observations 

 must be attended to : 



The acid must be agitated from time to time. 



It happens sometimes that, towards the end of the evaporation, 

 the fluid disengages a little gas, which is indicated by the vari- 

 ation of the mercury in the gauge. This disengagement is oc- 

 casioned, no doubt, by extrEineous substances which remain in 

 the fluid ; it may be stopped by the addition of two or three 

 drops of extremely dilute sulphuric acid. 



Sometimes the fluid will deposit some white flocculi of silex. 

 This should be separated. The fluid may be decanted by a pipe 

 with a very fine termination, and but a small quantity of it will 

 be lost. 



Until the fluid becomes very concentrated the evaporation goes 

 on very quietly, but when the oxygenated water scarcely con- 

 tains any more water, bubbles frequently rise, which burst with 

 difficulty. At first sight, it seems as if much oxygen gas 

 escaped, but on examining the gauge, it will appear very 

 trifling. It will scarcely become sensible in twenty-four hours, 

 and the alteration then observed is in part occasioned by the 

 disengagement of gas from the sulphuric acid, belonging to a 

 portion of the oxygenated water which has been evaporated. 



The fluid may be known to be in the most concentrated state 

 possible, when it gives four hundred and seventy-live times its 



