THE RIVER IRWELL AND ITS TRIBUTARIES. 247 



to a very great extent that further special self -purification 

 of the water by oxydation. The carbon dioxide is mostly 

 formed by the oxydation of the sewage and other carbon- 

 aceous contaminations present in the water. I have made 

 a great number of determinations of the amount of free 

 carbon dioxide gas in solution^ in the Irwell water, and 

 always found that on allowing the same water sample to 

 stand for a week (or even a day or two in summer), a 

 further amount of carbon dioxide had been formed and 

 dissolved in the water. This further amount was entirely 

 derived from the oxydation of the carbonaceous impurities 

 of the water. I ascertained on making further experiments 

 that an increase of temperature had a very great influence 

 upon the formation of carbon dioxide in sewage-polluted 

 water. The way I ascertained this was very simple. I 

 first determined very carefully the amount of free carbonic 

 acid gas (carbon dioxide) dissolved in the Irwell water, by 

 gently warming it in a flask to about 94° C, and drawing 

 all gas evolved through a standard solution of barium 

 hydrate. When no further amount of gas was thought to 

 be coming ofi", the barium-hydrate flask was removed and 

 the amount of baryta still remaining not saturated deter- 

 mined by standard oxalic acid solution; then another 

 flask containing a farther charge of the barium hydrate 

 solution was attached to the apparatus as before, and the 

 water sample again heated in its flask for half an hour at 

 94° C. ; if no more gas came off I at once proceeded to 

 heat the flask to 100° C, when a copious generation of 

 carbon dioxide always took place. If the carbon dioxide 

 came off" during the second heating to 94° C, then this 

 heating was continued for a considerable time until I 

 assumed nothing more did come ofi" (and in actual practice 

 it was not at all difiicult to be quite sure), then I titrated 

 the barium hydrate solution as before. From the experi- 

 ments thub made lam very strongly of opinion that determina- 



