Adeney — Dissolved Gases and Fermentative Changes. 559 



It will be seen from the foregoing remarks, that, although the method is open 

 to several sources of error, it is possible to keep the total error within such narrow 

 limits that valuable results may be obtained by it. But in order to effect this, it 

 is necessary to observe certain precautions in carrying out some of the steps in the 

 method, and to these precautions I will now briefly refer. 



Th.e Preparation and. Analysis of the Solutions and Dilutions. 



In the first place the tap-water employed was that supplied to Dublin from the 

 Vartry Waterworks. It is a good upland surface water, and lias proved a valuable 

 one for tbe purposes of my experiments, since it contains extremely small 

 quantities of dissolved matters, the total solids being usually not more than four 

 parts per 100,000 of water. It, however, invariably contains small quantities of 

 peaty colouring matter, and for this reason I have sometimes found it necessary 

 to use pure distilled water instead. The water was drawn from a large cistern 

 holding nearly 400 gallons. 



When making up a long series of dilutions for observation, it has, of course, 

 been impossible to determine the gases in each member of the series just before 

 bottling, since the time required for so many analyses Avould be so considerable 

 that it would be impossible to pi'event serious changes of temperature, and also, in 

 all probability, fermentative changes, especially if sewage-water happened to be 

 the subject of experiment, and the final results would therefore not be comparable. 

 When dealing, therefore, with a series of dilutions at the commencement of an 

 experiment, I have determined the dissolved gases and inorganic nitrogenous 

 bodies in the lowest dilutions, and in the tap-water employed for making the 

 dilutions, only, and calculated, from the data obtained, the quantities of dissolved 

 gases and inorganic nitrogenous substances for all the other dilutions made up. I 

 need scarcely remark that at the conclusion of an experiment each member of the 

 series of dilutions has been in every case directly analysed. 



I have adopted a similar method of preparation for all the dilutions I have 

 experimented with. The basis of each series of dilutions has been either 

 undiluted sewage-water, or a strong solution of other substances of known 

 composition and strength. Five litres, at least, of the lowest dilution have been 

 made, and aerated by violent agitation with air in a large glass vessel fitted with 

 a glass tap. After the aeration the solution has been allowed to rest for about 

 fifteen minutes, to allow air-bubbles to rise to the surface and escape. 



Two or more bottles have then been carefully filled, and allowed to stand 

 unstoppered for a few minutes, until again all air-bubbles have escaped. All the 



