TRANSACTIONS OF SECTION B. 817 



pumice moistened with the same liquid. The number of corks and connections in 

 this arrangement increases the chance of leakage. The [J-tube must be supported 

 in an upright position both when in use and afterwards, that the acid may not 

 come in contact with the corks ; if too much acid is poured in, the bend becomes 

 blocked by a plug of liquid ; there is no means of telling when the acid has become 

 less efficient by dilution ; nor is it easy to recharge the tube with fresh acid. 



The form of drying tube shown avoids these defects. It is at once wash-bottle 

 and drying tube ; it has one cork, and stands upright ; the pumice- can be well 

 drenched with sulphuric acid, the excess draining down and filbng the lower part, 

 through which the gas bubbles, to a convenient height ; dilution announces itself, 

 and the acid is easily renewed. The shape is that of a Gay-Lussac burette, with a 

 constriction about 2 inches from the bottotn. A piece of pumice, large enough to 

 block the constriction, is first dropped in, and the tube is tilled to near the top 

 with small fragments of pumice. In charging with acid cai-e is taken not to wet 

 the upper part of the tube ; next day the level of the acid in the lower part of the 

 tube is marked with a strip of gummed paper. The small side tube which enters 

 the large tube near the bottom is the inlet for gas ; when the moisture absorbed 

 has raised the level of the acid about 2 mm, above the mark the acid in the lower 

 part is poured off through the small tube, and fresh acid is poured in through the 

 pumice. The inlet and outlet tubes are made of the same height, so that a series 

 of similar drying tubes may readily be joined together. 



8. Standards of Purity Jor Sewage Effluents.^ By Dr. S. Rideal. 



The author discussed the 'Standards of Purity for Sewage Effluents ' which 

 local authorities have hitherto had in mind in deciding as to whether they should 

 be permitted to be discharged into a river or not. It was pointed out that the 

 majority of these standards are arbitrary and artificial. The ' Oxygen consumed' 

 and ' Incubation ' tests, lately in dispute at Manchester, bacterial and chemical 

 figures, and the ' Fish test ' were passed in review. But since the nitrogen is signi- 

 ficant of the more dangerous forms of pollution, a calculation of the proportion 

 between its oxidised forms, which are harmless, and the unoxidised, which are 

 liable to occasion smells and to be otherwise deleterious, will denote the extent to 

 which an effluent has been purified. The figure obtained, termed the ' percentage 

 of purification,' ranges from none for raw sewage to 976 for a deep well in chalk. 

 Moreover, as several observers have proved, the oxygen of the nitrate and nitrites 

 is in an ' available ' form, and is capable, with the help of bacteria, of supplementing 

 the free dissolved oxygen of river water in destroying the remaining organic 

 matter. From these considerations a formula is deduced which embodies all the 

 natural data for the conditions of discharge of an effluent into a stream, including 

 the flow and aeration of the latter, and the volume, oxygen required, and 'avail- 

 able oxygen ' of the effluent. The result is a 'factor of safety,' C, which must 

 never be below unity. The application of the formula shows that the Thames, 

 with C = l-08, has so narrow a margin that it often has had to be supplemented, 

 especially in warm weather, by the addition of oxidising agents, while the Exe, 

 on the other hand, with C = 7'9, has a large margin for natural purification. 



It is concluded that ' an effluent which has been properly prepared, and is in an 

 active state of wholesome bacterial change, under the above conditions of free and 

 potential oxygen, if clear and nearly free from odour, may be safely discharged 

 into any river of moderate volume.' 



9. Action of Aminonia on Gun-cotton. By W. R. Hodgkinson and 

 Captain Owen, R.A., Artillery College, Woolwich. 



Gun-cotton that has been thoroughly washed with alcohol and ether ignites or 

 explodes when heated to between 180° and 185°. Analysis of such gun-cotton gives 

 figures agreeing very closely indeed with those required by a cellulose tri-nitrate. 



' Printed in extenso in Sanitary Record, xxii. 490. 



