196 Profs. P. F. Frankland and Marshall Ward. 



surface soil — we know far too little about them to say much as to the 

 comparison — the water is falling more and more out of the dangerous 

 stages. 



At still deeper levels, even these anaerobic forms are left behind, 

 and the thoroughly filtered liquid may now subside into a closed sub- 

 terranean basin, where it may remain pure for any length of time.* 

 so far as living organisms are concerned, provided no fissures or 

 direct prolongations of surface waters allow of contamination from 

 above. f 



It is obvious from the foregoing that the two great sources of con- 

 tamination of our water-supplies are the air and the surface waters. X 



* The remarkably impure deep well water examined by Rohn and Wichmann 

 (' Mitth. d. Oesterr. Versuchstat. f. Brauerei u. Malzerei,' H. 2) must surely have 

 been connected with surface waters ! 



f A recent examination (1891) marie by one of us of the water from deep wells 

 in tbe chalk of the Kent "Waterworks Company showed tbe number of micro- 

 organisms revealed by the gelatine test to vary from 4 to 76 and to average 32 in 1 

 cubic centimeter. In all cases the water was taken directly from tbe pumps and 

 before it had undergone any storage. 



For collected results of the bacteriological examination of spring and well waters, 

 see especially Hueppe (" Die hygienische Beurtheilung d. Trinkwassers vom bio- 

 logischen Standpunkte," Schilling's ' Journ. f. Grasbeleuchtung und Wasserver- 

 sorgung,' 1887), also Tiemann-G-artner (' Untersuchung. d. Wassers,' Braunsch- 

 weig, 1889, p. 498). 



X It is hardly necessary now to insist on the importance of the air, and its dust, 

 in this connexion. Reference may be made to the following in confirmation : — 

 Angus Smith ('Air and Rain,' 1872), Tyndall ('Floating Matter of the Air,' 

 1881) ; Percy F. Frankland (" The Distribution of Micro-organisms in Air," ' Roy. 

 Soc. Proc.,' 1886, No. 245, p. 509 ; " Some of the Conditions affecting the Distribu- 

 tion of Micro-organisms in the Atmosphere," ' Soc. of Arts Journ.,' 1887, vol. 35, 

 p. 485 ; " A new Method for the Quantitative Estimation of Micro-organisms in the 

 Atmosphere," ' Phil. Trans.,' 1887, B, p. 113) ; G-race C. and Percy F. Frankland 

 (" Studies on some new Micro-organisms obtained from Air," ' Phil. Trans.,' 

 1887, B, p. 257) ; Percy F. Frankland and T. Or. Hart (" Further Experiments 

 on the Distribution of Micro-organisms in Air," ' Roy. Soc. Proc.,' vol. 42, 1886, 

 p. 267) ; Miquel ('Annuaire de l'Observatoire de Montsouris,' 1877 to 1891, and 

 'Manuel Pratique d' Analyse Bacteriologique des Eaux,' 1891). Also Aitken in 

 'Proc. Roy. Soc. Edinb.,' vol. 16, 1886, p. 139 ; 'Trans. Roy. Soc. Edinb.,' vol. 15, 

 February 6, 1888 ; and as regards contamination by surface waters see Koch (' Rede 

 zur Stiftungsfeier der militararztlichen Bildungsanstalten,' 1888, p. 25), Pjagge 

 and Proskauer ('Zeitschr. f. Hyg.,' vol. 2, p. 479), Soyka ('Deutsche Viertel- 

 jahrschr. f. offentl. G-esundheitspflege,' 1888, p. 638), Wolfflmgel (' Arb. a.d. 

 Kaiserl. Gresundh.-amte,' 1886, p. 546), and Fraenkel ('Zeitschr. f. Hyg.,' 1889, 

 p. 23). 



As regards filtration, Percy F. Frankland (" On the Removal of Micro-organisms 

 from Water," ' Boy. Soc. Proc.,' 1885, " New Aspects of Filtration and other 

 Methods of Water Purification : The Gelatine Process of Water Examination," 

 ' Journ. Soc. Chem. Ind.,' 1885 ; " Water Purification : its Biological and Chemical 

 Basis," ' Proc. Institut. of Civil Engineers,' vol. 85, 1885-86 ; " Filtration of Water 

 for Town Supply," ' Trans, of the Sanitary Institute of Great Britain,' vol. 8, 1886 ; 



