TRANSACTIONS OF THE SECTIONS. 217 



good water, not always perhaps of chemical purity, but at any rate free from any 

 great contamination of animal and espocially of huuinn excreta. Tliis possibility 

 threatens to disappear in the United Kingdom generally ; and especially so ^^•itll 

 regard to the manufacturing districts and to the east of England, not only from the 

 mere increase of the population, but much more from the higher cidtivation of the 

 land. The moorlands are everywhere being broken up for the plough ; fallow- 

 ing has given place to heavy manuring and to sewage irrigation, both of which are 

 freely applied to pasture as well as to arable land. The population of bullocks 

 and sheep has also increased with the human population. Tlie result is that tlie 

 rain is contaminated as soon as it reaches the ground. The surface di'ainage, 

 instead of being water naturally distilled, flowing otF clean grass or moss, is the 

 washings of manure. The spring- water, again, is not pure rain-water which has 

 passed tlirough a rock-filter and has taken up some mineral ingredients, but is 

 simply these manure washings more or less completely filtered. In our streams 

 the water derived from both these sources undergoes fresh exposure and cleansing 

 by aquatic vegetation, but at the same time fresh contamination. The mere 

 statement of the problem in this way carries with it, almost axiomatically, the 

 inference that the effective character of filtration is a matter for quantitative 

 investigation, not for assumption as perfect and complete. We know, moreover, 

 that some of these natural filters have been overtasked. 



Let us now turn aside to consider what is the work to be done, and what Ls, so 

 far as we are able to understand it, the work actually done by filtration. 



I believe I am right in saying that with the exception of the strong corrosives, 

 which act like weapons rather than as medicaments, no one really knows what 

 poisoning is. We must take it as au expression used to summarize the imlmown 

 and possibly inscrutable chain of events of which we only see the primary cause 

 and the ultimate effect. We may perhaps go one step further in respect of the 

 pijisonous effect of organic sewage in its unfiltered form. It contains, for one 

 thing, the dead products of organic decay. A grass filter or an earth filter very 

 rapidly renders this part of the sewage innocuous by oxidizing it. Then it con- 

 tains germs of animal life, some of which, unless intercepted or killed, prey para- 

 sitically on the larger mammalia. Thirdly, it contains vegetable germs, closely 

 allied, it would seem, to the moulds and other small fungi ; these, finding a resting- 

 place in our bodies, grow and destroy or spoil the cells of which our own gl'o^vth 

 consists, much in the same way that the yeast fungus modifies the worts of beer, 

 or that the common moidd spoils the flavour of a pot of jam. The effect of such 

 spores upon us is called zymotic disease. The first class of impurities is pretty 

 easily dealt with. Probably the means already exist of calculating at what point 

 any given filter will or wiU not be overcharged in respect of its defecating function 

 by the oxidation or entanglement of dead matter. But the question of the filtra- 

 tion of living germs is altogether more obscure. We know that many of them 

 are cauglit and effectually intercepted by both surfrtce and underground filtration ; 

 but we do not know in what proportion this intercepting takes place, eitlier on the 

 average of all germs or with reference to each kind of germ which may be present 

 — different questions not always sufficiently distinguished. Then we also know 

 that the life of some germs is destroyed if their de^elupment be too long retarded, 

 liateman, Michael Scott, and others afterwards have described the remarlcable 

 effect which storing water in dark tanks has in keeping it clear, not only while it 

 remains in darkness, but even under sub.*queiit exposure to light. Now we have 

 at present very little quantitati^-e or well-digested knowledge on these subjects. In 

 fact, little more is known of them than is contained in the crude statement which 

 I ha^•e just laid before you. We have no series of experiments to show what 

 or how many germs escape a given process of filtration or stm-age ; and it is not 



n pr 

 of: 



evei-y germ that we need be afraid of: the greater part of them, probably, are quite 

 innocuous. All that the chemists have been able to give u^ is a dubious estimate 

 of the total quantity of organic matter (whatever that term may mean) which the 

 infiuent and effluent waters severally contain. They do not and caimot tell us in 

 what form the matter exists, whether dead or alive, animal or fungoid. Now for 

 many pui-poses the information so given is about as useful as it would be to know 

 that there is animal and ACgetable life in a gi-ven field, without being told whether 



