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it altogether from our houses. The ground in the neighbourhood of dwellings is in constant danger of becoming contaminated with noxious matter--the soakage from midden heaps or ashpits, or even from fæcal matter, is constantly passing into the ground of towns, and the sewers themselves may, and not unfrequently do, leak and allow their contents to drain away into a porous soil. If any of these substances contain the germs of fevers, dysentery, diarrhoea, or cholera, we have seen how readily through the agency of our household fires they may get into the houses themselves. You probably all know that the severe attack of typhoid fever from which the Prince of Wales narrowly escaped with his life came from sewer-gases that were drawn into his well-warmed bedroom from a neighbouring closet; but, after what I have said as to the movement of the ground air, it will be apparent that not only sewer-gas, but vapours from the soil itself, are almost equally potent for harm. We shall presently have abundant proof that many diseases do actually come from this source.
6. Ground Water.—The next important agent in the soil is the ground water. Except in very hot climates, or occasionally in very hot summers in this country, all soil is more or less damp, and generally even in the hottest weather we have only to dig a few inches into the ground before we find a certain degree of damp
But at a certain depth below the surface-a depth that varies in different soils—we come at last to a point at which the ground is evidently saturated with water. Any space made by the spade speedily fills with water flowing into it from the porous soil around.
This is the surface of a sort of underground lake that is everywhere spread beneath us, only, unlike a lake, it is not level, but rises to different heights below the surface of the ground. In a marsh it rises quite to the surface, and in a sandy soil its depth depends upon the distance of some more impervious layer of rock or clay. The level of the ground water, then, being determined by the nature of the underlying strata, may be at very different depths below the surface, and in some cases is at a very great depth. It also varies very much in height at different seasons. Its height is, in fact, dependent upon the rainfall and upon the obstructions to its course, and ultimately it tends to fall into the nearest water
It is, however, usually much higher at some distance from the river towards which it flows, because of the slowness of the rate at which it percolates through the soil. In Munich the
rate of movement towards the Isar river is only 15ft. daily, and the rapidity of its rise and fall differs very much in different places and at different seasons of the year. In some places it scarcely changes its level ; in others, as in India, where the rainfall at the rainy season is excessive, it may vary to the extent of 17ft. (as at Sangor, in India). In Munich its greatest variation is noft
. This ground water is always highly charged with carbonic acid gas, though not to the same extent as the ground air, and it might be supposed that it obtains its supply of this gas from the ground air, were it not that its charge of the gas is not always greatest when nearest to the more pervious part of the soil.
7. The Relation between Soil and Disease.--It is now time to enquire what practical conclusions we can draw from all these details about the air and water in the soil. They are none of them without significance, and especially with reference to our immediate purpose of finding out the influence of soil upon the health of the human beings who crawl about over its surface.
An old friend of mine—the late Isaac Taylor, a former resident in Manchester, for whose memory I have a profound respectonce said to me, "When people talk about going from home for change of air, what they really mean is change of earth. The air we breathe is approximately the same everywhere out of doors (diagram); it is the character of the underlying ground that we change." It behoves us, then, to enquire how this change may affect us.
Mr. Taylor thought that there might be differences in the electrical currents passing through the earth, and there are unquestionably differences in the receptivity of soils to heat. But I do not propose now to enquire into these still unsettled problems. It more concerns us to see how far the soil can promote or hinder the growth of those organisms that we now know to have such a pernicious effect in causing disease.
Now, all soils, besides air and water, contain a certain proportion of organic matter, i.e., certain compounds of carbon and nitrogen and hydrogen, as well as purely mineral matter, and in order that even the lowest forms of life may exist there must be present a certain degree of warmth, a certain amount of air, some moisture, and often some organic particles to serve as food. These are the conditions under which life may exist, and they are all found in soil. Different soils will take up different degrees of warmth from the heat of the sun; but, in most ground, during summer and autumn, there will be sufficient heat, at any rate in this climate, for gas, which
the hatching and growth of the microscopic germs of putrefaction and disease. It is supposed by Dr. Parkes that it is from the former of these actions that proceeds the carbonic acid we have found to exist in such large proportion in ground air. This question, which is still unsettled, is, however, of much less importance than the question of the origin of different diseases in the soil. To this point we will now turn our attention. The disease that has been longest known to be closely connected with the state of the ground is intermittent fever, marsh fever, or ague.
8. Malaria.--If you glance at Dr. Lombard's map of the distribution of malaria—as it is called—that is, air charged with the elements of ague, you will see how widely it is spread, chiefly around the coasts of hot countries and at the mouths of rivers, especially those where there are marshes. It is well known that such marsh lands are particularly unhealthy when their surface, previously saturated with water, is exposed to the direct heat of the sun, and thus the soil is for a time partially dried up. Ferguson says, "that this is an indispensable condition in hot climates." (Buck, “Hygiene and Public Health," vol. 1., p. 436.) Digging up the soil also, excavating for canals, building dykes, the clearing
and preparation of lands for their first cultivation, by exposing earth, containing matters that have long remained in a quiescent state, to the energetic action of the sun, all these operations increase the danger from malarious poison. Thorough, deep drainage, on the other hand, as we have already seen, removes the danger of infection.
It has been doubted whether it is inhaling the air or drinking the water from marshes that has the most influence in producing fever, but the instance already quoted from Galton, and other cases in which fever was only produced when the wind was blowing over a marsh, are sufficient to prove that whatever the water may do, the air is able to convey the disease.
The true cause of marsh fever has now been found to be an organism-one of the numerous race of Bacillus- the bacillus malaria. It is found in the shape of small shining movable spores, which when injected into the blood grow into peculiar dumb-bell shaped rods.
These creatures have been found in both stages in the blood of persons suffering from ague-as rods in the cold stage, and as spores in the intermissions of the disease. We can readily understand now how they may exist in marshy soil, how they are set free by turning over the ground, and how they may be conveyed into men's bodies by means of drinking water or in the air.
9. Diarrhæa and dysentery constitute another group of common diseases causing an enormous mortality, especially amongst children, and having much to do with the condition of the soil. In his report to the Privy Council in the year 1859, Dr. Headlam Greenhow showed that in some districts the mortality from these diseases is nine times as great as in some others, and he shows that this excessive mortality is coincident (1) with the tainting of the atmosphere with the products of organic decomposition, especially of human excrement, and (2) with the habitual drinking of impure water.
Both these conditions have ultimately to do with the state of the ground, for in most instances the impure water spoken of was drawn from wells, and these were often contaminated by admixture with ground water, tainted and poisoned with substances that had passed into the soil.
Leicester is a town that has for some time borne an unenviable notoriety as a place where the infant mortality from diarrhea is excessively high, and successive medical inspectors have attributed this notoriety to the fact that parts of the town are built upon made ground, such as I described just now.
There is also a further reason for ascribing these diseases to some zymotic or fermentative element in the fact that it is only in the summer months that there is any great prevalence of these complaints. In the year 1859 I pointed out, as a result of an enquiry into the influence of atmospheric changes upon disease, that diarrhoea did not begin to increase materially in prevalence until the mean temperature of each week was 60°, and this observation has since been confirmed by others. In a recent report by the medical officer of Leicester the temperature of the soil has been ascertained, and he also ascribes the excessive mortality of the town from this disease to the fermentative changes going on in the filth-sodden ground.
In all towns there exists this danger, and although in Manchester great improvements have been made in the abolition of the old privies and ashpits, we can hardly doubt that much of the ground where these have existed is still saturated with the overflowing refuse from these pits of abomination.
10. Ground water has also been shown to play a very important part in the production of typhoid fever and cholera.
I have here a chart showing the rise and fall of typhoid fever side by side with the height of the ground water, for a series of 16 years; and you will see that in every case in which the disease was prevalent the ground water was lower than usual, and not only so, but the rise of the ground water always seems to bring with it a diminution in the mortality from this disease.
Prof. Seidel, from data covering a period of eight years, has calculated that the probability is 36,000 to i that there is a connection between variations of level of soil water and the prevalence of the disease. To account for this fact, Prof. Pettenkofer supposes that the germs of typhoid fever, scarcely at all contagious in their recent state, need to be in contact for a time with soil favourable to their further growth. This they find in the ground recently abandoned by the ground water, and after flourishing there for a time they acquire such virulence that, upon rising into the outer air or passing into drinking water, they are able to give the disease to human beings.
It must be pointed out in reference to this theory that there are cases, such as the outbreak of typhoid fever at Zurich, of which I show a diagram, in which the converse of the above-mentioned observation has taken place, the disease rising and falling with the ground water.
In either case, however, we cannot doubt that the ground water has had something to do with the disease; probably, in the one case, draining from a large area into the wells, and in the other rising through infected ground, and also ultimately getting into the drinking water.
There are now many scores of well-authenticated cases in which typhoid fever has been traced to the water from wells. In cholera also the condition of the soil has an important influence upon
the spread of the disease. In England there is a well-founded belief that one of the chief agents in conveying this disease is drinking water. The notorious outbreak of cholera in London, that was traced to the Broad Street pump, shows the power of water in spreading this disease. This, the most terrible outbreak which ever occurred in this kingdom, carried off in ten days upwards of 500 people, all residing within 250 yards of this notorious pump. “The mortality in this limited area,” says Dr. Snow, "probably equalled any ever caused in this country, even by the Plague, and it was much more sudden, as the greater number of the cases terminated in a few hours. It would also, undoubtedly, have been