visitation of that town by cholera, in the year 1866, three-fourths of the deaths from this disease occurred within 200 or 300 feet of such watercourses, some of them having been allowed to fill up; and others having been converted into sewers that "do not sufficiently carry off the land drainage, or which, being of the flat-bottomed and leaky kind constructed in the last century, may pass their contents into the surrounding soil."

But before we go into the evidence proving the connection between disease and the condition of the soil, it will be well to enquire in what way this influence may be exerted, and how poisonous vapours can be generated, and can exhale into the air so as to reach the dwellers in houses.

And, first, we must cease to regard the earth beneath us as exclusively solid matter. It is everywhere more or less porous, and permeable by both air and water. Sand will take up 25 per cent of its own weight of water; finely precipitated chalk, 85 per cent; pure clay, 70 per cent; garden earth (humus), 181 per cent; and turf, 200 per cent, or double of its own weight of water. (Schübler.) Even a well-baked brick will take up as much as half a pint of water. Naturally, where water will pass, air will pass so much the more readily. The whole art of drainage depends upon this porosity of the soil.

5. Ground Air.-But the ground air is not exactly like the air of the atmosphere around us. It is true that it is a mixture of the same gases-oxygen, nitrogen, and carbonic acid; but in atmospheric air there are usually only four parts per 10,000 of carbonic acid, and 20 per cent by volume of oxygen. In ground air the oxygen at a depth of a few yards diminishes in some cases to 15 per cent, and the carbonic acid may rise to as much as 8 per cent-200 times more than in atmospheric air. This tendency to substitute carbonic acid for oxygen increases the deeper we get into the ground.

Prof. Pettenkofer, to whom we owe much of our knowledge of underground physics, found that in the neighbourhood of Munich the carbonic acid varied from 1.58 per 1,000 volumes, at a depth of 5 feet, to 18.38 at a depth of 13 feet. At this last-mentioned depth, then, the air would be quite irrespirable, and it would at once extinguish a light. It was at Dresden that Fleck found the large proportion of 80 parts of this gas per 1,000. There are a good many cellars that go nearly as deep as 13 feet into the ground, and the cellar air often to a great extent passes into the house

above. You may imagine the danger of breathing such impure air as this, and how important it is to keep it out of the house!

It is perhaps the duty of the lecturer on foundations to tell you how to do this. But I may mention that there are three ways of managing it: (1) by putting a thick layer of concrete under the cellar floor; (2) by having a cavity under this floor communicating with the kitchen flue; and (3) Dr. Richardson's, in his "City of Hygeia," by building the house entirely above ground, without any cellars, and supported upon arches. I may mention that in the new model wards recently built at the London Fever Hospital, two of these plans are combined, for the arches supporting the wards are built upon a layer of concrete, extending several feet beyond the area of the building.

66

In the model bye-laws, which I have already quoted, the tenth clause runs as follows: 10. Every person who shall erect a new domestic building shall cause the whole ground surface or site of such building to be properly asphalted, or covered with a layer of good cement concrete, rammed solid, at least six inches thick."

I am told that this precaution cannot add much to the cost of a house, since a cubic yard of concrete only costs from ten to fifteen shillings, and at six inches thick this would cover sixty-four square feet. This clause is also absent from the Manchester byelaws. But the ground air is by no means stationary-it is continually in motion. A good proof of this fact is given by Pettenkofer, when he shows that it is possible to preserve a bird alive in a glass cylinder, enclosed at a depth of several inches of gravel; and he mentions that some years ago, in Saxony, two men who were shut up in the shaft of a well for ten days, owing to the falling in of the earth, were kept alive and were not much the worse for it when they came out again. ("On the Relations of the Air to the Soil," p. 75.)

He also gives the experiment of blowing through gravel into a tube connected with a manometer, and this experiment I will show you. It proves at least the possibility of the air moving through one kind of soil.

And there is evidence still more to the purpose that ground-air both can and does travel long distances to enter into dwellinghouses. Just now I mentioned the possibility of air coming from cellars into a house: the following circumstance narrated by Pettenkofer shows how far it may travel when sucked in by the in-draught caused by warm and ascending air inside a bonsa,

visitation of that town by cholera, in the year 1866, three-fourths of the deaths from this disease occurred within 200 or 300 feet of such watercourses, some of them having been allowed to fill up; and others having been converted into sewers that "do not sufficiently carry off the land drainage, or which, being of the flat-bottomed and leaky kind constructed in the last century, may pass their contents into the surrounding soil."

But before we go into the evidence proving the connection between disease and the condition of the soil, it will be well to enquire in what way this influence may be exerted, and how poisonous vapours can be generated, and can exhale into the air so as to reach the dwellers in houses.

And, first, we must cease to regard the earth beneath us as exclusively solid matter. It is everywhere more or less porous, and permeable by both air and water. Sand will take up 25 per cent of its own weight of water; finely precipitated chalk, 85 per cent; pure clay, 70 per cent; garden earth (humus), 181 per cent; and turf, 200 per cent, or double of its own weight of water. (Schübler.) Even a well-baked brick will take up as much as half a pint of water. Naturally, where water will pass, air will pass so much the more readily. The whole art of drainage depends upon this porosity of the soil.

5. Ground Air.-But the ground air is not exactly like the air of the atmosphere around us. It is true that it is a mixture of the same gases-oxygen, nitrogen, and carbonic acid; but in atmospheric air there are usually only four parts per 10,000 of carbonic acid, and 20 per cent by volume of oxygen. In ground air the oxygen at a depth of a few yards diminishes in some cases to 15 per cent, and the carbonic acid may rise to as much as 8 per cent-200 times more than in atmospheric air. This tendency to substitute carbonic acid for oxygen increases the deeper we get into the ground.

Prof. Pettenkofer, to whom we owe much of our knowledge of underground physics, found that in the neighbourhood of Munich the carbonic acid varied from 1.58 per 1,000 volumes, at a depth of 5 feet, to 18.38 at a depth of 13 feet. At this last-mentioned depth, then, the air would be quite irrespirable, and it would at once extinguish a light. It was at Dresden that Fleck found the large proportion of 80 parts of this gas per 1,000. There are a good many cellars that go nearly as deep as 13 feet into the ground, and the cellar air often to a great extent passes into the house

above. You may imagine the danger of breathing such impure air as this, and how important it is to keep it out of the house!

It is perhaps the duty of the lecturer on foundations to tell you how to do this. But I may mention that there are three ways of managing it: (1) by putting a thick layer of concrete under the cellar floor; (2) by having a cavity under this floor communicating with the kitchen flue; and (3) Dr. Richardson's, in his "City of Hygeia," by building the house entirely above ground, without any cellars, and supported upon arches. I may mention that in the new model wards recently built at the London Fever Hospital, two of these plans are combined, for the arches supporting the wards are built upon a layer of concrete, extending several feet beyond the area of the building.

In the model bye-laws, which I have already quoted, the tenth clause runs as follows: 10. Every person who shall erect a new domestic building shall cause the whole ground surface or site of such building to be properly asphalted, or covered with a layer of good cement concrete, rammed solid, at least six inches thick."

I am told that this precaution cannot add much to the cost of a house, since a cubic yard of concrete only costs from ten to fifteen shillings, and at six inches thick this would cover sixty-four square feet. This clause is also absent from the Manchester byelaws. But the ground air is by no means stationary-it is continually in motion. A good proof of this fact is given by Pettenkofer, when he shows that it is possible to preserve a bird alive in a glass cylinder, enclosed at a depth of several inches of gravel; and he mentions that some years ago, in Saxony, two men who were shut up in the shaft of a well for ten days, owing to the falling in of the earth, were kept alive and were not much the worse for it when they came out again. ("On the Relations of the Air to the Soil," p. 75.)

He also gives the experiment of blowing through gravel into a tube connected with a manometer, and this experiment I will show you. It proves at least the possibility of the air moving through one kind of soil.

And there is evidence still more to the purpose that ground-air both can and does travel long distances to enter into dwellinghouses. Just now I mentioned the possibility of air coming from cellars into a house: the following circumstance narrated by Pettenkofer shows how far it may travel when sucked in by the in-draught caused by warm and ascending air inside a house,

"In December, 1859, the chaplain of St. Ulrich's church, at Augsburg, was suddenly seized with a serious illness, the nature and cause of which were inexplicable. The sisters of mercy who nursed him were, one after another, seized with the same symptoms-pain and congestion of the head, fainting, etc. The symptoms always became aggravated when the weather got colder. After some time, no improvement having appeared, a friend who came to see him one day exclaimed on entering the room'There is an escape of gas.' This was denied by all connected with the house, and declared by the doctor to be immaterial, as the patient was now pronounced to be undoubtedly suffering from fever. At length, however, a person was sent for to the gas office, and he at once said there was an escape, but confessed himself unable to discover its source. The patient, however, acted on the hint, and having left the house, in spite of the doctor, recovered in a few days. No sooner was he gone, and the windows of his apartments thrown open, and the fire let out, than his next neighbour got attacked by the very same symptoms. He, too, recovered at once by a rapid flight from the house.

"It was impossible to examine the underground pipes at the time, as the ground was frozen hard. However, at the end of six days this was done, and an escape was discovered in the main pipe, which ran in the centre of the street, some twenty feet distant, the gas escaping in such quantities as to burn briskly when lighted.

"The coldness of the weather had necessitated larger fires in the house, and the increased heat developed a current of air from the ground into the house, the gas being sucked up with it. When the patient had left the house, his room was allowed to cool, and the current of air and gas was thus diverted to the apartments of his neighbour."

But it will be readily understood that if coal gas can thus travel long distances underground, and that it may be drawn into houses by the suction of the warm air, any other air that the ground may contain will with equal facility be thus conveyed.

And there is further the certainty that this air may carry with it not only vapours, but any minute particles that can be floated along in its current. That there are such particles, germs that are the active causes of diseases, has also been abundantly proved; you will see, then, how important it is either to see that the ground air is perfectly pure, or, as that is well-nigh impossible, to exclude