gaseous impurities, or by contaminating the well; and, finally, we have the tank containing a quantity of pasty, putrescent matter, which is constantly emitting noxious gases and vapours. The latter nuisance can be much diminished by covering the cesspool with a perforated frame, upon which is piled a quantity of charcoal; but the evil attending soil percolation—an evil increased when waterclosets deliver into the tank-cannot be avoided, unless we construct cesspools with impervious bottoms and sides, and take care that they are emptied frequently, the contents being partially deodorised by clay or charcoal, carted away and applied as manure.

The storage of animal excreta in cesspools near to houses and wells is to be regarded as the most dangerous of all modes of temporarily disposing of sewage, and is attended with exceptional risk when the water supply is drawn from a well in the neighbourhood of the sewage tank.

The water-carriage system, by which the sewage is swept through carefully constructed drains out of the house into the street sewer, is undoubtedly the most convenient and least dangerous plan, though, as we have already stated, it is the most wasteful method. A well-constructed and ventilated water-closet, connected with a good general sewerage system, ought not to cause any unpleasant smell in a house; but should deodorants be required, in the opinion of the Sanitary Officer, liquids alone ought to be employed, and these such as cannot exert a corrosive action on the leaden or iron tubes, or metallic apparatus of the closet. Condy's fluid is an exceedingly convenient deodorant for this purpose, or a mixed solution of the sulphates of copper and iron, solution of alum, of chloralum, of sulphate of zinc, sulphite of lime, carbolic acid, or of nitrate of lead, may be employed, but acids and chloride of lime are objectionable.

Town sewers may be conveniently deodorised and disinfected by means of chloride of lime, chloralum, perchloride of iron, the sulphites, or carbolic acid. When sewer gas escapes from badly-trapped channel drains, or from other

openings, the plan of Dr. Stenhouse should be adopted, of obliging the issuing gas to pass through a layer of wood charcoal. This is laid in trays, and covered, so as to be protected from wet. These air filters are very effective, and last for a long time.

(b.) Treatment or Disposal of the General Refuse from a Town.-Whether the town sewage is removed by carting from cesspools into suburban tanks, or the refuse is carried there by water, the Medical Officer will have to advise as to the final treatment, unless it is immediately discharged into the sea. In the latter case no special treatment is necessary, unless it may be deodorisation, and this is always best accomplished in the cesspools or the sewers by the addition of any of the agents we have above enumerated.

But it may not be possible to send the refuse into the sea, or even into a river. In such a case the general plan which, according to our rather large experience in the matter, is almost invariably applicable in a more or less modified form is the following :-Prepare a crude sulphate of alumina by acting upon every 2 cwt. of a dry and highly argillaceous clay with 1 cwt. of strong oil of vitriol. The mixture must be thoroughly effected by means of thick iron shovels, though the acid will act upon the metal. Let the mass stand for ten days or a fortnight, in order to render the action as complete as possible. Then, in order to treat 100,000 gallons of sewage, place in the bottom of a tank capable of containing this bulk of liquid 12 cwt. (or 2 cwt. if the sewage be very strong) of the sulphated clay. The latter is then stirred up with 100 gallons or so of water, in order to get as much sulphate of alumina as possible into solution. The sewage is then pumped, or allowed to flow in. When the 100,000 gallons have been delivered, the mixture is stirred with long poles, and the whole allowed to rest for some hours. The alkaline sewage precipitates the alumina from the sulphate, and this, in falling to the bottom, carries with it suspended matters, leaving a clear liquid, usually almost inodorous. The patent process of Anderson differs from the above only

S

in the addition of lime to the mixture of sulphated clay and sewage. When complete separation has taken place, the clear liquid is run off from the tank, without disturbing the precipitate, and the latter collected as a mud, and allowed to drain on a porous bed, then kiln-dried, and sold as manure, its manurial value being about thirty shillings a ton; or it may be burnt, and the residue used for making another quantity of sulphated clay.

The treatment of the clear liquid has now to be considered. This solution contains most of the urea present in the sewage, since this is a very soluble body, and is not precipitable by any known process; moreover it is the constituent of the sewage of highest manurial value, on account of the large proportion of nitrogen it contains. Though we are not able to precipitate this urea from the effluent water from our tanks, percolation of the liquid through soil serves to remove it in great part, since it is easily decomposed into ammoniacal compounds, which the soil retains with extraordinary power, while a portion of the constituent nitrogen is oxidised to nitrous and nitric acids during the passage of the water. The best mode of disposing of the effluent water from the precipitating tank is by irrigation over land, for all offensive matter has been removed from the sewage, and the water distributed does not create a nuisance. The land irrigated with the effluent water yields good crops on cultivation, and is unobjectionable as a "sewage farm." Our experience of the Edinburgh and the Croydon farms, where untreated sewage is distributed, is such as to lead us to recommend the above-described precipitation of offensive solid matters in tanks prior to the flooding of land with the liquid.

It may be asked, Can the effluent water from the precipitating tank be allowed to flow into a river? We say, Certainly not before it has percolated through a stretch of rather porous soil, and so undergone another and most important purifying process. After sufficient filtration of this kind, it may be allowed to flow into a relatively large and rapid river.

CHAPTER XXIII.

CONTAGION AND DISINFECTION.

Theories of Contagion-Gaseous and Zymotic Hypotheses-Contagion Particles-Chauveau's Experiments on Vaccine Lymph-Chemical Agents used as "Disinfectants "-Deodorants-Antiseptics-True

Disinfectants-Treatment of Excreta, etc.-Disinfection of a RoomChemical Treatment of Articles of Clothing-Disinfection of Bedding, Clothing, etc., with the aid of Heat-Dry Heat-Steam Chamber for Disinfection.

DR. BURDON SANDERSON, in the introductory remarks to his report "On the Intimate Pathology of Contagion," says:"The question of the mode of existence of contagious matter lies at the foundation of all scientific inquiry as to the means of obviating or counteracting contagion." It is, therefore, advisable to devote some attention to the consideration of contagion before referring to disinfection, or the destruction of contagion.

Many and contradictory opinions have, from time to time, been held with regard to the intimate nature of contagion. The oldest view appears to have attributed it to the effect of poisonous gases inhaled or absorbed, or to the poisonous effects of putrid animal or vegetable matters. This view was closely connected with the zymotic theory of contagion; for some believed that these poisonous gases, etc., acted not as direct poisons, but by setting up changes which gave rise to specific poisonous.effects.

The more recent zymotic view was that a certain ferment was introduced into the body, which produced a peculiar form of fermentation, differing for each different disease. This ferment acts until the substance capable of being acted upon is exhausted, or its action put a stop to by some external agent. Neither of these views is held to any great extent by scientific pathologists of the present day, and in their

place has been substituted the hypothesis that the contagious particles of disease are Hiring organisms. Dr. Sanderson

*The notions which at present float in the minds of the medical community as to the nature of contagion are vague and indefinite. They are, for the most part, derived from what is commonly observed as to the mode of communication of small-pox and cow-pox. As regards the latter, it is held by almost all experts in vaccination that vaccine is a transparent liquid, and that it is most active when most transparent; whence it is most natural to infer that the contagious principle is soluble. As regards variola, it is known that the morbific influences can be exercised at a distance from their source, and through the air. From this undoubted fact it is assumed almost as if it followed from it, that the infective poison is volatile, and that it possesses the physical properties of a vapour."

Dr. Parkes makes a very practical classification (under three heads) of the various views at present held with regard to the nature of contagion, as follows:

1. The particles are supposed to be of animal origin, born in and only growing in the body; they are, in fact, minute portions of bioplasm or protoplasm.—(Beale.)

2. The particles are supposed to be of a fungoid nature, introduced ab externo.

3. The particles of contagion are thought to be like Schizomycetes, ie., of that class of organisms which Nägeli has separated from fungi, and which form the lowest stratum of the animate world at present known to us. These bodies have the various names-Bacteria, Zooglœa, Microzymes, Vibrios, Monads, etc. The observations and experiments of Dr. Sanderson and others" afford strong ground for accepting the doctrine

that the contagious principle is neither soluble in water nor capable of assuming the form of vapour;" but, in making this statement, Dr. Sanderson should not be understood to convey the impression that contagia may not be carried mechanically by either air or water as dust or sediment, for, in fact, these methods for the conveyance of contagion are proved to exist.