Objections have been made to the perchloride, as it contains arsenic; but the amount of this is small, and as it falls with the deposit it is never likely to be dangerous.

8. Lueder & Leidloff's Powder consists (according to Leuchtenberg's analysis) of ferric sulphate, 36 per cent.; ferrous sulphate, 16; free sulphuric acid, 4; calcium sulphate and other substances, 44. It has been highly commended, but, from experiments made at Netley, it does not seem very powerful.

9. Lead Nitrate, or Ledoyen's Fluid, is made by dissolving 1 pound of litharge in about 7 ounces of strong nitric acid and 2 gallons of water; a little of the water is mixed with the litharge; the acid is gradually added, and then the rest of the water. This quantity will deodorize a moderatesized cesspool. It acts rapidly on hydrogen sulphide, and can be depended upon for this purpose.

10. Zinc Chloride.-Burnett's fluid contains 25 grains to every fluid drachm; 1 pint is added to a gallon of water (1 to 8). It is usually said to decompose hydrogen sulphide until the solution becomes acid, when its action ceases; but Hofmann finds that it does not act on free hydrogen sulphide, but on ammonium sulphide, forming zinc sulphide and ammonium chloride. It destroys ammoniacal compounds and organic matter. The sulphates of zinc and copper decompose free hydrogen sulphide, with formation of metallic sulphide and water.

Burnett's fluid delays decomposition in sewage for some time; but a very peculiar odor is given out, showing that some change is going on. A good effect is produced on hydrogen sulphide by a mixture of zinc and ferrous sulphates (Larnaudès' mixture) which also lessens for the time the peculiar sewage smell.

11. Zinc Sulphate.-This forms part of the Universal Disinfecting Powder (Langston-Jones' patent), along with Cooper's salts, viz., calcium and sodium chlorides. This powder has the advantage of being inodorous, but it is not a strong deodorant. It, however, gets rid of fecal odor to some extent, and is efficacious against H,S.

12. Potassium permanganate prevents putrefaction for a short time, and removes the odor from putrefying sewage, but it requires to be used in large quantity.

13. Preparations from coal-tar; carbolic acid (phenol or phenic acid, or phenyl-alcohol (CHO)); coal-tar creosote, and cresylic acid (cresol or cresyl-alcohol (C,H,O) ), in various admixtures. These substances are all excellent sewage deodorants and arresters of putrefaction.

2

The last few years have seen an extraordinary development in the manufacture of these substances. Phenol or carbolic acid is now obtained in great purity, and is sold in crystals, and also in a liquid form. All the

To later samples some calcium borate was added. Probably the addition of ferrous sulphate might improve it.

It is perhaps unfortunate that phenol and cresol, which are rather alcohols than acids, should have been termed carbolic and cresylic acids. If the terms phenol and

cresol could be used instead it would be better.

preparations may be conveniently classed under the three divisions of crystals, liquids, and powders.

(a) Crystals.-Carbolic acid, more or less pure, is the only substance under this head; it is so slightly soluble in water (only in the proportion of 5 per cent.) that it is not so useful as a deodorant as the impurer kind. When mixed with sewage it acts slowly and not so perfectly as the impurer kinds. When exposed to the air it liquefies, and is slowly given out into the air, and is then supposed to be useful as an air purifier.

(b) Liquids.-Carbolic acid, more or less impure, dissolved in water, simply, or with a little alcohol and cresylic acid (cresol), forms the liquid carbolic acids. In the market they are found almost colorless, or highly colored. The various liquids contain from 10 to 90 per cent. of phenol. Cresol, though crystalline and colorless when pure, is usually found in the market as a dark liquid. Some of it, no doubt, exists in most samples of carbolic acid. Owing probably to the way they mix at once with the sewage, the liquid acids are more deodorant than the crystallized acid, and restrain putrefaction for a long time. Carbolic acid, however, does not act on hydrogen sulphide, though it will restrain the processes which produce it.

Samples of so-called carbolic acid are sold, which are only impure tar oils, and almost destitute of deodorizing power. Sometimes a nauseous sulphur compound is also present.

Mr. Crookes' gives the following rules in order to determine the presence of the tar oils :-

"Commercial carbolic acid is soluble in from 20 to 70 parts of water, or in twice its bulk of a solution of caustic soda, while oil of tar is nearly insoluble, but if the amount of carbolic acid be increased, some remains undissolved.

"To apply the tests-1. Put a teaspoonful of the carbolic acid in a bottle, pour on it half a pint of warm water, and shake the bottle at intervals for half an hour, when the amount of oily residue will show the impurity; or dissolve one part of caustic soda in 10 parts of warm water, and shake it up with 5 parts of the carbolic acid. As before, the residue will show the amount of impurity.

"These tests will show whether tar oils have been used as adulterants, but to ascertain whether the liquid consists of a mere solution of carbolic acid in water or alkali, or whether it contains sulpho-carbolic or sulphocresylic acids, another test must be used based on the solubility of these, and the insolubility of carbolic acid in a small quantity of water. In this case proceed as follows:-2. Put a wineglassful of the liquid to be tested in a bottle, and pour on it half a pint of warm water. If the greater part dissolves, it is an adulterated article. Test the liquid in the bottle with litmus paper; if strongly acid, it will show the probable presence of sulphoacids; whilst if alkaline, it will show that caustic soda has been probably used as a solvent."

If the quantity of carbolic acid has to be estimated from a liquid, it must be distilled at a given temperature. Carbolic acid boils at 184° C. (= 363° Fahr.), cresol at 203° C. (= 397.4° Fahr.).

In using the liquid acid, 1 part is mixed with 50 or 100 of water, according to the strength of the acid, and thrown down drains or into cesspools, or sprinkled with a watering-can over dung-heaps.

'Third Report-Cattle Plague Commission. Carbolic acid can be distinguished from creosote by its solubility in glycerine (Morson).

(e) Powders.-The two principal carbolic acid powders are M'Dougall's and Calvert's, but there are several others in the market known under various names.

M'Dougall's and Calvert's powders are widely different in composition. The former is strongly alkaline from lime, and makes the sewage alkaline. It consists of about 33 per cent. of carbolate of lime and 59 per cent. of sulphite of magnesia, the rest being water.

Calvert's powder is carbolic acid, about 20 to 30 per cent., mixed with alumina from alum works, and some silica.

The quantity of these preparations which must be used depends on the degree and duration of deodorization wished for. For the daily solid excreta (4 ounces) of an adult at least from 30 to 70 grains of the crystallized acid, 60 drops of the strong liquid (90 per cent. of acid), or ounce of the dilute carbolic acid, sold at 1s. per pint, are necessary, if the sewage is to be kept in an unaltered state for 10 to 20 days, but a smaller amount is sufficient for 2 or 3 days. Dr. Sansom, who does not rate the effect of carbolic acid so highly as a deodorant, also finds that much larger quantities must be used than is usually stated. Half an ounce of either Calvert's or M'Dougall's powder for 4 ounces of sewage has a preservative effect for 18 to 20 days; ounce or less is effectual for 3 or 4 days, but if the stools contain urine much more is necessary."

2

Smaller quantities can, however, be used, if diminution, but not entire removal of smell and putrefaction is desired. Quicklime 5 parts, and carbolic acid 1 part, make a good deodorizing mixture. If hydrochloric acid is added, and then water, the lime is deposited, and the carbolic acid floats on the surface, and its amount can be determined.

14. The Süvern Deodorant.-The water flowing from sugar factories has long been a source of annoyance and ill-health; it contains quantities of vegetable organisms (Oscillaria alba or Beggiatoa), which act like ferments, and rapidly decompose the sulphates in the water, and liberate hydrogen sulphide. Herr Süvern, to remedy this, proposed a preparation of coal-tar thus prepared. A bushel and a half of good quicklime is put in a cask and slaked; it is well stirred, and 10 b of coal-tar are thoroughly mixed with it, so that the coal-tar may be thoroughly divided. Fifteen pounds of magnesium chloride dissolved in hot water are then thoroughly mixed with the mass, and then additional hot water is added, enough to make a mass of just sufficient liquidity to drop slowly from a stick inserted in it and then pulled out. The magnesium chloride forms deliquescent calcium chloride, magnesia being liberated, and it is found that this prevents the caking of the deodorant and the adherence to pipes. This deodorant has come into considerable use for cesspools, drains, etc. The Müller-Schurr deodorizer has been already noticed.

15. Dr. F. T. Bond (of Gloucester) has introduced a new deodorant in the form of powder and liquid, consisting essentially of metallic salts,

1 See Dr. Parkes' experiments in the Army Medical Department Report, vol. viii., p. 318.

Op. cit., p. 203.

Dr. John Day (of Geelong) published a paper in the Australian Medical Journal (June, 1874), on the comparative value as disinfectants of carbolic acid and mineral oils, such as gasolene and kerosene. He prefers gasolene, and finds it may be used for papered walls, furniture, clothing, and flooring. It must be used with caution near lights, as it is very inflammable. Dr. Day attributes its action to its strong oxidizing properties; paper brushed over with it gave the reaction of peroxide of hydrogen after more than a year.

4 Trautman, Die Zersetzungsgase, 1869, p. 35.

alum, and terebene (a hydrocarbon derived from turpentine by treatment with sulphuric acid). Terebene has a pleasant odor, and so far is superior to carbolic acid; its deodorizing powers are very considerable. The preparations in the form of powder are various, the chief being ferralum and cupralum, the latter being most frequently employed. It consists of copper sulphate, aluminum sulphate, a little potassium dichromate, and terebene. It is a very powerful deodorant, counteracting ammonia and hydrogen sulphide, and at least masking fecal odor as much as carbolic acid. Some objections were formerly made to it on account of a tendency to deliquescence, due to the presence of sodium chloride. This has now been remedied, and the preparation keeps well.

The substance advertised as Sanitas is a hydrocarbon derived from turpentine acted upon by steam. It has the advantage of being easily miscible with water, but it is not very powerful.

16. The remarkable power shown by salicylic acid in arresting fermentation, and its value in the antiseptic treatment of wounds, would seem to indicate it as a good agent, but it is at present too expensive for use on a large scale.

General Conclusion.-It must be remembered that deodorization is only possible within certain limits, and that in a number of cases only partial results can be obtained, unless very large quantities of the deodorant are used.' The most effectual appear to be the terebene preparations, especially the cupralum, and carbolic acid and its preparations. Of these the cupralum has the advantage of destroying hydrogen sulphide and neutralizing ammonia, which are only masked by the others. Chloride of lime and chloride of soda are also powerful, but have themselves a sickly odor, very disagreeable to many persons. The Süvern deodorant is probably the next best, and after that the ferric chloride (FeCl ̧).

1 In experimenting with the very offensive infusion of linseed, it was found almost impossible to get rid of odor without using very large quantities of the deodorants.— [F. de C.]

CHAPTER XX.

STATISTICS.

AN accurate basis of facts, derived from a sufficient amount of experience, and tabulated with the proper precision, lies at the very foundation of hygiene, as of all exact sciences. Army surgeons have already contributed much important statistical evidence as to the amount and prevalence of different diseases, and it is evident that no other body of medical practitioners possess such opportunities of collecting, with accuracy, facts of this kind, both among their own nations and others. As they have to make many statistical returns, it seems desirable to make a few brief remarks on some elementary points of statistics, which are necessary to secure the requisite accuracy in collecting and arranging facts. But it is, of course, impossible to enter into the mathematical consideration of this subject; for a separate treatise would be required to do justice to it.

SECTION I.

A FEW ELEMENTARY POINTS CONNECTED WITH GENERAL STATISTICS.

1. The elements of statistical inquiries are individual facts, or so-called numerical units, which having to be put together, or classed, must have precise, definite, and constant characters. For example, if a number of cases of a certain disease are to be assembled in one group with a definite signification, it is indispensable that each of these cases should be what it purports to be, an unit not only of a definite character, but of the same character as the other units. In other words, an accurate diagnosis of the disease is essential, or statistical analysis can only produce error. If the numerical units are not precise and comparable, it is better not to use them. A great responsibility rests on those who send in inaccurate statistical tables of diseases; for it must be remembered that the statist does not attempt to determine if his units are correct; he simply accepts them, and it is only if the results he brings out are different from prior results that he begins to suspect inaccuracy.'

It is in vain to conceal the fact that many persons look at tables of diseases collected indiscriminately as worse than useless, from errors in diagnosis. Even in the army returns, which are all furnished by qualified practitioners, there is reason to doubt the correctness of the earlier tables especially. But it is believed that the army returns of diseases are now gaining in accuracy, and it cannot be too strongly urged on medical officers that perfect accuracy in diagnosis is a duty of the highest kind. It is much better to have a large heading of undetermined diseases than, when in doubt, to put a case of disease under a heading to which it has no unequivocal pretensions. It is greatly to be regretted that, from the abridged form in which they are now published, much valuable information is now no longer obtainable from the Army Medical Reports.