by using Mr. Morton Festing's excellent form of "ozonecage." 5. Excessive humidity and a high temperature interfere with this test by causing evaporation of the iodine; while (6) long exposure to air and light causes bleaching; this effect is also produced by (7) sulphurous acid, the product of coal combustion, and a constant impurity in the atmosphere of large towns. 8. The prevalence of what has been called a "true antozone period," generally coincident with a north wind, also affects the test, but this subject requires elucidation. Ozone, moreover, seems to exert some imperfectly understood action on the starch itself, besides liberating the iodine from the iodide of potassium. Add to all this the defective character of ozonometers, their varying scales of division, the difference between various observers in their power of distinguishing shades of colour, errors which have resulted from differences in the quarter of the compass to which the tests were exposed, differences of elevation, &c., and when we remember that this starch and iodide of potassium test is that which has been by far the most generally used, we shall not be surprised to meet with the following desponding results: 1. According to Dr. Richardson, the tests for atmospheric ozone are very unsatisfactory. 2. According to Professor Heaton, the greater number of the observations thereon are worthless. 3. According to Boehm, tests prepared alike give different results. 4. According to Scoresby Jackson, all the ozonometric methods which have been proposed are more or less objectionable. 5. According to Cloez, ozonometric observations are utterly destitute of any value. 6. According to Parkes, the uncertainty of the whole subject forbids us to draw any conclusions thereon. 7. According to Admiral Fitzroy, no clear and satisfactory results have been obtained. 8. According to Dr. Stark, ozonoscopes merely register the amount of moisture present in the air. 9. According to the Scottish Meteorological Society (January 14th, 1869), no means whatever have yet been devised for the estimation of ozone. Under such circumstances it need not astonish us that the beneficial or deleterious effects of ozone upon the animal economy in curing or causing disease of any kind, the share it takes in spreading or destroying the infection of contagious disorders, and the part it plays in renovating the atmosphere, are all still as vigorously debated as the influence of climate in phthisis, the action of bromide of potassium, and the pathology and treatment of diabetes. The chief "air-purifiers" are ozone, nitrous acid, and peroxide of hydrogen. The sum of these three may be measured by the amount of iodine liberated from a solution of pure neutral iodide of potassium. In order to ascertain this, slips of moderately thick Swedish filteringpaper must be dipped in a solution of the above salt, strength about 15 per cent, then carefully dried and kept in a dark place, not exposed to air. When used, one or more of these slips must be placed in an ozone-box through which a current of air of known velocity is drawn by an aspirator for the length of time determined on, and, as moisture favours volatilisation of the liberated iodine, they must be as much as possible protected by means of several layers of wire gauze, further corrections of this source of error being drawn from the hygrometric readings. The slip (or slips) of paper is then removed and carefully compared with the chromatic scale. Next, as a portion of the liberated iodine is often oxidized into iodic acid which gives no indication of its presence by colorisation, this must be decomposed by means of tartaric acid spray, and the slip again compared with the chromatic scale. If, however, we require to ascertain the amount of pure ozone present in the atmosphere, we must resort to the celebrated iodized litmus test, which was some time ago suggested by Bernays and Hornidge, but first actually employed by Houzeau. It is thus conducted : Blue litmus having been boiled in distilled water, the solution is allowed to stand for twenty-four hours; then In decanted, and divided into two equal parts. One of these is reddened by means of the smallest possible amount of pure sulphuric acid, and then mixed with the other. The whole is again divided, one part reddened by a similar addition of sulphuric acid, and mixed with the other; this process is again and again repeated until a persistent vinous red hue is obtained, the stability of which is ascertained by a mark caused by it not becoming blue on drying. This solution is allowed to stand and then filtered. It ought to contain one seventy-fifth part by weight of the dry extract. Care must be taken to secure the precise vinous red tint, as any undue excess of acid (which produces a hue resembling that of the skin of an onion) renders the test much less delicate. The very slight excess of acid always present is useful, as it prevents the formation of iodate of potash. this are to be dipped the slips of Swedish filtering-paper described above; but the solution of iodide of potassium ought to contain only about 1 per cent. of the salt. If a slip of the paper impregnated with litmus be dipped into this latter and exposed to the air, the ozone, if present, will liberate a portion of the iodine from the iodide of potassium, replacing it in combination with the potassium, thereby forming potash. The alkalinity thus produced will be appreciable by the depth of the blue tint assumed by the litmus paper, which is thus a quantitative test indicating the amount of ozone present in the atmosphere. The chief source of error in this test arises from the fact that the reddened litmus has a tendency to assume a more or less bluish tinge in damp air, even apart from the influence of ozone, owing to fermentative changes set up in the drug itself by moisture. Moreover, the test is not quite so delicate as could be wished; but still it not merely furnishes incomparably the best means hitherto known of estimating atmospheric ozone, but, in the present state of our knowledge, it is the sole trustworthy method at our disposal. Smyth's ozone box, as modified and improved by Dr. Fox, seems at once the simplest and the best receptacle for the test-papers during the experiment. It is described and delineated pp. 262 and 264 of that gentleman's treatise on ozone, and at p. 255 of the same work he gives a diagram and description of the whole apparatus that he employs in his experiments. It would be useless to attempt any description of these here without the aid of diagrams, and I have already lingered too long on this part of the subject. We may now reasonably hope that, with the aid of these improved methods and apparatus, Dr. Fox and other sagacious and accurate observers will ultimately succeed in rescuing this much vexed subject of ozone from its present unfortunate position as one of the opprobria chemiæ. At present, regarding the subject from a medical point of view, it must be confessed that even after the labours of such men as Berzelius, Schönbein, De la Rive, Frémy, Williamson, Baumert, Andrews, Tait, Soret, Faraday, Houzeau, Scoutetten, and numerous others, extending over a period of nearly thirty years, we can say but very little which is either definite or positive. The relation between ozone and cholera epidemics is as yet undetermined. In London, during the epidemic of 1854, Glaisher found the normal amount of ozone in districts where deaths from cholera were numerous, and no ozone whatever in districts where not a single death occurred from this disease. The observations of Seitz, at Munich, during the same year are in harmony with those of Glaisher. Similarly, Dr. Day, of Geelong, found an abundant ozonic reaction around the houses of cholera patients. In the United States, during 1851, Peter was unable to trace any connection between the absence or presence of ozone and the prevalence of cholera, and in this negative opinion he is supported by E. Boeckel, of Strasburg, Strambio of Milan, and one set of observations by Simonin of Nancy. Moreover, Fournet maintained that in Lyons, the "city without ozone," cholera is not more frequent or severe than elsewhere, and the negative view is further supported by Petenkofer and Schifferdecker. Dr. Moxon thinks that the prevalence of simple diarrhoea is increased by the preponderance of ozone. On the other hand, the observations of Smallwood in Canada; those of Cook in Bombay, extending from 1863 to 1865; those of Moffat at Newcastle in 1863, and throughout England and Wales in 1866, and those of Simonin at Nancy in 1855, all favour the supposition that the times of the appearance and prevalence of cholera coincide with those of the absence or deficiency of ozone. Billard, of Corbigny, thinks that the diminution of ozone is the first cause of a chloera epidemic. The views of these observers are further supported by experiments at Berne, Marseilles, and Berlin. I have endeavoured to sum up the evidence on both sides as fairly as I can, but who shall decide when such doctors disagree? Schönbein has Next, as regards catarrhal affections. demonstrated that catarrh can be set up in animals by forcing them to breathe air largely impregnated with ozone, and Mr. Blackley has observed a condition closely resembling hay-fever to be thus produced. At the same time, the amount of ozone present in the atmosphere is, even when at its maximum, wholly insufficient to give rise to any such morbid states in a previously healthy subject, while it is, of course, amply abundant to cure any existing disease to which it is homœopathic. Accordingly, we learn from the results of observations by Seitz at Munich that the number of cases of catarrh decreases in proportion as the amount of atmospheric ozone increases. The observations of Seitz are corroborated by those of Dr. W. W. Ireland, at Umballa, and Mr. Harris at Worthing. It is true that Heidenreich, E. Boeckel, and Spengler hold a diametrically opposite opinion, and affirm that the period of a catarrhal epidemic is coincident with that of an excess of atmospheric ozone; while Faber, Wunderlich, Schifferdecker, T. Boeckel and others deny that any connection whatever exists between these periods. But it must be remembered that many influences contribute to the production of ozone, and that these influences act in different proportions in different places. Among the most powerful and widely distributed of these agencies are cold and damp, and since the maximum of atmospheric ozone is on all sides admitted to be wholly insufficient to produce catarrh, it is only reasonable to assign the comtemporaneity of periods of high ozonic reaction and catarrhal epidemics to the influence of cold and damp in pro |