to find any larger proportion of unhealthy, badly fed, or overworked mothers or nurses at one part of the year than another. It is quite certain that if any one or all of these agencies exerted a prominent or controlling influence in determining attacks of serous diarrhoea and cholera infantum, such attacks would be met with frequently at all seasons of the year. Yet, both the records of the commencement of attacks and the statistics of mortality, show that the prevalence of all grades of these two forms of disease is restricted almost entirely to the ninety days intervening between the last week in June and the last in September.

Thus, in Chicago, in 1877, only 2 deaths from cholera infantum are reported in the statistics of the Health Department during the months of November, December, January, February and March; 8 in April; 6 in May; 23 in June; 246 in July; 163 in August; 69 in September; and 13 in October. Again, in 1875 and 1876, I obtained the date of the commencement of 351 cases of serous diarrhoea and cholera infantum, of which 61 commenced in June, 197 in July, 66 in August, and 27 in September, and none during the remaining months of those years. The ratio of prevalence thus found to exist in the various months of 1875-6-7, in Chicago, will be found to fairly represent the ratio every year, and in all the Northern and Eastern cities of our country. If we turn our attention in another direction, we will be met by still greater difficulties in accounting for the prevalence of these bowel affections on the supposition that they are produced by the causes to which they have usually been attributed. For instance, the mortuary statistics show that the diseases under consideration prevail but little in cities so located that there is only a short range of temperature between the warmest days of summer and the coldest days of winter, and where, from sea breezes or otherwise, the summer nights are cool.

There is no evidence within our knowledge which shows that the milk distributed in San Francisco and New Orleans is any purer or of better quality than in Boston and Chicago. Neither are the nursing mothers any more free from mental and physical infirmities, nor the sanitary conditions of the dwellings, sewers, etc., more perfect in the two former than in the two last-named cities. Yet an examination of the mortality statistics of these several cities shows a ratio of only about 5

deaths from cholera infantum annually for every 10,000 inhabitants in San Francisco, and 7 in New Orleans, while Boston gives about 25 and Chicago 30 deaths from the same cause for every 10,000 of their inhabitants. The foregoing facts show conclusively that there must be some efficient cause or causes which determine the prevalence and fatality of the diseases under consideration that are not common to all large cities and all aggregations of civilized people.

Their prevalence at certain seasons of the year only, and chiefly in certain climatic regions, shows conclusively that they are dependent on causes which are operative under some circumstances not common to all civilized communities.

To determine what these circumstances are, I commenced, many years since, to keep a record of the date of beginning of all attacks of serous diarrhoea and cholera infantum coming under my observation, in connection with the coincident meteorological conditions of the atmosphere, and for three years coincident records of a similar character were kept by active practitioners in Cairo, Illinois; Davenport, Iowa; and Omaha, Nebraska. Reports giving the results of these investigations were made in the Medical Section of this Association, and published in the Transactions for 1875, 1877 and 1879, to which I must refer you for details. Those investigations were sufficient to establish the following important conclusions:—

First.—That the prevalence of the affections under consideration is limited principally to the months of July, August and September, commencing with the first wave of high atmospheric heat that continues day and night for more than five days, which in the latitude and altitude of Chicago is sometimes the last week in June, but more frequently the first week in July, and continuing more or less during the succeeding ninety days.

Second. That while the number of deaths from these affections in any city or given community will be nearly the same. in the two first months after they begin,-that is, July and August, the date of the initial symptoms, or beginning of the disease in three-fourths of all the cases, will be in July, very few originating after the first of August. Many cases that commence in July, and partially recover, are found to relapse or become worse during certain waves of high temperature in August; and a large percentage of those attacked in July

continue wasting with the disease until relieved by death throughout the months of August and September.

Third. That it is not simply high or extreme heat of temporary duration, such as that of a single day or of any number of days with cool nights, which favors the development of serous diarrhoea and cholera infantum, but continuous high temperature day and night through several days in succession.

And if, in addition to the high heat, the air be stagnant, as from lack of winds, or from obstructions, as in large and compactly built cities, or from defective ventilation of dwellings, the morbific effects are greatly increased. This explains why these affections are more numerous and fatal in cities than in rural districts, and why they prevail so little in even large cities located in warm climates, provided the location be such as to afford cool breezes at night, as is the case in San Francisco and New Orleans, already alluded to.

Fourth. That while the great majority of attacks which occur in any given summer are found to have their beginning in July, or during the first thirty or forty days after the first wave of protracted high temperature for the season, they are not equally distributed over the whole of the month, but are almost all traceable to a limited number of days and nights coincident with the waves or periods of continuous high temperature. From observations extending over twenty years in Chicago and three years in Cairo, Davenport, and Omaha, 1 have found that the special waves or periods of high temperature day and night vary in duration from three to fourteen days. When they do not extend beyond three days, the effect on the number of attacks of serous diarrhoea and cholera infantum is slight. In Chicago, many summers have passed without a single period of continuous high heat of more than three or four days' duration, and such have uniformly been accompanied by a low ratio of infant mortality from bowel affections. In a majority of the seasons, however, there have been found three of those periods of continuous high temperature between the 25th of June and the 31st of July, each from five to seven days in duration. And the attacks of serous diarrhoea and cholera infantum have increased so rapidly that the number of deaths from these alone has caused the aggregate mortality of July to be more than double that of June. Much more rarely a season has occurred in which one of these periods of continuous heat

day and night has commenced during the last week in June or first week in July, and continued with but little variation for two or three weeks. Such seasons have uniformly been characterized either by a prevalence of epidemic cholera or an extraordinary mortality from the serous fluxes in children.

Having thus traced the origin of that part of infantile mortality caused by the affections just named to the coincidence of continuous high atmospheric heat with a minimum of atmospheric currents, let us inquire, for a moment, how this combination of circumstances can affect the living human body.

First.-We have the physical law that the higher the temperature of the air the rarer it becomes, and the less oxygen is contained in each cubic inch. Consequently an individual breathing a given number of times per minute, and a given number of cubic inches of air at 80° F. would receive into the air cells of his lungs much less oxygen per hour than one breathing the same number of times and the same number of cubic inches of air at 60° F.

Again, a still or stagnant atmosphere, whether from the absence of winds or currents without or of ventilation within, becomes more rapidly exhausted of its oxygen and impregnated with impurities from the breathing of living beings than one actively changed by currents and free ventilation.

Second. The physical law of expansion by increase of temperature applies to living as well as to dead matter. Consequently continuous high heat, acting on the living human body, tends to increase the distance of the atoms or molecules from each other, and thereby lessen the force of vital affinity or general tonicity of the tissues, while it increases the excitability or susceptibility to impressions.

Third. The capacity of the blood for taking up oxygen or holding it in suspension, depends much on the proportion of saline elements it contains, and under a continuous high temperature the increase of cutaneous exhalation rapidly diminishes the free salts of the blood, especially the chloride of sodium, and thereby directly lessens its capacity to receive the oxygen from the air-cells of the lungs in exchange for its carbonic acid gas. It is hardly necessary to add, that on the degree of oxygenation and decarbonization of the blood depends the sensibility and natural action of the vaso-motor and all other portions of the nervous structure of the body. Here, then, we

have in these waves or periods of high summer heat the coincidence of less oxygen to the cubic inch of inspired air; less capacity of the blood to take up and hold it in solution; less general tonicity of the textures of the body, with increased excitability of the mucous membranes and cutaneous surfaces from the direct stimulus of external heat. The two first of these conditions, by lessening the oxygenation and decarbonization of the blood, directly diminishes the influence of the vaso-motor nerves over the tone of the vessels of the morbidly excitable mucous surfaces of the alimentary canal, and thereby favors serous exudation instead of either natural secretion or absorption. Thus, by first establishing the coincident conditions under which serous diarrhoeas and cholera infantum actually occur, and, second, by analyzing these conditions by the application of known laws of physics and physiology, we are enabled to see clearly the exact pathological conditions inducednamely, a morbidly sensitive condition of the mucous membrane of the alimentary canal, in conjunction with such a diminution of general tonicity and special impairment of vaso-motor nerve influence as to impair the natural secretory actions, and directly establish more or less exudation of the serous elements of the blood. In a large proportion of these cases the resulting serous exudation is only sufficient to render the natural evacuations thinner and more abundant, constituting the mildest form of summer complaint." From this we have all grades of severity up to an entire suspension of secretory action, and so rapid an exudation as to cause the copious vomiting and purging of an active cholera morbus; so copious, indeed, sometimes, as to exhaust the water and salts of the blood, and induce fatal collapse in a few hours. The essential pathological conditions are, general impairment of tonicity of the tissues with deficient oxygenation of the blood, and special impairment of the vasomotor nervous influence over the vessels of the mucous membranes of the stomach and intestines. The exudation constituting the discharges results from these pathological conditions, and has no necessary connection with any grade of inflammation, catarrhal or otherwise. Inflammation of portions of the mucous membrane often supervenes as a complication during the progress of protracted cases.

But ilio-colitis and recto-colitis or dysentery seldom occur until later in the season, when warm days are followed by cool