presence in the blood of something in the nature of a ferment, which I then supposed to be fungoid, those symptoms presenting, as it appeared to me, considerable analogy to the results produced by ferments upon saccharine solutions. Sir Joseph did not take any notice of this part of my letter, the bulk of which had reference to questions relating to the botany of New Zealand, and there, so far as I was concerned, the matter ended; and I only mention this now in order to show why I take so much interest in the important investigations to which I have alluded.

It must be apparent that the results established by these investigations render it of the utmost importance that there should be a more general knowledge than now exists, of the nature of the minute organisms in question, of their mode of action on man and the domestic animals, and of the circumstances which lead to or favour their introduction into the body, more especially, as regards the latter point, with a view to the adoption of all possible measures to prevent it.

The first to notice the presence in the blood of the class of organisms referred to was Davaine, who, in 1850, found in that of animals which had died of splenic fever quantities of minute rods, to which he gave the name of bacteridia; but it was only in 1863, when Pasteur had discovered the part played in fermentation by some of the organisms now forming members of the class termed microbes, that Davaine was led to suppose that these bacteridia were the actual cause of the splenic fever.

This disease, commonly known by the name of anthrax, affects man as well as animals, and is one of the most deadly to which cattle and sheep especially are subject. In these animals it is generally produced by inoculation, as, for example, by the bites of flies which have fed upon the carcases of beasts that have died of the disease, or by the poison coming otherwise into contact with some accidental abrasion of the skin, or with punctures of the mucous coat of the mouth caused by the prickles of plants on which they may have fed. The period of incubation of the disease is extremely short, an ox, apparently healthy at his return from his work, having been known to exhibit symptoms of the disease soon after, and to die within an hour after exhibiting the first appearance of infection.

In order to discover why this disease was so common in some districts in France in which it had been found difficult previously to assign a cause for its spread, Pasteur carried on some experiments, with the aid of two other specialists, and found that sheep permitted to feed on grass upon which bacteridia taken from the blood of diseased animals had been intentionally spread, speedily manifested all the symptoms of

the disease, and died. The glands and tissues of the back of the throat were found to be in the condition which would naturally result from inoculation by means of wounds, slight, it might be, of the surface of the mucous membrane of the mouth, and, in order to ascertain whether this was the likely cause, grass mixed with thistles, bearded ears of barley, and other prickly forms of fodder, on which bacteridia had been scattered, was given to other sheep, the result being that mortality amongst them was rapidly induced.

Experiments were then made in order to ascertain how long the vitality of the bacteria was maintained after the death of an infected animal, and it was found that, while by far the greater number were destroyed by the putrid fermentation of the carcases of the diseased sheep, a proportion became disengaged by the gas generated during decomposition, and that these, drying up, produced spores which retained their vitality for a long period, and were so minute and light as to be capable of being transported by even the weakest currents of air.

As regards the human subject, it has been found that anthrax, appearing in the first instance in the form of malignant pustules, affects shepherds, butchers, tanners, and others who handle the flesh and skins of tainted animals, the disease usually resulting rapidly where there is any scratch or wound in the face or hand.

Instances have occurred in which the disease has apparently been introduced into man through the mouth or lungs, but human beings are apparently less subject to contract it in this manner than the herbivora, for the flesh of animals killed after the microbe has become fully developed in the blood is often eaten with impunity in the farmhouses on properties on which the disease has been prevalent. It may, however, be said that in this case the microbes are effectually destroyed by the cooking processes to which the flesh is subjected.

The important researches which were induced by and followed upon the discovery of the effects of the inoculation of sheep with Bacillus anthracis have, as already mentioned, clearly demonstrated that nearly if not all the most serious febrile diseases which attack man and the domestic animals have their origin in the introduction into the system of special forms of microbes.

This has been established in the cases (amongst others) of cholera, typhus, and typhoid fevers, small-pox, recurrent, yellow, scarlet, and intermittent or malarious fevers, and in croup, and also in such minor diseases as measles, whooping-cough, &c., in man, and in the cases of the rouget, or swine-fever, of glanders as affecting horses, of rabies as affecting the

dog and cat, and of fowl-cholera, and in many other diseases affecting the domestic animals.

But whilst we have a special form of microbe concerned in each of these diseases, there are general characters and modes of reproduction common to the entire class, and these have particularly to be considered in relation to the manner in which they are disseminated and to the precautions which have to be taken against infection.

The latter points, however, can only be discussed after we have acquired a knowledge of the former, and I now proceed to give such information in this respect as I have been able to gather from the works of Pasteur and others.

For some time after the discovery of these singular organisms it was matter of doubt, even in the minds of the most careful observers, whether they belonged to the animal or vegetable kingdom, many, indeed, inclining to the belief that they constituted a link connecting the two; but later and more accurate investigations have established that they are purely vegetable, and are, in effect, special forms of Algæ. The classification now recognised divides them into eleven genera, most of which have received names characteristic of their respective outward forms; and to one or other of these nearly all the specific diseases I have already referred to have been assigned.

As several of these genera, however, are polymorphic, it is not impossible that some diseases may be produced by microbes belonging to more than one genus, although the impression as yet arrived at by investigators is adverse to such a supposition.

But, however varied may be the external forms of these pathogenic organisms, they all possess certain characters in common to which I will now shortly refer.

In the first place, they all appear under the microscope as minute cells of a spherical, oval, or cylindrical shape, sometimes single, sometimes united in pairs or in articulated chains or chaplets either straight, curved, or spiral. The diameter of the largest does not exceed two micro-millimetres, and that of the smallest is not more than a fourth of that size, so that at least 500 of the former and 2,000 of the latter must be placed end to end in order to reach the length of a millimetre, requiring, in fact, a magnifying-power of from 500 to 1,000 diameters, and sometimes even a still higher power of the microscope, to make them clearly visible.

“One very common bacterium,” says Trouessart, “may be found everywhere, and can be easily procured for microscopic observation, namely, Bacterium termo, or the microbe of impure water. This bacterium is not injurious to health, since there is no potable water in which it is not found in greater or less

quantity. In order to obtain numerous specimens it is enough to take half a glass of ordinary water from a spring or river, and to leave it for some days on a table or chimney-piece, the vessel being uncovered to allow the access of air. We may soon observe that a thin coating is formed on the surface of the water which looks like a deposit of fine dust, but which, however, consists of myriads of bacteria. If we take a drop of this water and place it under a cover-glass in order to examine it under a microscope with a magnifying-power of about 500 diameters, we shall, as soon as the instrument is properly focussed, see a really surprising spectacle.

“The whole field of the microscope is in motion: hundreds of bacteria, resembling minute transparent worms, are swimming in every direction with an undulatory motion like that of an eel or snake. Some are detached, others united in pairs, others in chains or chaplets, or cylindrical rods, which are partitioned or articulated: these are only less mature, or younger, than the first. Finally, we see a multitude of small globules which result from the rupture of the chaplets. All these forms represent the different transformations of Bacterium termo, or the microbe of putrefaction. Those which are dead appear as small, rigid, and immovable rods.”

In all the various forms of microbe, however, each cell consists of a cellulose wall containing protoplasm. These cells, so long as they continue active, multiply by fission with enormous rapidity, precisely in the same way as in the case of ferments. When, however, the liquid upon which they subsist is dried up, the cell-wall dries also, and the protoplasm within contracts and forms a spore, which, being set free by the rupture of the wall, gives rise to fresh microbes by germination when placed under suitable conditions.

Pathogenic microbes, then, are extremely minute cells, invisible except under very high powers of the microscope, having walls composed of cellulose, enclosing particles of protoplasm, multiplying rapidly whilst active by fission, and, when no longer capable of increasing in this manner, producing spores which, on finding a suitable soil, produce a fresh crop by germination.

Microbes themselves and their spores, as well as the spores of moulds and ferments, are extremely light, and float in the atmosphere amongst the innumerable other particles of matter which reveal their presence to the naked eye by reflecting light, as may be observed when a ray passes through a narrow opening into a darkish room. Elaborate investigations have been made in Europe—chiefly in France—with a view to obtain statistical information as to the hygienic condition of the air of towns, according as it is more or less charged with these minute organisms.

The apparatus used for the purpose is so constructed as to enable the observer to collect, on thin plates of glass coated with glycerine, the particles of dust, &c., floating in the atmosphere, the collecting-plates being transferred, at fixed intervals, to the objective of a microscope in order that the collected matter may be examined. By means of this process calculations have been made in the various places of observation of the number of microbes or their spores ordinarily present in given volumes of air, and it has been found that during the damp weather of the winter months, and that part of the summer in which the increased temperature is accompanied by moisture, they are comparatively few in number, the maximum being reached in the months of April, May, and June, (answering to our months of October, November, and December,) when the summer air is hot and dry, and during cold dry weather in winter, the chief reason for the observed difference in numbers being that during the moister seasons the microbes and their spores settle on the ground, on the leaves of plants, &c., where, however, the pathogenic forms are to the full as dangerous as when floating in the atmosphere. The difference in the number of spores found by means of these observations under favourable and unfavourable conditions was considerable, the number under the former frequently reaching 35,000 in a cubic metre of air, whilst under the latter it did not exceed 7,000. But, whether we take into consideration the higher or the lower number, it is clear that the risk of infection is extremely great, especially in places where diseases originating from the introduction of these organisms into the system are known to exist.

As instances of the extreme danger resulting from such conditions I might cite the case of hospitals in which large numbers of patients formerly died in consequence of erysipelas supervening upon surgical operations, owing to the atmosphere being thickly pervaded with the microbes of that disease, and the case of whole families having been sacrificed, chiefly through ignorance, but too often through that pig-headed disregard of the results of scientific investigation which persons, otherwise intelligent, frequently display in relation to such diseases as phthisis and small-pox. Fortunately, as regards our hospitals, the disastrous results produced by erysipelas are now avoided by the use either of Guerin's protective dressing—adopted in consequence of Tyndall and Pasteur's researches into the nature of air-germs—or, as is more generally the case, of Lister's antiseptic treatment of wounds.

It will have been gathered from the foregoing remarks that the microbes of the various diseases I have already referred to—which, however, by no means exhaust the list of those that owe their origin to the action of pathogenic germs—

must often be disseminated in enormous numbers by currents of air and otherwise, in localities inhabited by persons afflicted with diseases so generated where no special arrangements are made to prevent it; and it must be apparent that we have little reason to wonder, looking to the efficiency of the natural modes of dispersion, and the facilities afforded by the structure of human beings and of herbivorous animals for their introduction into the system, at the occasional rapid spread of the more malignant of these diseases, such as small-pox, cholera, malignant typhus, &c., in places deficient in the possession of proper preventive agencies.

These agencies are of two kinds—first, such as free those who are unavoidably exposed to danger of infection from liability themselves to contract the special disease; and, second, such as are calculated to prevent the general spread of infection in places where the disease is present.

Of the first kind are vaccination, as in the case of small-pox, and inoculation with the virus of other specific diseases, so attenuated by culture as to reduce it to the condition of a vaccine. The methods by which the latter effect is produced are very interesting, and are entirely due to the wonderful perception of Pasteur, who has demonstrated their efficiency in the cases of rabies, fowl-cholera, swine-fever, and others, though no success has yet marked the efforts made in such diseases, as typhoid fever, the glanders of horses, and the infectious pneumonia of horned cattle, owing to the extreme difficulty of attenuating the microbes of these diseases by culture. But success is not despaired of in these cases, and the process may in time be also applied to others of these forms of disease in man. Pending this, however, it is our clear duty to adopt measures of the second class above referred to.

The most important of these are,—first, the isolation of diseased patients, a thing which presents some difficulty in private dwellings; and, second, the immediate exposure to destructive processes of the excreta of the patients,—and when I speak of excreta I use the term in its largest sense.

Everything which has come into or which has been even liable to the chance of contact with such patients should be exposed to some treatment which has been proved to be destructive to microbes and their spores, before uninfected persons are subjected to the risk of contagion or infection, especially where the actual destruction of tainted articles, such as body-linen, bedclothes, &c., is inexpedient or impossible—as, for example, in the instance of persons who cannot afford the sacrifice. All the surfaces of the rooms in which they have lain should be subjected to similar treatment. In order to show the importance of this, Trouessart

cites the investigations of Wood and Formad, two American physicians of the highest standing, in relation to an outbreak of croup in 1881 at Ludington, a small town on the borders of Lake Michigan. The principal industries carried on there were derived from the neighbouring forests, an immense quantity of the trees of which had been sawn into planks in numerous sawpits within the area of the town. The greater portion of the town stands on a height, but one quarter is built on low marshy ground, which has been partly filled up with sawdust. In this quarter the soil is so saturated with moisture that when a small hole is dug it fills with water immediately, and consequently cellars are unknown. It was in this quarter that the epidemic broke out, and was most severe, almost all the children having been attacked by it, and not less than a third of them having actually died. When Wood and Formad began their investigations Formad went to Ludington to study the epidemic and collect materials for experiments. In all the cases of croup he found the blood full of micrococci belonging to Micrococcus diphthericus—some detached, others united in the form of zooglœa—that is, agglutinated in small masses,—and others, again, in the colourless corpuscles of the blood. All the organs, and especially the kidneys, were likewise filled with them. With the materials which he gathered he and Wood made experiments in cultures, and were able to inoculate rabbits with croup. These inoculations were made subcutaneously in the muscles and trachea, and were followed by the production of false membranes, the animals soon dying with all symptoms of diphtheria, and the blood proving to be full of micrococci. An examination of living animals showed that the micrococcus first attacked the colourless corpuscles, within which their vibratile motion could be observed. The corpuscles changed in appearance, the granules disappeared, and each corpuscle became so full of the micrococci that they could no longer move. In fact, the micrococci grew until they caused the rupture of the corpuscle, and then escaped in the form of an irregular mass, which constituted the zooglœa. Corpuscles filled with micrococci were found in the false membrane, in the small vessels—which they dilated and completely obliterated—and even in the marrow of the bones. Cultures made in flasks afforded important results. A comparison of the sowings made with micrococci collected at Ludington with those found in the ordinary diphtheritic angina, (then and still common at Philadelphia,) showed a great difference in the vitality and virulent properties of microbes derived from these two sources. The former multiplied rapidly and energetically, succeeding each other up to the tenth generation; while those from Philadelphia only went to the fourth or fifth generation,