AMONG the sciences astronomy might formerly have been regarded as the one which most strikingly appealed to the imagination of the public; it may be doubted, however, whether astronomy is still first favorite. It has, at any rate, a formidable rival in bacteriology -the science of that infinitely minute life which, as recent research has shown, is everywhere so abundant. Astronomy, it is true, may impress our minds in a more profound manner by the conception it presents of the vastness of the universe; yet the marvels and mysteries of the micro-organic life of our globe are certainly little less impressive. That in an area not larger than a penny-piece we may have a minute world as densely populated as Europe itself, with its three hundred and fifty millions, is surely no less calculated to excite our wonder than the conception of the enormous dimensions. of those vast worlds, so far removed from our planet, which it is the province of astronomy to describe.

The extreme minuteness of bacteria, their ubiquity, the rapidity with which they reproduce themselves, the enormous importance of the functions they perform, and their rôle as propagators of many of the deadly diseases which afflict humanity, all serve to invest them with the deepest interest. There is, for the human mind, an intense fascination in the study of these "invisible friends and foes," which are present, in their teeming millions, in the air we breathe, the food we eat, the water we drink, and in the soil beneath our feet; and on whose action our comfort, our health, and even our very existence itself may be said to depend.

The strides which our knowledge of bacteriology has made within recent years are well known to all.

Not

merely has the ubiquitous microbe been shown to be a potent agent in the propagation of disease; it is being demonstrated, more and more every year, to

be implicated in many industrial processes of the most diverse nature. Bacteriology has already done much to revolutionize not a few of our large industries, and it bids fair to revolutionize many more. Among processes in which microbes play an important part may be mentioned the fermentative industries, so widespread in extent and involving such an enormous amount of capital. Any one who has even the most elementary knowledge of brewing knows of what assistance a knowledge of bacteriology has proved to the brewer. Alcohol, in whatever form it occurs, is the product of minute life; hence the light which the study of the nature and habits of alcohol-producing microbes is able to throw on its manufacture is great. Again, such a widely used article as vinegar is another product of microbic life; while in such industries as tobacco, linen, jute, hemp, leather, citric acid, opium, indigo, and many others, bacterial life is more or less implicated. There are also certain processes in Nature-of such importance that the continuity of vegetable and animal life may be said to be dependent on them-that are caused by the agency of germ life. Such are the processes of the decomposition and. putrefaction of organic matter. Few people, probably, ever reflect on the significance of such processes in Nature's economy, or realize that these processes are the chief agencies at work in effecting that vast circulation of matter which is constantly going on. To grasp the true significance of this great law, it must be remembered that the total amount of matter on the earth's surface available for the formation of fresh animal and vegetable life is limited. Modern science has taught us that matter is not destroyed; all that can happen to it is change of form. It goes through a great variety of changes, it is true, but the sum total always remains the same. In effecting this great

cycle of change bacteria are every day being recognized to a greater extent as the most potent agents.

It is only just, therefore, to the much-maligned microbe to emphasize this highly beneficent rôle it performs, since in the popular mind it is chiefly associated with disease. Such a view is really most unfair to the useful microbe, which, after all, is ten thousand times more abundant than his pathogenic (or disease-producing) brother. Even with regard to diseaseproducing microbes it must be remembered that while they produce disease they have also furnished man, in the so-called anti-toxines they give rise to, with a means of defence against the very diseases they cause.

In this paper we purpose to attempt to show briefly how beneficent their action is in the greatest as well as the oldest of all human industries, agriculture. The article has been suggested by the announcement, which has recently been made, of a highly interesting development of agricultural bacteriology-viz., a further application of the practice of soil inoculation -a practice which was first introduced. some two years ago.

Before, however, describing this latest development of agricultural bacteriology, reference may be made to some of the functions which the microbe has already been shown to perform in agriculture.

In the first place, it must be stated that the action of bacteria in agriculture is chiefly, if not almost wholly, beneficent. No doubt there are certain terrible diseases, affecting the live stock of the farm, which are due to germ life; but, on the other hand, the important services which they render in other departments of farmingmore especially in what may be termed agriculture proper, i.e. the tillage of the soil and the growth of crops-must be held to far outweigh their inimical action.

Till recently the soil was wont to be regarded as exclusively composed of dead, inert matter; now we know better. Much of it, it is true, is composed of such matter; yet so intimately and

so inextricably is this inert matter permeated with microbic life that the soil can no longer be regarded as so much dead material. The truth of this may be illustrated by the statement that the abundance of this micro-organic life is such that it has to be reckoned in hundreds of millions per ounce of soil. What the various functions of this teeming microscopic life are we do not fully know as yet. Many of these minute denizens of the soil, however, have been proved to be the active agents in the putrefaction and decomposition of that organic matter which all soils contain in greater or less abundance, and for the original formation of which bacteria, as it has recently been shown, are probably responsible. To them the fertility of the soil is originally due: on their continued action the maintenance of that fertility also depends, since they elaborate, in a variety of ways, the food materials of the plant, converting it into forms suitable for assimilation.

As we have said, the soil is, in a sense, the product of their action through long ages. Till recently it was believed that a necessary condition of their development was the presence of organic matter. According, however, to an interesting discovery, made within the last few years, certain soil organisms, it has been shown, are able to subsist on a purely mineral diet. This latter class are of very wide occurrence, and are found even on bare rock surfaces.

The microbic inhabitants of the soil may be divided into different classes according to the nature of the products they give rise to. While many of them exercise an oxidizing action-some actually producing oxygen itself-by assimilating the organic matter and giving rise to carbonic acid and water, others exercise a reducing action. The former are beneficial to the processes of agriculture; the influence of the latter is distinctly inimical, as they cause a loss to the soil of that highly valuable fertilizing ingredient, nitrogen. We have, however, every reason to believe that the influence of this latter class is more than neutralized by the work

of a recently discovered type of microbe, whose function it is to enrich the soil by "fixing"-as it is technically called the free nitrogen gas in the air, and thus render it available for the requirements of the plant.

Although we know as yet but little of the exact methods in which the decomposition of the material of the soil is effected, we have reason to believe that it is due to a highly interesting system of co-operation on the part of these minute soil workers. While some are instrumental in initiating the first stages of decomposition, others carry on its development through successive stages.

The abundance of this minute life is, as has already been indicated, very great. It is, however, almost entirely limited to the surface portion of the soil, and its occurrence in abundance may be said to be confined to the first eighteen inches of soil, although bacteria have been found at much greater depths than that indicated. There are other factors which influence the development of microbic life in the soil, among them being its temperature, the amount of its moisture, and its physical and chemical condition.

A short description of some of these bacteria may now be given.

Those most abundant belong to the first class, viz. those exercising an oxidizing influence, and which give rise to such products as carbonic acid and water. The functions which such bacteria perform are very important, and their influence on plant growth cannot be well over-estimated. Carbonic acid is the most important solvent of the mineral matter of the soil, and by its help the roots of plants are enabled to absorb their food in suitable form. Owing to a variety of considerations, which it is impossible here to enter into, the supply of what is termed "available" (i.e. for the plant's needs) nitrogen in a soil is the factor which most largely determines its fertility; accordingly it is the organisms which have to do with the elaboration or fixation of this highly valuable plant food that possess most interest for the agriculturist.

Among these nitrogen bacteria a large and important class are implicated in the process known as nitrification.

Nitrogen occurs in the soil in different forms. It has for a number of years been held that it is only, however, when it is converted into nitric acid or, more strictly speaking, nitrates that it is available as a food for green-leaved plants. As by far the largest proportion of it exists in other forms, it is obvious that a process of conversion of these other forms into nitrates must go on in the soil. This process is known as nitrification. For long it was believed to be a purely chemical act; but in the year 1877 the important discovery was made that it was really the result of micro-organic action. Since then it has been made the subject of much research, with the result that we now know that the process takes place in several stages. Nitrogen in the organic form is first converted into ammonia compounds; these, in their turn, are converted into nitrites, a less highly oxidized form; while these last are finally converted into nitrates. All three stages in the process are effected by separate classes of bacteria, their joint action furnishing an excellent example of the principle of co-operation.

That such a process as nitrification takes place has long been known; indeed, considerable information had been collected regarding the conditions. favorable to its development long before its connection with organic life had been surmised. The process had been used in the manufacture of that important constituent of gunpowder saltpetre.* Since, however, the discovery of its true nature has been made, the conditions favorable to its development have been studied much more closely; and as the outcome of this re

*The most striking example of nitrification on a large scale is furnished by the Nitrate Fielos of Chili, which consist of gigantic deposits of impure nitrate of soda (caliché). The discovery of the true nature of the process of nitrification has thrown an interesting light on the question of the origin of these deposits. The present writer has discussed the question in Blackwood's Magazine, March, 1892.

search great light has been thrown on the value of tillage operations in promoting soil fertility.

The nitrification bacteria belong to the oxidizing class. The first stage is effected by bacteria which are abundant in the air, in rain-water, and in the surface of the soil, and which flourish best at temperatures between 80 and 90 degrees Fahr. "To spherical corpuscles the larger of which barely reach a diameter the thousandth of a millimetre, and the smaller ones being so minute as to be hardly discernible in photographs, although shown there with a surface one million times greater than their own"-is due the second stage in the process, the conversion of ammonia into nitrites. The third and final stage is effected by ferments on an average four times as minute as those effecting the second stage. A peculiar interest attaches to the two last-mentioned classes of bacteria-which have been respectively called nitrosomonas and nitrobaeter-inasmuch as they differ from all hitherto discovered bacteria. For the purposes of isolating them it was found necessary to cultivate them in a purely mineral medium. This statement derives its significance from the fact that it is subversive of what has hitherto been regarded as a fundamental law of vegetable physiology, viz. that the power of deriving carbon from a purely mineral source is alone possessed by greenleaved plants.

Space does not permit of the description of the various conditions which influence this interesting and, from the economic point of view, highly important process. It must suffice here to say that temperature and moisture are among the most important. The process takes place most rapidly in warm weather, a fact which may be held to explain partly the superior fertility of the soil in tropical countries. Rarely in such a climate as our own are the heat conditions at their maximum favorable point. Whenever the temperature approaches freezing-point the process ceases. Moisture is also a most important factor-the absence of water in the soil or an excess of it being equally

unfavorable. The limitation of their occurrence to the superficial layers of the soil is due to the fact that the bacteria effecting the process require for their abundant development a plentiful supply of air. It is on this account that they are not found in waterlogged soils. Their susceptibility to poisonous substances, such as certain compounds of iron (compounds which are apt to be produced when the soil is not properly aerated), and to sulphur compounds, serves to explain more clearly than was previously realized the inimical action of such a body as gas-lime.

And here a very interesting practical question presents itself. Since the fertility of a soil may be said to depend, to no small extent, on the abundance of these nitrifying ferments, is it possible, it may be asked, in the case of a soil which from some cause or another may have had its valuable microbic life killed out, to re-seed the soil? To this it may be answered that numerous experiments have demonstrated in a striking manner the value of inoculating a sterile soil with nitrifying bacteria. This has been effected by strewing over the soil materialsuch as an old garden soil-rich in nitrifying ferments. Indeed, there can be little doubt that the value of farmyard manure, to a certain extent, may be ascribed to the fact that it supplies the soil with abundance of such organisms. But the principle of soil inoculation has been more systematically worked out in the case of other classes of organisms, viz. those which fix the free nitrogen of the air.

The discovery of this type of microbic life in certain excrescences or nodules on the roots of leguminous plants, such as peas, beans, etc., was made in the year 1886; and it has since been discovered that quite a number of different kinds of organisms are implicated in the process. Indeed, it seems highly probable that each different kind of leguminous plant has its own special kind of ferment. These invade the roots from the soil, giving rise to the formation of nodules, where they multiply with great rapidity and

stimulate the growth of the plant cell. Three stages in the process may be defined. The first is that during which the bacteria live as parasites at the expense of the plant cell. Gradually, however, the struggle for existence becoming very intense, they are converted into a passive state, and the cells are filled with bacterium-like bodies. The plant then absorbs the contents of the nodules. We know comparatively little, as yet, of the exact mode in which the nitrogen is fixed. That the process is the result of the joint action of the bacterium and the plant, and is an example of what is technically known as symbiosis, is, however, clear.

We have said that the significance of this discovery is great. For one thing, it points to a very important method of economically enriching our soils with nitrogen. It has also thrown great light on the reason of the beneficial results of certain practices long in vogue among agriculturists, such as the rotation of crops, and more especially the long-observed extraordinary capacity of certain leguminous crops, such as clover, for obtaining nitrogen-a fact which had been noticed as early as the time of the Romans. But what, from the economical point of view, is even more important is, that it suggests to the future agriculturist a mode of enriching the soil in nitrogen which possesses many advantages over the present custom of using expensive nitrogenous fertilizers. This consists in the inoculation of the soil with pure cultures of nitrogen-fixing bacteria. Already two important steps have been taken in the development of this methor of inoculation on a practical scale. Two years ago Professors Nobbe of Tharand, a distinguished authority on plant physiology, as the result of lengthened researches on the subject, took out a patent for the preparation of pure cultures of the different nitrogen-fixing bacteria of leguminous crops; and such pure cultures, known under the name of nitragin, have been during that period manufactured on a commercial scale by a large German chemical firm. These pure cultures are sent out in little vials, each vial containing suf

ficient of the pure culture for the inoculation of an acre of soil. Evidence is yet wanting to show how far such inoculation on a practical scale has been accompanied with success. Many small experiments, however, have demonstrated its value in a striking man

ner.

Within the last month or two a further development has been witnessed in the introduction by Herr Caron of Ellenbach, a German landed. proprietor, of another bacterial culture in similar form and prepared by the same German firm. This new preparation is known as alinite, and is designed for inoculating the soil with another class of nitrogen-fixing bacteria, and is recommended for use in connection with the other great class of agricultural crops, viz. grain crops; so that we have now pure cultures suitable for use in the case of all the common agricultural crops. From researches carried

out by Dr. Stocklasa of Prague, it would appear that alinite consists of a pure culture of the bacillus megatherium.

The mode in which these pure cultures are applied to the soil is simplicity itself. It consists either in inoculating the seed of the crop, which is to be sown, with the culture by immersing it in a watery solution of the culture; or in inoculating the soil, which may be most conveniently done by mixing a quantity of sifted dry soil with the pure culture and spreading this over the fields. When we reflect that in a vial barely a couple of inches in length and less than a quarter of an inch in diameter there may be contained the means of enriching an acre of ground in its most valuable of all fertilizing constituents we realize the great advantage such a process possesses over the more costly and troublesome mode of strewing large quantities of artificial manure. It must not, however, be concluded that this interesting application of bacteriological methods in agricultural practice is beyond its experimental stage. The evidence in support of the practical value of alinite is almost nil, while that in favor of nitragin is still of a very