258 



KNOWLEDGE. 



[NOVEMBEE 1, 1898. 



The smell of freshly-turned earth is often regarded by 

 country lovers as one of the panaceas for the ills of the 

 flesh, and " follow a plough-share and you will find health 

 at its tail " has proved a sound piece of advice to many a 

 weakly town-sick one, over whose head the threatenings of 

 consumption hung like the sword of Damocles, though it is 

 possible that it is the fresh air, and more especially the 

 sunshine, which are the saving media, and not the mere 

 smell. 



But what do we know about this characteristic smell of 

 the soil '.' Can we regard it as the mere attribute of the 

 soil as a simple substance, such an attribute as is, for 

 instance, the peculiar smell of leather, or the odour of 

 indiarubber ; or can we go deeper and find that it is really 

 an expression of complexity below '? 



Strangely enough this is the case, for the smell of damp 

 earth is one of the latest signposts we have found which 

 lead us into a world which, until recently, was altogether 

 beyond our ken. It points us to the presence, in the 

 ground beneath us, of large numbers of tiniest organisms, 

 and not merely to their presence only, but to their activity 

 and life, and reveals quite a new phase of this activity. 

 A handful of loose earth picked up in a field by the hedge- 

 row, or from a garden, no longer represents to us a mere 

 conglomeration of particles of inorganic mineral matter, 

 "simply that and nothing more"; we realise now that it 

 is the home of myriads of the smallest possible members 

 of the great kingdom of plants, who are, in particular, 

 members of the fimgus family in that kingdom, plants so 

 excessively minute that their very existence was undreamt 

 of until a feW| years ago. 



Some faint idea of their relative size, and of the numbers 

 in which they inhabit the earth, may be gleaned from the 

 calculations of an Italian, Signor A. Magiora, who, a short 

 time ago, made a study of the question. He took samples 

 of earth from different places round about Turin and ex- 

 amined them carefully. In ordinary cultivated agricultural 

 soil he found there would be eleven millions of these germs 

 in the small quantity of a gramme, a quantity whose small- 

 ness will be appreciated when it is remembered that a 

 thousand grammes only make up about two and a quarter 

 pounds of our English measure. Thus, a shovelful! of 

 earth would be the home of a thousand times eleven 

 millions of bacteria — but the finite mind cannot grasp 

 numbers of such magnitude. In soil taken from the street, 

 and, therefore, presumably more infected with germs, he 

 calculated that there was the incredible number of seventy- 

 eight million bacteria to the gramme. Sandy soil is com- 

 paratively free from them, only about one thousand being 

 discovered in the same amount taken from sandy dunes 

 outside Turin. 



But though the workers were hidden yet their works 

 were known, for what they do is out of all proportion to 

 what they are ; in fact they perform the deeds of giants, 

 not those of veriest dwarfs. " By their works shall ye 

 know them" might be a fitting aphorism to describe the 

 bacteria of the soU. And the nature of their deeds is 

 widely various, for though the different groups are members 

 of one great family, yet, like the individuals of a human 

 family that is well organized, they have each of them their 

 special vocation. In the spring time, when the sun warms 

 the chiUy earth, they act upon the husks that have pro- 

 tected the seeds against the rigours of the winter, and 

 crumble them up so that the seedling is free to grow ; they 

 break down the stony wall of the cherry and plum which 

 has hitherto imprisoned the embryo ; and then, when the 

 young plant starts, they attach themselves to its roots, 

 assist it to take in all sorts of nutriment from air and soil, 

 and thus help it in its fight through life, and when its 



course has run they decently bury it. They turn the green 

 leaves and the woody stem and the dark root back into the 

 very elements from which they were built up ; they effect 

 its decay and putrefaction, and resolve it into earth again. 

 " Dust to dust, ashes to ashes," is the great life work of 

 the earth bacteria. 



But up to the present the fresh smell of the earth, the 

 smell peculiar to it, has not been in any way associated 

 with these energetic organisms, and it is quite a new 

 revelation to find that it is a direct outcome of their 

 activity. Among the many bacteria which inhabit the 

 soil, a new one, hitherto imknown, has been just recently 

 isolated and watched. It lives, as is usual with them, 

 massed into colonies, which have a chalky-white appearance, 

 and as it develops and increases in numbers it manifests 

 itself by the familiar smell of damp earth, hence the name 

 that has been given it — Clailutlirir odorifcra. Taken singly 

 it is a colourless thread-like body, which increases numeri- 

 cally by continuous sub-divisions into two in the direction 

 of its length. It derives its nutriment from substances in 

 the soil, which either are, or have been, touched by the 

 subtle influence of life, and in the processes of growth 

 and development it evolves from these materials a compound 

 whose volatilizing gives the odour in question. This 

 compound has not yet been fully examined ; it is not 

 named, nor have all its properties been satisfactorily 

 elucidated, but two facts concerning it stand out clearly. 

 One is that it is the true origin of the smell that we have 

 hitherto attributed to earth simply ; and the other, that it 

 changes into vapour under the same conditions as water 

 does. Therefore, when the sun, shining after the rain, 

 draws up the water from the earth in vapour form, it 

 draws up, too, the odorous atoms of this newly-found 

 compound, and these atoms, floating in the air, strike on 

 our olfactory nerves, and it is then we exclaim so often, 

 " How fresh the earth smells after the rain." 



Though moisture, to a certain extent, is a necessary 

 condition of the active work of these bacteria, yet the 

 chief reason why the earthy smell should be specially 

 noticeable after the rain is probably because this com- 

 pound has been accumulating in the soil during the wet 

 period. We only smell substances when they are in 

 vapour form, and since the compound under consideration 

 has precisely the same properties in this respect as water, 

 it will only assume gaseous form when the rain ceases. 

 The bacteria have, however, been hard at work all the time, 

 and when the sun shines and "drying" begins, then the 

 accumulated stores commence their transformation into 

 vapour, and the strong smell strikes upon our senses. 

 For the same reason we notice a similar sort of smell, 

 though in a lesser degree, from freshly-turned earth. This 

 is more moist than the earth at the surface, and hence, on 

 exposing it, evaporation immediately begins, which quickly 

 makes itself known to us through our olfactory nerves. 



It may also have been remarked that this particular 

 odour is always stronger after a warm day than after 

 a cold one, and is much more noticeable in summer than 

 in winter. This is because moderate warmth is highly 

 conducive to the greater increase of these organisms, and, 

 in fact, in the summer they are present in far larger 

 numbers and exhibit greater vitality than in the winter, 

 when they are often more or less quiescent. 



Two other characteristics of Cladothrix odorifera are 

 worthy of notice as showing the tenacity with which it 

 clings to life. It is capable of withstanding extremely 

 long periods of drought without injury ; its development 

 may be completely arrested (for water in some degree is a 

 necessity with all living things, from highest to lowest) 

 but its vitality remains latent, and with the advent of 



