Kitabı oku: «The History of Salt», sayfa 5

CHAPTER VI
EFFECTS ON ANIMAL AND VEGETABLE LIFE

As salt is one of the principal constituents of the blood, and as it is present in the various tissues of the body, and as its ingestion is necessary for the animal economy, for the maintenance of its health, and consequently for the due development of the several organs, and the invigorating effects it exerts over their functional activity, we will now consider it in the relation it holds to animal and vegetable life.

By the great majority of land animals salt is evidently an article much relished, for in those districts where salt springs and lakes are prevalent, many quadrupeds and birds are invariably to be seen.⁵⁰ They frequent these spots in great numbers, and very seldom migrate to those districts which are deficient in salt, or, if they do, very speedily return; these animal instincts are indicative that they are aware of its bracing qualities, and experience the salubriousness of the atmosphere, which naturally is impregnated with a fair amount of salt, which has risen through the media of exhalations from the water or evaporation of the same.

In the Ruminantia the beneficial and, indeed, the salutary action of salt is remarkably observable, for it counteracts in this class of animals the deleterious effects of rainy weather, damp pasturage, and damaged fodder. It also imparts a consistency to the fat, and renders the meat more palatable and wholesome. All cattle, without an exception, thrive best if they are supplied with salt; and they will consume no small quantity. Horses will, on the average, consume daily six ounces; cows, four ounces, and will, it is said, secrete a larger quantity of milk, and of a much richer quality, than those from which salt is usually withheld. Sheep will consume half an ounce daily, and they are not affected with the rot, as is so frequently the case in low-lying marshy districts where they drink water in which there are myriads of the fluke-worm, embryonic and developed, especially after heavy rains or inundations, as, for instance, a river overflowing its banks. It is a fact which farmers and graziers should by no means lose sight of, that these worms are totally destroyed by giving sheep a certain amount of salt during moist and wet seasons, and in those localities which are generally in a state of humidity.

In marine animals common salt is a necessary constituent of their drink, and in fact it is the preserver of their life; but it is injurious, if not certain destruction, to many fresh-water fish, though some live both in the sea and fresh water – as the salmon, sturgeon, and some species of lamprey. The male salmon, on entering the mouths of rivers in order to spawn, follow the females, and fecundate the ova which they have deposited in little pools, or kinds of nests. They, therefore, are hatched in rivers. After the first year they remove to the sea, and, remaining in it for about two months to ten weeks, return to fresh water. Such is the alternate fresh and salt-water life of the salmon, showing us that some fish can live in the sea and breed in fresh water.

Reptiles and animals of an inferior class are deprived of life by the action of salt water; and such organisms as the amœba, hydra, rotifer, and others of a similar grade which we see in stagnant ponds, are speedily killed if put into water in which salt is dissolved; this is also the case with earth-worms, snails, and indeed all insects as a general rule, especially if generated by animal and vegetable decay.

Owing to the antagonism of salt to life produced by putrefaction, it is frequently rubbed into meat to prevent it from being attacked by putrescent larvæ; and even if decomposition has commenced, it arrests for a long time its further progress. We all know what an intense irritant it is to leeches, and how they immediately vomit if some salt is sprinkled upon them when they are engorged with blood.

Land shells are rapidly killed by sea-water, and so are their eggs; this fact has been demonstrated by Darwin, who says: “Their eggs, at least such as I have tried, sink into it and are killed.” From experiments performed by Baron Aucapitaine, we find the above corroborated. He placed in a box, pierced with holes, one hundred land shells belonging to ten different species, and then immersed it in sea-water for a fortnight; only twenty-seven recovered.

These experiments are conclusive, and prove that salt destroys life of an inferior grade, probably owing to the fact that, generally, it is calculated to produce results of a nature somewhat disposed to become an annoyance, or even inimical to the vitalisation of superior organisms, and tends to arrest their progress and due development. We must remember that these two experiments of Mr. Darwin and Baron Aucapitaine were with sea-water, consequently the other salts which it holds in solution (the sulphates of soda and magnesia), and the organic matter which it contains, very probably hastened the progress.

The Batrachians, a class of animals allied to the reptiles, but undergoing a peculiar metamorphosis, have an antipathy to salt, and consequently cannot live in salt water; it is death to them sooner or later.

We cannot say that reptiles, as a rule, frequent fresh water in preference to salt, some being found only in sea-water, and in those parts of the ocean where there is a greater quantity of saline matter than in others. There is the marine Chelonia, for instance, commonly known as turtles (Chelones); one sub-group, the common green turtle, so well known for its palatable qualities, is composed of species altogether herbivorous, and of gregarious and innocent habits, “These animals may be seen in herds at the bottom of the sea, quietly browsing on the weeds growing there. Sometimes they enter the mouths of large rivers, and are occasionally seen to make their way ashore, apparently in search of food.”⁵¹ Like the salmon, it is a habitat of both fresh and sea water, though under different conditions; one frequents fresh-water for food, the other for breeding. Another sub-group comprises turtles of carnivorous habits, active, and, when attacked, fierce; such is the loggerhead turtle and the hawksbill; the latter is the animal which furnishes the arts with the elegant substance called tortoise-shell. There is also a genus of carnivorous habits, called the Sphargis, or coriaceous turtle.

There are likewise the river tortoises (Tryonices), which are conspicuous tenants of the Ganges, the Euphrates, the Niger, the Nile, the Mississippi, and the Ohio. These reptiles are next in size to the turtles, some being three feet long; they are very fierce, and do not even scruple to attack the young alligators. They live principally on fresh-water fish and small reptiles; sometimes they will venture into sea-water in quest of food, though not far, as we may suppose. There are also the Emydes, which are sometimes called fresh-water tortoises, sometimes marsh tortoises, which are of many different species. They haunt lakes, marshes, and small rivers in Asia, Africa, and Australia, but more particularly America, where the proper habitat is represented. In the North American rivers there is found the Emysaura serpentina, which has a large head and crocodilian tail; it feeds on fishes and small birds. Another species, called Chelys fimbriata, or Matamata, belongs exclusively to the rivers of Guiana.

We thus see that the Chelonia, which are remarkable for the box-like case in which most of them are enclosed, are inhabitants of the sea, while their near relations, the tortoises, are only partially aquatic in their habits.

Reptiles are therefore neither land, sea-water, nor fresh-water animals, if we view them as a whole; but if we divide them into orders, we shall be able to see at once which are fresh-water, which are terrestrial, and which are inhabitants proper of the sea. Firstly, there is the Amphibia (doubled-lived), which live and breed in fresh water, such as rivers, lakes, ponds, and ditches, and which are killed if put into salt water. Secondly, there is the Ophidia (snake-like order), which are peculiar to the land, though there is a fresh-water snake in the East Indies, and which the natives will boldly attack with sticks. The Sauria (lizards) next claim our attention. The alligator is a native of North America, and is very abundant in the Mississippi. It is very seldom seen near the mouths of rivers, and in winter it buries itself in the mud, and continues in a torpid state till spring. Then there are the crocodiles, which are natives of Africa, the West Indies, and America. Their habits are somewhat similar to those of the alligator, frequenting the creeks of rivers by night in search of food; they are sometimes seen near the mouths of rivers, but not as a rule. We have already remarked upon the Testudinata, or the turtle kind.

Reptiles, therefore, either frequent the land or the water; some are purely aquatic, others purely terrestrial, the remainder are both; one order is altogether marine, though frequently they are seen on shore, where they are caught.

Salt water is death to one order, but affords the means of life to another; to yet another order, with but few exceptions, both salt and fresh water are deleterious, and, in fact, death; whilst still another order frequents both elements, just as the chances of obtaining food may direct them.

Such animals as the hippopotamus, the rhinoceros, the tapir, and the elephant, and a few others belonging to the Pachydermata, frequent the banks of rivers and fresh-water lakes, where they wallow in the mud, and now and then, as fancy takes them, splash about in the water; but they, like the crocodile, have never been known, as far as I can gather, to make for salt water, and therefore they are seldom, if ever, seen near the mouths of rivers, or by the coast.

Salt is therefore not avoided, almost as a rule, either by animals or birds; and in those districts where salt lakes are situated (to which interesting fact I have already alluded) are to be invariably seen, not only great numbers of animals, but large flocks of birds of different kinds, showing conclusively that they possess an instinctive preference for those localities where the atmosphere is more or less filled with saline matter, than for those places where it is entirely absent. It is but seldom that animals frequent those spots which are injurious to them; they take good care to avoid them, if possible, and if they detect anything deleterious, whether it be in the air, soil, or water, they migrate to more genial quarters; instinct indicates this necessity, and they accordingly act upon it. It is strange that mere animal instinct should be superior to human reason, and that animal sagacity should be more far-seeing than human forethought! Nothing is more strongly confirmative of this anomaly, if I may call it so, than the partiality which animals entertain for those districts which abound with salt lakes, and the antipathy, or utter indifference, with which some people regard that substance which keeps the body pure, healthy, and, I may say, clean, and which plays such a highly-important part in the animal economy.

In the vegetable kingdom salt is by no means an inconsiderable item, and as an agricultural agent it is most invaluable, though its operation therein varies in a remarkable degree; in small quantities it is injurious only to a few plants, while to some it appears to be beneficial in every way. In moderation it is an excellent manure, especially if the soil is of a sandy nature; but in large quantities it is decidedly pernicious to all plants, without an exception, though unequally so. According to experiments made by Dr. Balfour and other eminent botanists, it appears that a solution of the chloride of sodium does not act so deleteriously as solutions of other inorganic substances, and the same effect is observable with a solution of the phosphate of soda: the strength of these solutions, we are told, varied from half a grain to five grains to the ounce of water; the sodium combined with the chlorine forming the chloride of sodium, and with the oxygen forming soda; the potassium, combined with the chlorine, forming the chloride of potassium, and with the oxygen forming potassa. The combinations take place, according to Johnston, in the living plants owing to the natural affinities of these inorganic substances.

Darwin writes: “In botanical works, this or that plant is often stated to be ill-adapted for wide dissemination, but the greater or less facilities for transport across the sea may be said to be almost wholly unknown. Until I tried, with Mr. Berkeley’s aid, a few experiments, it was not even known how far seeds could resist the injurious action of sea-water. To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days. It deserves notice that certain orders were far more injured than others; nine Leguminosæ were tried, and, with one exception, they resisted the salt-water badly; seven species of the allied orders, Hydrophyllaceæ and Polemoniaceæ, were all killed by a month’s immersion. For convenience’ sake, I chiefly tried small seeds, without the capsules or fruit; and as all these sank in a few days, they could not have been floated across wide spaces of the sea, whether or not they were injured by the salt-water. Afterwards I tried some larger fruits, capsules, etc., and some of these floated for a long time. It is well known what a difference there is in the buoyancy of green and seasoned timber; and it occurred to me that floods would often wash into the sea dried plants or branches with seed capsules or fruit attached to them. Hence I was led to dry the stems and branches of 94 plants with ripe fruit, and to place them on sea-water. The majority sank quickly, but some which, whilst green, floated for a very short time, when dried floated much longer; for instance, ripe hazel-nuts sank immediately, but when dried they floated for 90 days, and afterwards when planted germinated; an asparagus-plant with ripe berries floated for 23 days, when dried it floated for 85 days, and the seeds afterwards germinated; the ripe seeds of Helosciadium sank in 2 days, when dried they floated for above 90 days, and afterwards germinated. Altogether, out of the 94 dried plants, 18 floated for above 28 days; and some of the 18 floated for a very much longer period. So that as 64/87 kinds of seeds germinated after an immersion of 28 days; and as 18/94 distinct species with ripe fruit (but not all the same species, as in the foregoing experiment) floated, after being dried, for above 28 days, we may conclude, as far as anything can be inferred from these scanty facts, that the seeds of 14/100 kinds of plants of any country might be floated by sea-currents during 28 days and would retain their power of germination.”

We have thus sufficient evidence before us to prove that salt or sea water does not totally destroy the vitality of seeds when they are in a dry state, that some of them will float for 90 days, and when planted subsequently will germinate; but that when not dry they will sink immediately. We may, therefore, justly conclude from the result of these experiments that salt is not noxious to vegetable life, neither does it destroy the latent principle of procreation which exists in them; and that though the process of germination may be retarded, and kept in a state of abeyance, it is not virtually annihilated, as one would feel inclined to predict, by the prolonged immersion of seeds in salt-water, be they dried or fresh.

Darwin’s experiments were afterwards verified, for he states that subsequently M. Martens tried “similar ones, but in a much better manner, for he placed the seeds in a box in the actual sea, so that they were alternately wet and exposed to the air like really floating plants. He tried 98 seeds, mostly different from mine; but he chose many large fruits and likewise seeds from plants which live near the sea; and this would have favoured both the average length of their flotation, and their resistance to the injurious action of the salt water. On the other hand, he did not previously dry the plants or branches with the fruit; and this, as we have seen, would have caused some of them to have floated much longer. The result was that 18/98 of his seeds of different kinds floated for 42 days, and were then capable of germination. But I do not doubt that plants exposed to the waves would float for a less time than those protected from violent movement as in our experiments. Therefore it would, perhaps, be safe to assume that the seeds of about 10/100 parts of a flora, after having been dried, could be floated across a space of 900 miles in width, and would then germinate. The fact of the larger fruits often floating longer than the small, is interesting; as plants with large seeds or fruit which, as Alph. de Candolle has shown, generally have restricted ranges, could hardly be transported by other means.”

Darwin’s experiments show us that salt or sea water does not entirely extirpate the life which is dormant in seeds, and those of Martens prove that seeds may be immersed in sea-water itself and yet retain the power of germination; and that when dry they may even float for 900 miles, and germinate when planted; developing into plants at the usual period of time allotted by nature!

In Cheshire it is a custom to let out the water of the salt-springs after rain, in order to improve the character of the soil and make it more productive. If we call to mind the preservative properties of salt and the purifying action which it possesses, with regard to animal and vegetable substances, we need not at all be surprised at the above use to which it is put by the agriculturists of Cheshire. The reader, perhaps, would like to know why it is used after rain. After a heavy shower, and more especially in the country, every insect leaves its little secluded habitation: the bee is once more on the wing; the spider resumes his usual central position in his web; flies of all sizes buzz here and there in search of food or for more secure homes; every bush is alive with its usual occupants; the lofty tree is once more the tenement of song; the caterpillar crawls on his solitary way; the ant trudges along on the gravel-path; the snail emerges from his retreat and plods slowly to another home; and the earth-worm raises itself on the lawn; all with one accord hail the reappearance of sunshine, and show signs, however feeble, of joy that the rain-cloud has passed and that the landscape has resumed its beauties, and the sky its gold and azure. The earth after rain, and particularly in spring and summer, teems with almost reanimated life, both with that which is harmless and with that which is hurtful, so that the Cheshire custom is one which cannot be too highly recommended, for when the soil is saturated with moisture, a soluble salt like the chloride of sodium, already in a state of solution, sinks in more rapidly, and permeates it more thoroughly than if it were merely sprinkled over the surface; and such insects as are associated with or which live in the earth are speedily eliminated, or are forced to seek shelter at a greater depth, where they ultimately die by reason of their inability to obtain their proper sustenance or the unsuitableness of their new abode.

There is a plant called Halimodendron which only grows in the dry, naked salt-fields by the river Irtysh, in Siberia; it is a genus of the Leguminosæ, and has purple flowers. Saltwort, or Salsola, (salsus, salt) is chiefly maritime, and the kelp of our shores is principally obtained from it. At one time the carbonate of soda was derived from this kelp or barilla, the ashes being obtained from burning sea-weeds and a species of Salsola; but now it is almost invariably made from common salt, by adding sulphuric acid, and so converting the chloride of sodium into a sulphate, and afterwards, by combustion with chalk and small coal, resolving it into a sulphide, and then into a carbonate. It is manufactured on a very large scale, and is an important staple of commerce. From it is obtained a most important drug, the bicarbonate of soda, the efficacy of which everyone, more or less, has once in a lifetime experienced.

This kelp has been put to a fraudulent use, for Sir Robert Christison tells us that disease has been traced to an impure kind of salt, in which, when investigated, the hydriodate of soda was detected, resulting, he says, from an inferior salt obtained from kelp.⁵²

In all those districts which are intersected by salt marshes, there is almost a complete absence of miasmatic effluvia, though, as a natural consequence, the vegetation is not of that rank luxuriance which is invariably to be seen in other marsh lands; because, whenever the soil is in a state of moisture, it is always covered with all kinds of weeds and useless plants, which altogether stop the growth of those which are of utility to the agriculturist.

In the case of salt marshes it is the reverse, and the neighbourhood is perfectly free from those endemic diseases which are prevalent in such localities as the fen-country, and other similar districts; for the atmosphere is pure, and the soil comparatively dry, and intermittent fever is unknown.