Chapter III: Part 3

In this connection I would say that while few of our native birds change the color of their plumage as an adaptation to the seasons, our pretty thistle bird, or American goldfinch, undergoes a radical change. In summer he has a bright yellow body with black markings and a black head, while in winter his plumage is all pale brown or sparrow-color, and we often fail to recognize in our somber winter resident the brilliant goldfinch of our summer. These little birds are gregarious in the winter, and as they fly in small flocks into the trees by the roadside they are frequently mistaken for sparrows, and in fact are usually called tree sparrows.

There are few things connected with the study of natural history more interesting than the tendency in animals to develop conditions suitable to their environment, and it is surprising to see for how long a time an acquired habit will sometimes survive after its usefulness has ceased.

The common chimney swallows always build their nests in chimneys that are unused during their breeding season. They make a semicircular nest of sticks, which they glue to the inside wall of the chimney with a secretion from their mouths. It is interesting to see the swallows gather the sticks for their nests, for they do not alight on the ground, but, while flying, break off dead twigs from trees without stopping in their flight.

This habit of building in chimneys must have been acquired in a comparatively short time, for there were no chimneys in this country before the arrival of the white man, and for a long time afterward the settler had but one chimney in his house, which must have been used, at least for cooking purposes, in the summer. So perfect is this habit that the swallow looks and acts as though he were made for the chimney; his color is a sooty black, so that he does not tarnish his coat by rubbing against the chimney walls; the feathers of his tail end in hard spikes, that he can use them to prop himself against the wall. I have been interested on a summer evening watching these swallows in hundreds circling around a church chimney in Plattsburg, until finally the birds in the center began to enter the chimney, the circle growing smaller and smaller as they apparently poured down in the vortex of a whirlpool of swallows. Many birds have acquired a habit of associating with man, and we rarely find them, except during the season of their flight, far away from houses.

The barn swallows always place their nests under the eaves or cornices of some building, usually a barn. These nests are built of mud gathered by the birds from wet places on the ground, and carried in their mouths to the sites chosen by them. Many of our farmers have an unkind feeling for the barn swallows, as they think the mud-daubed nests on the new red paint are not an artistic addition; but if our cattle could give an intelligent opinion they would welcome the birds, for all swallows are entirely insectivorous, and they must eat many flies and mosquitoes that otherwise would be left to torment our animals.

Birds that build in inaccessible places seem to rely upon that for security, and apparently make little effort to conceal their nests, while those building on or near the ground are generally careful to hide them, and they display considerable cunning in preventing discovery. Robins, for instance, after the young are hatched, never drop the eggshells over the side of the nests to the ground, where they would attract attention and cause one to look directly overhead and thus find the nest, but take the broken shells in their bills and carry them off, dropping them while flying. Frequently birds are very shy and easily frightened away from their nests, but after they are well established they sometimes show a good deal of tenacity in staying by them until the young are ready to leave.

Some years ago we opened an old ore mine, where a pair of phœbe birds had placed their nest on a shelf a few feet overhead, a projecting rock protecting it from the flying stones of the blasts that were fired several times a day, and the men were working so near that they could almost touch it with their hands. These birds did not desert their nests until the young were old enough to leave. The site was not used the following year, as is usually the case with the phœbe bird.

No bird has insinuated himself into our affections more deeply than the bluebird. He charms us as he flits through the air like a painted arrow, reflecting the sunlight from the metallic luster of his wings, while he pours out his inspired song “in notes as sweet as angels’ greetings when they meet.” He comes to us before the unfolding of the first bud of spring, sings to us until our hills and mountains are covered with the richness of their summer verdure, and stays with us until this verdure is changed to all the beauty of its autumnal glory. I am very sorry, but I believe our bluebirds are gradually though steadily decreasing in numbers. Some years ago two pairs nested in our yard, one pair in a hole in an old apple tree and one pair in a box, but for several years these nesting places have been unoccupied, and I know of a number of other former nesting places that have been vacant for years.

Twenty years ago the wild pigeons were quite plentiful in the fall of the year in this part of our State, but each fall they came in decreasing numbers, and for the last four or five years I have not seen a single bird.

There is no sweeter songster than the shy hermit thrush, and I am much pleased in believing that his numbers are increasing. In former years they were not often heard; now, as our spring afternoons decline into twilight, his charming notes come to us from almost every suitable point.

For the first eight or ten years of my residence in Au Sable Forks I did not see a turtle dove, and now I see them nearly every summer.

Our American eagle is occasionally seen in the Adirondacks, and some years ago a large female golden eagle was caught in a steel trap near my home and came into my possession, where she occupied a slatted hencoop, and whenever curiosity led a hen to poke her bill through the slats her head was taken off very quickly. I was afraid that if I kept the eagle I would turn vivisectionist or become too cruel for a hunter, so I presented her to the Zoölogical Gardens in Central Park.

In birds of prey the female is the larger and finer bird, while the reverse is true with other birds; but there is a striking exception in the noble woodcock. No bird is held in higher appreciation by the sportsman, and a female woodcock in full plumage is as rich in coloring and as beautiful in marking as any bird I know. He lies well for the dog, is rare sport for the gunner, and has no equal for the palate. He nests in our alder thickets or on wet marshy ground, and around my home it is the work of a man to get him. He is nocturnal in his habits, feeding at night and pushing his long, slender bill into the soft ground, leaving holes that to the casual eye look like worm holes, but which are easily recognized by one familiar with his habits.

Cow blackbirds are common to this locality during the summers, and they are found in our pastures with the cattle. I have never found their eggs in the nests of other birds, but they are Mormonistic in their habits, one often having as many as a dozen wives, and I have known the crow blackbird to have more than one mate.

Some years ago an article went the rounds of the newspapers telling of a man catching a flock of crows by soaking corn in alcohol and leaving it for the crows to eat, and when they became drunk he caught them. I tried bread crumbs soaked in whisky on English sparrows, but they would not eat them, and I finally got a crow, and though I kept him until he was very hungry I could not get him to eat corn soaked in whisky, and he found no difficulty in picking up every unsoaked kernel and leaving the others. You may draw your own moral, but I am satisfied that the crow will not eat food saturated with alcohol. He is either too uncivilized or too intelligent.

Orioles and other birds sometimes give us much annoyance by eating the green peas from our gardens, and, except in the case of English sparrows, we do not like to shoot them. I once killed a hawk and roughly stuffed it with straw, putting it on a pole near my pea vine, where the birds collected in numbers to scold and peck at it, but they were afraid to touch the peas, and finally left mine for those of my neighbors across the street.

The Acadian owl is a pretty, cunning-looking little bird, not much larger than a robin. He is the smallest of our owls and quite tame, and is not often seen around my home. Some two years ago, while hunting with my brother we saw one of these little birds on the limb of a tree not far from the ground, and we concluded to try and snare him. We cut a long pole and made a slip noose with a shoe string, and while my brother kept the owl’s attention by standing in front of him I slipped the noose over his head from behind. When we had the owl we wanted to tie him, and since we could not spare the shoe string for that purpose, my brother decided to tie him with his watch chain. He snapped the catch around one leg, and while trying to fasten the other leg the owl made a flutter and got loose, and the last we saw of him he was sailing over the tops of the trees with the watch chain hanging to his leg.

I have always taken an interest in birds because I have loved them, but it does not follow that I know much about them. Some one said that the more we know men the less we love them, but that man was an old cynic and doubtless told an untruth. Certain it is that the more we know our native birds the more we love them, and it is one of the encouraging signs of the day that it has become fashionable for young people to take an increasing interest in the birds and wild flowers of their own country, and a young person would hardly be considered accomplished to-day who is entirely ignorant of at least the common names of the flowers that bloom in our fields and woods and the birds that pour out their ecstatic music from our trees and hedges.

* * * * *

HERBERT SPENCER’S work on Education has been translated into
Sanskrit by Mr. H. Soobba Row, who gives as his reason for
publishing a version in an “unspoken” language that the pundits,
for whom the version is primarily intended, “can more easily
appreciate the ideas conveyed in Sanskrit than perhaps in any other
vernacular.”

THE STRUCTURE OF BLIND FISHES.

BY CARL H. EIGENMANN,

PROFESSOR OF ZOÖLOGY, INDIANA UNIVERSITY.

THE COLOR OF THE AMBLYOPSIDÆ.--The three species of Chologaster are colored, with varying intensity, from C. cornutus, which is darkest, to C. Agassizii, in which the color is faintest. The color cells are in all cases arranged in a definite pattern. These are determined by the underlying muscles. The pattern consists of three longitudinal bands on the sides, following the line where the muscle segments are angularly bent, and cross-stripes along the line separating successive segments.

The general color of Typhlichthys is cream and pink. It is abundantly pigmented. In younger specimens the pigment is arranged in more definite areas about the head. In the old it is more uniformly distributed, being, however, specially abundant about the brain. The pigment pattern of the body is precisely as in Chologaster, except that the individual pigment cells are minute and their aggregate not evident except under the lens.

The retention of the color pattern of Chologaster in Typhlichthys is not less interesting than the retention of similar habits. It is perhaps due to different causes. The color pattern in Chologaster is determined by the underlying muscular structure, and the retention of a similar pattern in Typhlichthys is due to the same underlying structure, rather than to the direct hereditary transmission of the color pattern.

Amblyopsis is flesh-colored, ranging to purple in the gill region, where the blood of the gills shows through the overlying structures, and over the liver, which can be seen through the translucent sides and ventral wall. About the head and bases of the fins the color is yellowish, resembling diluted blood. The surface of the body is slightly iridescent, and the surface of the head has a velvety, peach-bloom appearance.

The general pink color of Amblyopsis is due to the blood. It is not due to any abnormal development of blood-vessels in the dermis. In the fins, where the blood-vessels are near the surface, the general effect is a yellowish color. The surface vessels of the dermis also appear yellowish. It is only on account of the translucent condition of all the tissues, permitting the deeper vessels to show through a certain thickness, that the pink effect is produced. Amblyopsis has always been spoken of as white. The term “white aquatic ghosts” of Cope is very apt, for they do appear white in the caves, and their gliding motion has an uncanny effect. All alcoholic specimens are white.

The pigment cells can not be made to show themselves, even by a prolonged stay in the light. The old, if kept in the light, will not become darker, and a young one reared in the light until ten months old not only showed no increase in the pigmentation but lost the pigment it had at birth, taking on the exact pigmentless coloration of the adult. Pigment cells are late in appearing in Amblyopsis. When the young are two months old pigment is abundant. This pigmented condition is evidently a hereditarily transmitted condition. It disappears with age. Primarily this disappearance was probably individual. But, as in the flounder, the depigmentation has also become hereditarily transmitted, for even those individuals reared in the light lose the color. Numerous facts and experiments show that while pigment may be, and is, developed in total darkness, the amount of color in an individual animal depends, other things equal, directly on the amount of light to which it is habitually exposed.

The lower and upper surfaces of the flounder, the one protected and the other exposed to the light, give the most striking example, and the argument is clinched here by the fact, noted by Cunningham, that a flounder whose lower side is for long periods exposed to the light takes on color. Loeb has shown that in the yolk sacs of Fundulus embryos more pigment cells are developed if the embryos are kept in the light than when they are kept in the dark. However, in the body, and especially in the eye, the pigmentation was not affected by the absence of light.

The general absence of color in cave animals is conceded. Packard states, “As regards change of color, we do not recall an exception to the general rule that all cave animals are either colorless or nearly white, or, as in the case of Arachnida and insects, much paler than their out-of-door relatives.” Chilton has made the same observation on the underground animals of New Zealand. Similar observations have been recorded by Lönnberg, Carpenter, Schmeil, and Viré. Hamann enumerates a number of species living both in caves and above ground. In such cases the underground individuals are paler than the others. This confirms similar observations by Packard.

Poulton has mentioned that Proteus becomes darker when exposed to the light. This has been verified by others. Typhlotriton larvæ living at the entrance of a cave are dark, while the adult living farther in the cave are much lighter, but with many chromatophores containing a small amount of color. Epigæan fishes found in caves are always lighter in color than their _confrères_ outside.

We have thus numerous examples of colored epigæan animals bleaching in caves, and also bleached cave animals turning dark when exposed to the light. We have also animals in which the side habitually turned to the dark is colorless, while the side habitually turned to the light is colored. Finally, we have cave animals that are permanently bleached.

Natural selection can not have affected the coloration of the cave forms, for it can be absolutely of no consequence whether a cave species is white or black. It could affect the coloration only indirectly in one of two ways: First, as a matter of economy, but since the _individual_ is in part bleached by the direct effect of the darkness there is no reason why natural selection should come into play at all in reducing the pigment as a matter of economy; second, Romanes has supposed that the color decreases through the selection of correlated structures--a supposition he found scarcely conceivable when the variety of animals showing the bleached condition is considered.

FIG. 4 _a_, _b_, _c_.--Three views of _Amblyopsis_.]

Panmixia can not account for the reduction of the color, since it returns in some species when they are exposed to the light, and disappears to a certain extent in others when kept in the dark. Panmixia, Romanes thinks, may have _helped_ to discharge the color. In many instances the coloration is a protective adaptation, and therefore maintained by selection. Panmixia might in such instances lower the general average to what has been termed the “birth mean.” Proteus is perhaps such an instance. But in this species the bleached condition has not yet been hereditarily established, and since each individual is independently affected “the main cause of change must have been of that direct order which we understand by the term climatic.”

Since, however, the bleached condition, which in the first instance is an individual reaction to the absence of light, has become hereditarily established in Amblyopsis so that the bleaching goes on even when the young are reared in the light, it is evident that in Amblyopsis we have the direct effect of the environment on the individual hereditarily established.

THE EYES OF THE AMBLYOPSIDÆ.--The structure of the eyes has formed the basis of a separate, fully illustrated paper.[A] The prominent features in the eyes of the various species must, however, be known before the question of the origin of these forms and the causes of degeneration can be seriously considered. The eyes of the species of Chologaster are normally formed, possessing a lens, pupil, vitreous body, retina, and optic nerve, and all the eye muscles normal to the fishes. The eyes are functional. The retina is, however, very much simplified. The eye of papilliferus is, in this respect, more perfect than the eye of cornutus. In papilliferus the outer nuclear layer consists of two series of nuclei, the inner layer of about five series of nuclei, and the ganglionic layer of a complete single layer of nuclei except where the optic fibers pass between them, for an optic-fiber layer is not present. In Chologaster cornutus the outer nuclear layer has been reduced to one or two series, and the ganglionic layer to cells widely separated from each other or in rows and little groups, but no longer forming a complete layer. In Amblyopsis and Typhlichthys the largest eyes are not more than one twentieth the diameter of those of Chologaster, or one thousandth of their bulk; the lens is nearly, if not quite, obliterated; the same is true of the vitreous body and the optic nerve in the adult. Beyond this the eyes differ much. In Amblyopsis scleral cartilages are present and prominent, the pigmented layer is prominent, the outer and inner nuclear layers form one layer only, two or three cells deep. In T. subterraneus the pigmented layer is insignificant, and no pigment is ever found in it, while the outer and inner nuclear layers are still separate. In both these species the ganglionic layer forms a central core of cells. In Amblyopsis several or all the eye muscles are present; in Typhlichthys nothing is left of them.

[A] Archiv f. Entwickelungsmechanik, viii, pp. 545-617, Plates
XI-XV.

Scleral cartilages are not present in Chologaster or Typhlichthys; in Troglichthys they are very prominent, sometimes _several times as long as the eye_. While there is no pigment left in Typhlichthys, there is in Troglichthys. The eye in the former is about 0.168 millimetre in diameter, while the entire eye of the latter is but about 0.050 millimetre, or less than one third the diameter, and less than one ninth the bulk.

The entire eye of Troglichthys is smaller than many single cells, and I shall be pardoned for not going into the details of its structure here.

THE TACTILE ORGANS.--The tactile organs are among the most important in the consideration of the blind forms. Their minute structure will form the basis of a separate paper. The prominent tactile organs about the head of Amblyopsis have been mentioned by nearly every writer, and they have been figured by Putnam-Wyman[B] and Leidig,[C] but the figures of the distribution of the ridges are worthless. The description of Professor Forbes[D] of Chologaster papilliferus is the only systematic enumeration of the ridges that has appeared. The accompanying figures, drawn by me with the _camera lucida_, and verified and copied by Mr. U. O. Cox, give the exact extent and position of the ridges in Amblyopsis and Chologaster papilliferus. It will be seen that in the number and distribution of the tactile area the two forms agree very closely, the eyed form having the same number and distribution of ridges or rows that the blind forms have. In Chologaster papilliferus most of the ridges are much less prominent than in the blind species, being sunk into the skin. About the nose and chin, however, the ridges are as prominent as in the other species. In the small Chologaster cornutus there are no distinct ridges at all, the tactile organs being arranged as in other species of fishes. In specimens of the same size the papillæ are not more prominent in papilliferus than in cornutus. It is only in the oldest of papilliferus that the papillæ become prominent. The number of individual papillæ in each tactile ridge differs considerably with age (size), so that an exact comparison between the large Amblyopsis and the much smaller species of Chologaster and Typhlichthys can not be made. From a number of counts made by Professor Cox I take the liberty of giving the following: Ridge No. 6 contains, in Chologaster papilliferus, six organs; in Typhlichthys, eleven; in two specimens of Amblyopsis, respectively eighty-three and one hundred and six inches long, twelve and twenty.

[B] American Naturalist, 1872, Plate II, Figs. 1 and 2.

[C] Untersuchungen z. Anatomie und Histologie d. Tiere, Plate
III, Fig. 28.

[D] American Naturalist, 16, 1882, p. 2.

Aside from the tactile organs in ridges, there are many solitary ones not evident from the surface in Amblyopsis. When the epidermis is removed by maceration, the dermal papillæ on which these rest give the whole head a velvety appearance.

In the young, at least of Amblyopsis, each of the tactile organ of the ridges is provided with a club-shaped filament abruptly pointed near the end. They wave about with the slightest motion in water, and are so numerous as to give the whole head a woolly appearance.

To recapitulate the facts ascertained concerning the eye and tactile organs:

1. The eyes were degenerating and the tactile organs developing beyond the normal before the permanent underground existence began.

2. The eyes continued to degenerate and the tactile organs to increase after permanent entrance to underground waters.

3. In the degeneration of the eye the retina leads; the vitreous body and lens follow; the more passive pigmented layer and sclera remain longest; the bony orbit is not affected.

BEARING OF THE FACTS GAINED ON THE ORIGIN OF THE CAVE FAUNA.--The origin of the cave fauna and of the blind fauna are two distinct questions. This was first recognized by H. Garman. Before, the two questions were considered as one, and two explanations are prominent among those suggesting its solution:

1. The explanation of Lankester seems either a pleasantry or the most unwarranted speculation. He says: “Supposing a number of some species of Arthropod or fish to be swept into a cavern or to be carried from less to greater depths in the sea, those individuals with perfect eyes would follow the glimmer of light, and eventually escape to the outer air or the shallower depths, leaving behind those with imperfect eyes to breed in the dark place. A natural selection would thus be effected. In every succeeding generation this would be the case, and even those with weak but still seeing eyes would in the course of time escape, until only a pure race of eyeless or blind animals would be left in the cavern or deep sea.”

This process does not, of course, account for the degeneration of the eye beyond blindness. But, aside from this objection, the humor of his “glimmer of light” impresses itself very forcibly on one after spending a day in following the devious windings of a living cave, not to mention his tendency in cave animals, which are negatively heliotropic, to follow it. There are other objections. Fishes are annually swept into the caves, but they are not able to establish themselves in them. To do this they must have peculiar habits, special methods of feeding and mating before a successful colonization of caverns can become successful. Further, if the origin of the cave fauna is due to accident, the accident must have happened to four species out of six of the Amblyopsidæ, while none of the numerous other species of fishes about the caves met with the same accident.

2. The second explanation is that of Herbert Spencer:[E] “The existence of these blind cave animals can be accounted for only by supposing their remote ancestors began making excursions into the caves, and, finding it profitable, extended them, generation after generation, farther in, undergoing the required adaptations little by little.”

[E] Popular Science Monthly, vol. xliii, pp. 487, 488.

This second view has been modified by H. Garman in so far as he supposes the adaptations to do without eyes and consequent degeneration of eyes to occur anywhere where a species has no use for eyes, enumerating burrowing animals and parasitic animals, concluding that “the origin of the cave species (nonaquatic especially) of Kentucky were probably already adjusted to a life in the earth before the caves were formed.” In this modified sense, Spencer’s view is directly applicable to the Amblyopsidæ. Hamann goes so far as to suppose that darkness itself is not the primary cause of degeneration, but unknown factors in the animal itself.

The three things to be considered in this connection are (_a_) the habit of the cave form, (_b_) the modifications to enable the form to do without the use of light, and (_c_) the structure of the eye as a result of _a_ and _b_.

_a._ The prime requisite for a candidate to underground existence is a negative reaction to light. We found that even the epigæan Chologaster is negatively heliotropic.

_b._ It must also be evident that a fish depending on its sight to procure its food can never become a cave form. Sunfishes, which are annually carried into caves, belong to this class of fishes. They are always poor when found in the caves, and will never be able to establish themselves in them. On the other hand, there are no reasons why fishes detecting their prey either by smell or touch should not be capable of colonizing caves. The catfishes and Amblyopsidæ belong to the latter class. It is surprising that more catfishes have not established themselves in caves. Among the Amblyopsidæ even those with functional eyes depend on touch and vibrations for their food. Chologaster has well-developed tactile organs and poor eyes. It is found chiefly at the mouths of underground streams, but also in the underground streams themselves. The tactile organs are not different in kind from those of other fishes, and their high development is not more marked than their development in the barbels of the catfishes. The characters which distinguish Chologaster as a fish capable of securing its food in the dark are emphasized in Typhlichthys, and the tactile organs are still more highly developed in Amblyopsis. The eyes of the last two genera are so degenerate that it is needless in this connection to speak of degrees of degeneration. On account of the structure of their eyes and their loss of protective pigment they are incapable of existence in open waters. With the partial and total adaptation to an underground existence in the Amblyopsidæ and their negative reaction to light, it is scarcely possible that for this family the idea of accidental colonization can be entertained for a moment. Their structure is not as much due to their habitat as their habitat is to their structure and habit.

Typhlogobius lives in the holes of shrimps under rocks on the coast of southern California. It is a living example of the origin of blind forms in dark places remote from caves. Here again the “accidental” idea is preposterous, since no fish could by accident be carried into the devious windings of the burrows they inhabit. Moreover, a number of related species of gobies occur in the neighborhood. They live ordinarily in the open, but always retreat into the burrows of crustaceans when disturbed. The origin of the blind species by the gradual change from an occasional burrow seeker to a permanent dweller in the dark and the consequent degeneration of the eye is evident here at once. Among insects the same process and the same results are noted. We have everywhere the connection of diurnal species with dark-loving and blind forms, a transition the result of habit entered into with intent, but no evidence of such a connection as the result of accident. Also numerous instances of daylight species being swept into caves, but no instance of one establishing itself there.

This view accounts also for the wide distribution of the blind fishes. The ancestry of the Amblyopsidæ we may assume to have had a tendency to seek dark places wherever found, and incipient blind forms would thus arise over their entire distribution. The structural differences between Troglichthys and Typhlichthys argue in favor of this, and certainly the fearless, conspicuous blind fish as at present developed would have no chance of surviving in the open water. Their wide distribution after their present characters had been assumed, except through subterranean waters, would be out of the question entirely. The same would not be true of the incipient cave forms when they had reached the stage at present found in Chologaster. It will be recalled that Chologaster, and even the blind forms, have the habit of hiding underneath boards and in the darker sides of an aquarium. These dark-seeking creatures would, on the other hand, be especially well fitted to become distributed in caves throughout their habitat. S. Garman’s able argument for the single origin and dispersal of the blind fishes through epigæan waters was based on the supposition that the cis-Mississippi and trans-Mississippi forms were identical. The differences between these species are such as to warrant not only that they have been independently segregated, but that they are descended from different genera. The external differences between these species are trifling, but this was to be expected in an environment where all the elements that make for external color marking are lacking. The similarity between Typhlichthys and Amblyopsis is so great that the former has been considered to be the young of the latter.

Judging from the structure of the eye and the color of the skin, Troglichthys has been longest established in caves. Amblyopsis came next, and Typhlichthys is a later addition to the blind cave fauna.

* * * * *

“Those,” said Dr. J. N. Langley, in his sectional address on
Physiology at the British Association, “who have occasion to
enter into the depths of what is oddly, if generously, called the
literature of a scientific subject, alone know the difficulty
of emerging with an unsoured disposition. The multitudinous
facts presented by each corner of Nature form in large part the
scientific man’s burden to-day, and restrict him more and more,
willy nilly, to a narrower and narrower specialism. But that is not
the whole of his burden. Much that he is forced to read consists of
records of defective experiments, confused statement of results,
wearisome description of detail, and unnecessarily protracted
discussion of unnecessary hypotheses. The publication of such
matter is a serious injury to the man of science; it absorbs the
scanty funds of his libraries, and steals away his poor hours of
leisure.”

A HUNDRED YEARS OF CHEMISTRY.

BY F. W. CLARKE,

CHIEF CHEMIST, UNITED STATES GEOLOGICAL SURVEY.

[_Concluded._]

It is evident, from what has been already said, that chemistry and physics are near akin--indeed, they can hardly be separated. Avogadro’s law and spectrum analysis are but two illustrations of the relationship, but many other examples are equal to them in importance. Take, for instance, the action of light upon chemical substances; it may provoke union of elements, or effect the decomposition of compounds; upon the latter phenomenon the art of photography depends. That salts of silver are chemically changed by light was the fundamental observation, and upon this fact most photographic processes, though not all, are founded. Thus light, working as a chemist in the laboratory of the photographic plate, has become the useful servant of all arts, all sciences, and all industries--an indispensable aid to invention and research. On this theme a volume might be written; a bare reference to it must be sufficient here.

Still another branch of chemistry, recently developed but essentially an extension of the theory of valence, is also due to the study of optical relations. That different crystalline bodies differ in their behavior toward polarized light has long been known, and the polariscope is recognized as an instrument of great value in chemical research. To the analysis and valuation of sugars and sirups it is most effectively applied, and commercial transactions of great magnitude depend in part upon its testimony. Here is practical utility, but the development of theory is what concerns us now.

The discovery of isomerism, of the fact that very different compounds might contain the same elements united in the same proportions, was easily interpreted by the theory of valence in a fairly complete and satisfactory way. In the structural formulæ the different atomic groupings were clearly shown, but with one essential limitation--the arrangement was in a single plane. That is, the linking of the atoms was considered, but not their relations to tridimensional space. For the study of reactions, for the classification of compounds, the structural symbols sufficed; but human thought is not so easily satisfied, and more was soon required. One class of isomers was unexplained, and an explanation was demanded.

A typical example of the difficulty was offered by tartaric acid, which exists in two forms differing crystallographically and optically. One form, dissolved in water, twisted a ray of polarized light to the left, the other produced a rotation to the right, while the crystals of the two acids, similar in all other respects, also showed a right- and left-handedness in the arrangement of their planes. The crystal of one variety resembled the other as would its reflection in a mirror--the same, but reversed. These differences, discovered by Pasteur as long ago as 1848, the theory of valence could not explain; to interpret them, and other similar cases, the arrangement of the atoms in space had to be considered.

In 1874 two chemists, Van t’Hoff and Lebel, working independently, offered a solution of the problem, and stereochemistry, the chemistry of molecular structure in three dimensions, was founded. They proposed, in effect, to treat the carbon atom essentially as a tetrahedron, the four angles corresponding to the four units of valence or bonds of affinity. They then studied the linking or union of such tetrahedra, and found that with their aid the formulæ for tartaric acid could be developed in different ways, showing right- and left-handed atomic groupings. Other similar compounds were equally explicable. Thus the definite conception of a tridimensional, geometric atom led to a new development of structural formulæ, from which many discoveries have already proceeded. The fruitfulness of the speculation vindicates its use, but it is only the first step in a method of research which must in time be applied to all of the chemical elements. Probably the study of crystalline form will be connected with these chemico-structural expressions, and from the union some greater generalizations will be born. From the geometry of the crystal to the geometry of the molecule there must be some legitimate transition. With all their utility, our present conceptions of chemical structure are incomplete; they represent only portions or special phases of some great general law, but so far as they go, properly used, they are valid.

But light is not the only physical force involved in chemical changes; heat and electricity are far more important. Heat, in particular, is essential to every chemical operation; it provokes combination and effects decomposition; it appears in one reaction and vanishes in another; apart from thermal phenomena the science of chemistry could not exist. From the very beginnings of chemistry this interdependence has been recognized, and its study has led to notable discoveries and to great enlargements of resource. In the theory of phlogiston the connection between heat and chemical change was crudely stated, and when Lavoisier saw that combustion was oxidation, thermochemistry began to exist.

In every chemical change a definite amount of heat is either liberated or absorbed--a distinct, measurable quantity. This fact was established by Hess in 1840, and since then the thermal values of many reactions have been determined, notably by Thomsen in Denmark and Berthelot in France. The data are already numerous, but as yet they have not been co-ordinated into any general law. They are in great measure the raw material with which some future scholar is to build. One fact, however, is already clear--namely, that the heat of formation or of combustion of any compound is conditioned by its structure. Two isomeric substances may differ widely in their calorific constants, an observation which has repeatedly been verified. Thus the conception of structure, of atomic grouping, appears again a chief factor in a set of unsolved problems.

In the relations of chemistry to heat perhaps the greatest advances have been made in the extension of our resources, particularly in regard to the development and control of temperatures. At the beginning of the century the range of temperatures available to the chemist was narrowly limited--from the freezing point of mercury at one end to the heat of a blast furnace at the other. His command of heat and cold are now vastly greater than then, and the steps which have been taken are worth tracing.

At the lower end of the scale the greatest progress has been made through the liquefaction of gases. When a liquid evaporates, heat is absorbed, or, reversely stated, cold is produced, and the more rapid the evaporation the greater is the cooling effect. A command of more volatile liquids is therefore a command of cold, and the liquefied gases represent the extreme limit of our power in that direction.

Near the beginning of the century, by combining cold and pressure, sulphurous acid and chlorine were reduced to the liquid state. In 1823 Faraday succeeded in liquefying still other gases, and in 1835 Thilorier went even further and reduced carbonic acid to a snowlike solid. Liquid chlorine, sulphurous acid, and carbonic acid, stored in strong cylinders of steel, are now commercial products, manufactured and sold in large quantities like any other merchandise. They can be transported to long distances and kept indefinitely, to the great convenience of chemists and the furtherance of research.

In 1845 Faraday published the results of further investigations, when it appeared that all but six of the known gases had been reduced to the liquid state. Through cold and pressure lower and lower temperatures were gained, each step forward having given a foothold from which a new advance was possible. In 1877 Pictet and Cailletet simultaneously succeeded in liquefying four of the supposedly permanent gases; nitrogen yielded to the attacks of Wroblevsky and Olzewski in 1883, and hydrogen alone remained unconquered. In 1898 Dewar overcame this last obstacle, and in the year following he actually reduced hydrogen to an icelike solid which melts at only eighteen degrees centigrade above the theoretical absolute zero. Every gas has been liquefied, and probably the lowest degree of cold attainable by man has been reached. Within the century the work began and ended; the future can only improve the working methods and utilize the new resources. Liquid ammonia has long been used in the manufacture of artificial ice and for direct refrigeration; liquid air, with its temperature of two hundred centigrade degrees below zero, is now almost a commercial product, obtainable in quantity, but with its possibilities of usefulness as yet practically undeveloped. The infant Hercules will doubtless find no lack of tasks to do, each one more arduous and more helpful to man than any labor of his mythical prototype.

To the chemist the possibilities thus opened are innumerable. Pictet has shown that at the low temperatures which are now easily attainable all chemical action stops, even the most energetic substances lying in contact with each other quiet and inert. A greater control of the more violent chemical reactions is therefore within reach, doubtless to be utilized in many ways yet unforeseen. At the highest temperatures, also, chemical union ceases, and compounds are decomposed; each reaction is possible only within a limited thermal range, of which the beginning and the end are measurable. From future measurements of this sort new laws will surely be discovered.

The first step in the upward scale of temperatures was taken in 1802, when Robert Hare, an American, invented the oxyhydrogen blowpipe. With this instrument platinum, hitherto infusible, was melted, a result of great importance to chemists. Apart from recent electrical applications, platinum finds its chief use in the construction of chemical apparatus, and Hare’s invention was therefore of great assistance to chemical research. Later in the century electrical currents were utilized as producers of great heat, until in the very modern device of the electric furnace the range of available temperatures has been at least doubled. Temperatures of three to four thousand degrees of the centigrade scale are now at the disposal of the chemist, and these are manageable in compact apparatus at very moderate cost. Cheap aluminum is one of the products of this new instrument, and the extraordinary abrasive substance, carborundum, is another. New industries have been created by the electric furnace, and in the hands of Moissan it has yielded scientific results of great interest and remarkable variety. The rarest metals can now be separated from their oxides with perfect ease, and new compounds, obtainable in no other way, the furnace has placed at our disposal. This field of research is now barely opened; from it the twentieth century should gather a rich harvest.

With electricity also chemistry is nearly allied, and along some lines the two branches of science have been curiously intertwined. Like the other physical forces, electricity may either provoke or undo combination, and, like heat, it may itself be generated as a product of chemical action. The voltaic pile and the galvanic battery owe their currents to chemical change, and it is only since the middle of the century that any other source of electric energy has become available for practical purposes. It is not surprising, therefore, that many thinkers should have sought to identify chemical and electric force, the two have so much in common. It was with the galvanic current that Davy decomposed the alkalies, and since his day other electro-chemical decompositions have been studied in great number, to the development of important industries. To the action of the current upon metallic solutions we owe the electrotype and all our processes for electroplating, and these represent only the beginnings of usefulness. Even now, almost daily, advances are being made in the practical applications of electrolysis, and the forward movement is likely to continue throughout the coming century. From the curiously reversible chemical reactions of the secondary battery the automobile derives its power, and here again we find a field for invention so large that its limits are beyond our sight. From every peak that science can scale new ranges come into view. The solution of one problem always creates another, and this fact gives to scientific investigation its chief interest. We gain, only to see that more gain is possible; the opportunity for advance is infinite. Forever and ever thought can reach out into the unknown, and never need to weep because there are no more worlds to conquer.

It was the study of electro-chemical changes which led Berzelius to his electro-chemical theory of combination, and then to the dualistic theory, which has already been mentioned. In or about the year 1832, when the Berzelian doctrines were at the summit of their fame, Faraday showed that the chemical power of a current was directly proportioned to the quantity of electricity which passed, and this led him to believe that chemical affinity and electric energy were identical. Electrolysis, the electrical decomposition of compounds in solution, was a special object of his attention, and by quantitative methods he found that the changes produced could be stated in terms of chemical equivalents or combining numbers. One equivalent weight of zinc consumed in the galvanic battery yields a current which will deposit one equivalent of silver from its solution, or which, decomposing water, will liberate one equivalent each of oxygen and hydrogen. All electro-chemical changes followed this simple law, which gave new emphasis to the atomic theory, and furnished a new means for measuring the combining numbers.

In the early days of electro-chemistry the products of electrolysis were studied in the light of the dualistic theory. But as chemical investigation along other lines overthrew this hypothesis, a closer examination of electrolytic reactions became necessary. Electrical decompositions were dualistic in character, but the dualism was not that taught by Berzelius. When a salt, dissolved in water, is decomposed by the current it is separated into two parts, which Faraday called its _ions_; in Berzelian terms these were in most cases oxides, but this conclusion fitted only a part of the facts, and finally was abandoned. Whatever the _ions_ might be, they were not ordinary oxides.

Many and long were the investigations bearing upon this subject before a satisfactory settlement was reached. The phenomena observed in solutions, raised still another question, that of the nature of solution itself, and this is not yet fully answered. Two lines of study, however, have converged, within recent years, to some remarkable conclusions, the latest large development of chemical theory.

It has long been known that solutions of salts do not freeze so easily as pure water, and also that their boiling points are higher. In 1883 Raoult discovered a remarkable relation between the freezing point of a solution and the molecular weight of the substance dissolved, a relation which has since been elaborately studied by many investigators. From either the freezing-point depression or the elevation of the boiling point the molecular weight of a soluble compound can now be calculated, and many uncertain molecular weights have thus been determined.

Another phenomenon connected with solutions, which has received much attention, is that of osmotic pressure. A salt in solution exercises a definite pressure, quantitatively measurable, which is curiously analogous to the pressure exerted by gases. In a gas the molecules are widely separated, and move about with much freedom. In a very dilute solution the molecules of a salt are similarly separated, and are also comparatively free to move. The kinetic theory of gases, therefore, is now paralleled by a kinetic theory of solutions, founded by Van t’Hoff in 1887, which is now generally accepted. All the well-established laws connecting pressure, temperature, and volume among gases find their equivalents in the phenomena exhibited by solutions. In Avogadro’s law we learn that equal volumes of gases, under like conditions of temperature and pressure, contain equal numbers of molecules. According to the new generalizations, equal volumes of different solutions, _if they exert the same osmotic pressure_, also contain equal numbers of molecules. The parallelism is perfect. With these relations the freezing- and boiling-point phenomena are directly connected.

But, both for gases and for solutions some apparent anomalies existed. Certain compounds, when vaporized, seemed not to conform to Avogadro’s law, and called for explanation. This proved to be simple, and was supplied by the fact that the anomalous compound, as such, did not exist as vapor, but was split up, _dissociated_, into other things. For instance, ammonium chloride, above a certain temperature, is decomposed into a mixture of two gases--hydrochloric acid and ammonia--which, on cooling, reunite and reproduce the original compound. Twice as much vapor as is required by theory, and specifically half as heavy, is produced by this transformation, which is only one of a large class, all well understood.

In the case of solutions it was found that certain compounds, notably the acids, alkalies, and metallic salts, caused a depression of freezing point which was twice as great as ought to be expected. This fact was illuminated by the phenomena observed in gases, and soon it was seen that here too a splitting up of molecules, a true dissociation, occurred. These anomalous solutions, moreover, were electrolytes--that is, they conducted electricity and underwent electrolytic decompositions--while normal substances, especially solutions of carbon compounds, such as sugar, were not.

Van t’Hoff’s discoveries went far, but one more step was needed, and this was taken by Arrhenius in 1888. Electrolytic compounds, when dissolved, are actually dissociated into their _ions_, partially so in a strong solution, entirely so in one which is infinitely dilute, a statement which leads to some extraordinary conclusions. For instance, the _ions_ of common salt are sodium and chlorine. In a dilute solution the salt itself ceases to exist, while atoms of sodium and atoms of chlorine wander about, chemically separated from each other but still in equilibrium. Sodium sulphate may be regarded as made up of two parts--sodium and an acid radicle which contains one atom of sulphur to four of oxygen--and these parts, its _ions_, are severed apart during solution to move about independent of each other.

This theory of Arrhenius, the theory of electrolytic dissociation, is supported by many facts, and fits in well with the kinetic theory of Van t’Hoff. Electrolysis is no longer to be considered as a separating process, but rather as a sorting of the _ions_, which receive different electrical charges and concentrate at the two electrical poles. The phenomena of freezing and boiling points in solutions, and of the absorption of heat when solid salts are dissolved, all harmonize with the conclusions which have been reached. A complete theory of solutions is yet to be proposed; but these new doctrines, which are true so far as they go, represent a long step in the right direction. A final theory will include them, but they are not likely to be set aside.

As we near the end of the century we find one more discovery to note, from a most unexpected quarter--the discovery of new gases in the atmosphere. In 1893 Lord Rayleigh was at work upon new determinations of density, with regard to the more important gases. In the case of nitrogen an anomaly appeared: nitrogen obtained from the atmosphere was found to be very slightly heavier than that prepared from chemical sources, but the difference was so slight that it might almost have been ignored. To Rayleigh, however, such a procedure was inadmissible, and he sought for an explanation of his results. Joining forces with Ramsay, the observed discrepancies were hunted down, and in 1894 the discovery of argon was announced. Ramsay soon found in certain rare minerals another new gas--helium--whose spectral lines had previously been noted in the spectrum of the sun; and still later, working with liquid air, he discovered four more of these strange elements--krypton, xenon, neon, and metargon. By extreme accuracy of measurement this chain of discovery was started, and, as some one has aptly said, it represents the triumph of the third decimal. A noble dissatisfaction with merely approximate data was the motive which initiated the work.