Although these gases are perfectly permeable to all the rays of the luminous spectrum, to those of the obscure heat which was employed in these experiments they exhibit a great difference of absorbing power, nitrous oxide gas absorbing 355 times and ammonia 1,195 times as much as dry air. If the tube had been closed with partially athermanous glass instead of diathermanous rock salt, no such results could have been obtained, as the glass would have sifted out nearly all the rays of low refrangibility before they fell upon the gases whose powers of absorption were the subject of experiment. The investigations which have been made upon the subject of diathermancy have been of great advantage in arriving at theoretical conclusions in regard to the molecular constitution of matter. In undertaking to explain why radiant heat of low refrangibility passes so much more readily through elementary than through compound gases, the mind is obliged to form conceptions of the different conditions in which the atoms are arranged in these two classes of matter. In the elementary gas they must be so disposed as to allow the waves of heat to vibrate freely without accepting their vibrations, while in the compound gas they must be so arranged as to receive or unite with them, or, in common language, to absorb them. In one case, therefore, the mind conceives of the atoms as swinging in the ether singly, receiving but little motion from its vibrations; while in the other they are grouped together in compound masses or molecules, which offer more obstruction to the ethereal waves, and therefore transfer to themselves a corresponding degree of energy. Tyndall found the body ozone to be highly athermanous, a quality which greatly distinguishes it from common oxygen. It has been held that ozone is a compound of oxygen and hydrogen. Now, heat destroys ozone, leaving oxygen; but if it also

contains hydrogen, some aqueous vapor must also result from the disorganization of the ozone. This vapor remaining in the oxygen would impair its diathermancy. But the gas obtained by heating ozone is as diathermanous as oxygen obtained in the ordinary way; therefore it contains no aqueous vapor, and the ozone from which it was derived must be simply oxygen, with its atoms grouped together somewhat after the manner of a com

pound gas. The diathermancy of iodine to the obscure, and its opacity to the luminous rays, allows the visible to be divided from the invisible spectrum which lies beyond the red rays, by passing the light of an incandescent body through a hollow prism of rock salt containing a solution of iodine in bisulphide of carbon. The conclusion to be arrived at from a consideration of this fact is, in the opinion of Tyndall, that the luminous waves which are intercepted by the iodine are in accord with its dissolved atoms, and therefore can transfer their motion or energy to them. Transparency and diathermancy he therefore considers as synonymous with discord, and opacity and athermancy as synonymous with accord, between the waves of ether and those of the molecules of the body on which they fall, or through which they pass. The blackness of lampblack he ascribes to the accord between the vibrations of its atoms and the waves embraced within the luminous portion of the spectrum; and the luminous rays which it absorbs are the ones which it radiates when raised to a sufficient temperature. But lampblack is also diathermanous to the very extreme obscure rays of the spectrum; a fact which was shown by Melloni. Aqueous vapor, although perfectly transparent to the luminous rays of the spectrum, was found by Tyndall to be quite opaque to those of the dark spectrum. This is one of the most interesting facts connected with the whole subject of heat, and of the greatest importance, not only in a strictly scientific sense, but in its practical bearing upon questions of meteorology, and therefore upon the business of every-day life. The formation of clouds by the radiation and consequent loss of heat from vapor through the drier atmosphere above, as well as by the condensation produced by currents of cool air, and the formation of dew from the same cause, the equableness of moist climates and the cold of high mountains, could never have been well understood unless the subject of the comparative diathermancy of dry and moist gases, particularly of the atmosphere in its various hygrometric conditions, had been carefully investigated.

DIATOMACEÆ, minute plants growing in moist situations, in collections of fresh water or in the sea, consisting of frustules of various forms, the walls of which contain a large quantity of silex, and are often beautifully diversified and marbled by striæ or by dots. Notwithstanding the general resemblance of these curious vegetations to the species of desmidies, they

are clearly made distinct by the flinty fronds, singular striation, and absence of green coloring matter. Agardh asserts that many of these organisms have as much affinity with the mineral kingdom as with the vegetable, being in fact vegetable crystals, bounded by right lines and collected into a crystalliform body, and having no other difference from minerals than that the individuals have the power of again separating from each other. As in the case of the desmidieæ, there are solitary species, and others grouped so as to form lines and membranes. In some, the production of new plants from spores presents the same dissimilarity between the young and the adult forms. There are also numerous genera which can be accurately distinguished not only by the difference of form or outline, but by their own peculiar striations, markings, and dots. In both the single and the associated species there is a distinct pellucid peduncle or footstalk. This is sometimes considerably dilated above, or else forked, sometimes repeatedly. In this case each frustule remains attached, the base dilating as may be required. This arrangement gives a fan-like appearance of great beauty.

Echinella flabellata, a fan-like marine diatom.

But in the thread-like species it is only the corners that remain attached; as no stem or footstalk is visible here, it has been conjectured that it exists only in those plants which have grown from spores or in the seedling forms. Certain channels or apertures are so arranged as to convey the water to the inner cellular membranes, and thus to afford nutriment. The same curious conjugation to be seen in other alge has been detected in the diatomaces by Thwaites, and has been confirmed by Berkeley and Broome. It is computed that vast areas of solid earthy matter are due to the growth, presence, and decay of these minute organisms. Many of the most beautiful are found in the guano of commerce, doubtless swallowed in the food of birds, and still remaining in perfect preservation. In the United States, masses of several inches in thickness are found on the bottoms of ponds, composed of myriads of these organisms, which on being exposed to desiccation become as white and friable as chalk. Even peat bogs and meadows abound with them. The polishing powders sold under the name of tripoli are composed of these natural silicious frag

ments. The soundings on the shores of Victoria Barrier, Australia, in water whose average depth is 1,800 ft., were found by Dr. Hooker to be invariably charged with diatomaceous remains. These fossil species are often so identical with recent ones, that it would be scarcely too extravagant to admit the assertion of Ehrenberg, that species are to be found in a living state in situations where they have been propagated from times far anterior to the existence of man. The United States are rich in the diatomacem, both fossil and living. In the tertiary infusorial stratum of Richmond, Va., Ehrenberg detected 20 genera and 46 species, of which all were also European excepting two. This group of American forms is of peculiar interest, because the strata at Richmond are decidedly of marine origin, and consequently give at once a general view of these marine microscopic forms along the North American coast. Of the perfectly free diatoms we have many species of naviculacea remarkable for beauty, symmetry, or delicacy, or else for their striations. The largest, most common, and most easily distinguished is navicula viridis, of an oblong outline, found in every ditch and pond. It can be detected in great abundance in the ashes of peat, and in the deposits of infusorial earths. Its length is about of a line. Several of a sigmoid outline are very remarkable for the delicacy of their striæ, of which may be mentioned pleurosigma Baltica, P. hippocampus, but more particularly P. angulata. The lines of striation upon Nitzschia sigmoidea are about 1000 of an inch apart. In fragilaria we have long threads of frustules adhering with considerable firmness at their commissures; but in diatoma they adhere only at a single point, so as to form curious chains of divided or separated joints. Prof. Bailey describes bacilaria paradoxa as a very interesting species, presenting by its curious motions and its paradoxical appearance an object well calculated to astonish all who behold it. At one moment the needle-shaped frustules lie side by side, forming a rectangular plate; suddenly one of the frustules slides forward a little way, the next slides a little also, and so on through the whole number, each, however, retaining a contact through part of its length with the adjoining ones. By this united motion the parallelogram is changed into a long line; then some of the frustules slide together again, so that the form is then much like a banner. Similar motions are constantly going on, and with such rapidity that the eye can scarcely follow them. The cause of this motion is wholly unknown, but it is probably mechanical and not vital. Mr. Smith, in his work on the diatomaceæ, estimates this motion as being inch per second. In meridion vernale we have one of the most beautiful of the freshwater diatoms. It consists of spiral or helicoidal chains, to perceive which the specimens must be tilted on edge. It occurs in immense quantities in mountain brooks, covering every sub

merged stone, or twig, or spear of grass, in the early days of spring. Among the groups with vittate or ribbon-like fronds, we may notice striatella arcuata, occurring in vast quantities on

1. Bacillaria paradoxa. 2, 3. Meridion vernale. the filiform marine algæ, and covering them so much oftentimes as to make them glitter in the sunbeams as if invested with crystals. In still another natural group, where the striæ are no longer visible in the frustules or fronds, we find a multitude of microscopic objects, furnishing sources for fresh admiration whenever they are examined. In some of these the fronds, which are disciform, are marked with radiating lines, of which coscinodiscus, very common in a fossil state in the Richmond earth and elsewhere, is most beautiful. In C. lineatus the cellules of the frond form parallel lines in whatever direction they may be viewed, and C. oculus iridis gives curious colored rings. When perfect, the disk of coscinodiscus is covered with circular spots in rows corresponding with the radii. In consequence of this arrangement they also form beautiful spiral rows in other directions, so that the curves present no inconsiderable resemblance to patterns produced by engine-turning; at other times the spots are found to form three sets of lines, making angles of 60° and 120° with each other; and on others the spots are disposed without much apparent regularity, frequently having a star-like figure in the centre. The spots are so small on some of the disks as to be almost invisible even by the highest magnifying powers; on others they are quite large and hexagonal. In podiscus Rogerii (Bailey), the whole surface is so beautifully punctate that no engraving could do it justice. The most complicated markings on the coscinodiscus scarcely rival the elaborate ornaments of this truly elegant organism. It has proved very common in Virginia and Maryland in a fossil condition. The beauty of isthmia obliquata, detected in the mud of Boston harbor, can only be appreciated by ocular examination. The diatomaceæ enter largely into the food of the mollusca. Dr. Hooker found dictyocha aculeata in the stomachs of salpæ taken off Victoria Land, and remains of diatomaceæ occurred in the same ascidiums examined between the latitudes of the N. tropic and 80° S. The medusse are also in particular often filled with these forms.-See Bailey in "American Journal of Science and Arts," vols. xli., xlvi.;

"Proceedings of the Essex Institute," vol. i., pp. 33-48, and vol. ii., pp. 70, 71; Kützing's Species Algarum (Leipsic, 1849); Smith's "British Diatomaces (2 vols. 8vo, London, 1853-'6); and Berkeley's "Introduction to Cryptogamic Botany" (London, 1857).

DIAZ, Miguel, an Aragonese explorer, born after the middle of the 15th century, died about 1514. He took part in the second expedition of Columbus, and having arrived in Hayti in 1495, he became involved in a duel which forced him to flee to the southern part of the island, where he married the female ruler of an Indian tribe. From information given by her, and with the coöperation of Bartholomew Columbus, who was governor of the colony, he discovered the gold mines of St. Christopher, and afterward took a conspicuous part in the foundation of Nueva Isabella (afterward Santo Domingo) in the vicinity of the gold districts. He faithfully adhered to Columbus until his death.

DIAZ DEL CASTILLO, Bernal, a Spanish adventurer and chronicler, born in Medina del Campo, Old Castile, near the close of the 15th century. He went to seek his fortune in the new world in 1514, and joined the expeditions which sailed from Cuba to Yucatan under Fernandez de Cordova in 1517, and under Grijalva in 1518. He afterward attached himself to the fortunes of Cortes. In 1568 he was regidor of the city of Guatemala. When Gomara's "Chronicle of New Spain" appeared, Diaz began his Historia verdadera de la conquista de la Nueva España, the object of which was to correct the many misstatements of his rival, and to claim for himself and his comrades a share of the glory which Gomara gave almost wholly to Cortes. The work was finished in 1558, and was first published at Madrid in 1632. An English translation by Lockhart appeared in 1844.

DIAZ DE SOLIS, Juan. See SOLIS.

DIBDIN. I. Charles, an English song writer and composer, born in Southampton in 1745, died July 25, 1814. He was the 18th child of his parents, who intended him for the church; but he studied music, and at the age of 16 went to London, where he at first supported himself by composing ballads for the music sellers and by tuning pianos. In 1763-4 the opera of "The Shepherd's Artifice," written and composed by him, was produced at Covent Garden theatre, after which he appeared for several years as actor and composer. Among his most popular works were "The Padlock," "The Deserter," "The Waterman," and "The Quaker," produced at Drury Lane under the management of Garrick. Having quarrelled with the latter, he was for several years engaged in various theatrical speculations, and in 1789 instituted a species of musical entertainment, in which he was the sole author, composer, and performer. He called it "The Whim of the Moment.", So successful did the enterprise prove, that in 1796 he erected a small theatre