or cupule," for the support of the egg. It is composed almost entirely of fibrous tissue, invested with a layer of tesselated epithelium. In some instances when the eggs were but little advanced, numerous fusiform cells were detected among the fibres. It is vascular, two or three vessels reaching to the cup, where they ramify and form a somewhat extended capillary plexus. (Figs. 3 and 4).

The eggs vary according to the degree of development from the 0·09 to 0.15 of an inch in diameter, and are covered with an external homogeneous membrane, containing minute punetiform depressions-within this is a second, of a brownish color and composed of epithelium. The embryos which were the most advanced and just ready to hatch, had not as yet completely absorbed the yolk, and were coiled up within the membranes, which in consequence of the irregularities of the mass formed by the embryo, had no longer a spherical form.

The eggs are retained in connection with the cup apparently by adhesion alone, for as soon as the foetus escapes, the egg membranes become very easily detached from the pedicle, and this last as shown by some of the specimens undergoes absorp

tion.

The relation of the embryo to the parent in this singular mode of gestation cannot be determined very accurately, but the vascular plexus in the cup, seems to be more than is necessary for the mere nutrition of the part. The egg increases in size during incubation, those ova in which development had but slightly advanced measuring from 0.09 to 0:11 of an inch in di ameter, while those nearly mature measured from 0.14 to 0.15 of an inch. How this increase of size of the embryo over the original size of the egg is actually obtained I have no facts to show, but either of two suppositions are probable; it may be by absorption of materials from the water which surrounds it, or from the capillary plexus of the pedicles, and in this case in a manner analogous to that of Pipa.

Among the Siluroid fishes of Guiana there are several species, which at certain seasons of the year have their mouths and branchial cavities filled either with eggs or young, and as is believed for the purpose of incubation. My attention was first called to this singular habit by the late Dr. Francis W. Cragin, formerly U. S. Consul at Paramaribo, Surinam. In a letter dated August, 1854, he says, "the eggs you will receive are from another fish. The different fishermen have repeatedly assured me, that these eggs in their nearly mature state are carried in the mouths of the parent, till the young are relieved by the bursting of the sac. Do you either know or believe this to be so, and if possible, where are the eggs conceived and how do they get into the mouth ?"

In the month of April, 1857, on visiting the market of Para. maribo, I found that this statement, which at first seemed to be very improbable, was correct as to the existence of eggs in the mouths of several species of fish. In a tray of fish which a negro woman offered for sale, I found the mouths of several filled with either eggs or young, and subsequently an abundance of opportunities occurred for repeating the observation. The kinds most commonly known to the colonists, especially to the negroes, are Jara-bakka, Njinge-njinge, Koepra, Makrede and one or two others, all belonging either to the genus Bagrus or one nearly allied to it. The first two are quite common in the market and I have seen many specimens of them; for the last two I have the authority of negro fishermen but have never seen them myself. The eggs in my collection are of three different sizes, indicating so many species; one of the three having been brought to me without the fish from which they were taken.

The eggs become quite large before they leave the ovaries, and are arranged in three zones corresponding to three successive broods, and probably to be discharged in three successive years; the mature eggs of a Jara-bakka eighteen inches long, measure three fourths of an inch in diameter, those of the second zone one fourth; and those of the third or very minute, about one sixteenth of an inch.

A careful examination of eight specimens of Njinge-njinge about nine inches long, gave the following results:

The eggs in all instances were carried in the mouths of the males. This protection, or gestation of the eggs by the males, corresponds with what has been long noticed with regard to other fishes, as for example, Syngnathus where the marsupial pouch for the eggs or young is found in the males only, and Gasterosteus where the male constructs the nest and protects the eggs during incubation, from the voracity of the females.

In some individuals the eggs had been recently laid, in others they were hatched, and the foetus had grown at the expense of some other food than that derived from the yolk, as this last was not proportionally diminished in size, and the foetus weighed more than the undeveloped egg. The number of eggs contained in the mouth was between twenty and thirty. The mouth and branchial cavity were very much distended, rounding out and distorting the whole hyoid and branchiostegal region. Some of the eggs even partially protruded from the mouth.

The ova were not bruised or torn as if they had been bitten, or forciby held by the teeth. In many instances the foetuses were still alive, though the parent had been dead for many hours.

No young or eggs were found in the stomach, although the mouth was crammed to its fullest capacity.

The above observations apply to Njinge-njinge. With regard to Jarra-bakka, I had but few opportunities for dissection, but in several instances the same conditions of the eggs were noticed as stated above; and in one instance, besides some nearly mature fœtuses contained in the mouth, two or three were squeezed apparently from the stomach; but not bearing any marks of violence or of the action of the gastric fluid. It is probable that these found their way into that last cavity after death, in consequence of the relaxation of the sphincter which separates the cavities of the mouth and the stomach. These facts lead to the conclusion that this is a mouth gestation, as the eggs are found there in all stages of development, and even for some time after they are hatched.

The question will be very naturally asked, how under such circumstances, these fishes are able to secure and swallow their food. I have made no observations bearing upon such a question. Unless the food consists of very minute particles, it would seem necessary that during the time of feeding the eggs should be disgorged. If this supposition be correct, it would give a very probable explanation of the only fact which might be considered at variance with the conclusion stated above, viz., that we have in these fishes a mouth gestation. In the mass of eggs with which the mouth is filled, I have occasionally found the eggs, rarely more than one or two, of another species. The only way in which their presence may be accounted for, it seems to me, is by the supposition that while feeding, the eggs are disgorged, and as these fishes are gregarious in their habits, when the ova are recovered, the stray egg of another species may be introduced into the mouth among those which naturally belong there.

5.

Fig. 5 represents a nearly mature foetus of the natural size from the mouth of Bagrus, with the yolk sac partially included in the cavity of the abdomen.

ART. III.-Some Facts respecting the Nitrates; by JOHN M. ORDWAY.

WHILE studying the nitrates of the sesquioxyds I found it advisable, for the sake of comparison, to examine the protonitrates also, with reference to some points not generally taken into account in enumerating the properties of these salts. And as the nitrates are among the most common and important salts, it may be worth the while to exhibit these gleanings in fields often gone over but not yet entirely cleared. There are few new facts to be brought forward, but the chief object of this paper is to show the fitness of certain means for the illustration of some general truths already well known.

In most chemical text-books no good instances are given of the development of heat by mere solidification. It is indeed usually mentioned that water may be cooled many degrees below the freezing point and remain liquid, and that on congealing its temperature suddenly rises to 32° F. But the experiment is so troublesome to make, especially in the lecture room, that these truths commonly pass as matters of faith rather than of sight, and the important principles which they illustrate, often fail of being distinctly impressed on the mind of the student. Now many of the hydrated salts, and among them the nitrates, melt at points above the common temperature of the air, and are therefore well adapted for showing, at all seasons and with great ease and clearness, the inertia of bodies with regard to change of form and the liberation of sensible heat by crystallization.* Nitrate of lime is preeminently suitable for the exhibition of these properties, since after having been fused and heated above 150° F., it may be cooled in a glass vessel as low as 60°, and kept in the liquid state a long time, often for several days; but on dropping in a bit of the solid nitrate, crystallization immediately commences, and an inserted thermometer soon rises to 110° F.

A substance which may be had both liquid and solid at a temperature considerably below the melting point, is obviously very convenient for displaying the comparative densities and specific heats in the two forms, as complications caused by dif ferences of temperature, may be entirely avoided. Thus the specific gravity of a specimen of nitrate of lime in the liquid state, at 60° F., was found to be 1.79. Some of the same was poured into oil of turpentine, made to solidify, and cooled to

*In an excellent work published in 1857,-"Lehrbuch der physikalischen und theoretischen Chemie, von H. Buff. H. Kopp und F. Zamminer,"-hyposulphite of soda is mentioned as capable of affording a very striking example of the heat becoming free during fixation; but this salt is less easy to prepare than most of the nitrates.

60° F. Its density was now 1.90. The contraction may be rendered appreciable by the eye, if we cool to a certain degree some melted nitrate contained in a long necked flask, fill with an oil up to a marked height, effect the crystallization, and then cool to the same point as before.

To illustrate the absorption of heat during the liquefaction of solids, freezing mixtures are commonly employed in which one of the ingredients, ice, is already cold. The experiment is more striking when all the articles used are at the temperature of the surrounding air. Such may be the case if we take crystallized sulphate of soda and a sesquinitrate. A mixture of 36 grams of powdered pernitrate of iron crystals and 57 grams of fine Glauber's salt, liquefied and lowered thermometer from 65° F. to zero. It readily froze water contained in a test tube. In cold weather, 8 grams of the nitrate and 9.5 grams of the sulphate brought the thermometer from 22° to -10°. *

In manufacturing salts on a large scale, the hydrometer is a very useful and ready instrument for determining when a solution is of the right strength to crystallize. But the quantities operated on in the laboratory are generally so small that the hydrometer can hardly be made available. The bulb of a thermometer, however, requires but little depth of liquor, and hence to one who wishes to prepare in the small way any of the highly soluble salts, a knowledge of the boiling points of the desired products may be of great service. Thus, finding that crystallized protochlorid of tin melts at 107° F., boils at 251°, and may be cooled to 83° without becoming solid, we see that to make this article in midsummer the evaporation of the weak solution must be continued till the boiling point gets nearly or quite up to 251°.

It should be remarked that the melting and boiling points given below, do not pretend to absolute exactness. No two dif ferent lots of the same salt are likely to give just the same figures; for it is next to impossible to get most of the hydrated salts exactly dry,-neither effloresced nor retaining mother liquor in the interstices. For any particular specimen the point of fusion can be determined with great precision. But the boiling points are high and, unless very nice precautions are taken, there will be some loss of water in heating up. So to find the temperatures of incipient ebullition, crystals were taken that were not entirely dry to start with, and the correctness of the indications was judged of after ascertaining the solidifying points of the residues.

There is, of course, no definite limit to the cooling which a melted salt may undergo without beginning to crystallize.

Nitrate of iron is pretty corrosive, and should not be touched with the fingers.

I