particularly that of the more gelatinous, is so much diminished under the high pressure, the precipitate only occupying onethird to one-sixth of its bulk under ordinary circumstances, a filter of one-third to one-sixth of the size usually employed may be taken, and thus the amount of ash proportionately essened.

As the water air-pump suffers no injury from the presence of corrosive vapors or gases, we can equally well employ it to filter liquids containing nitrous acid, sulphurous acid, fuming nitric acid, chlorine, bromine, volatile chlorids, &c. In such cases I use a peculiar filtering arrangement, consisting of a cylindrical glass vessel, the lower end of which is drawn out before the blowpipe to the form shown in fig. 5; in this drawnout portion a thin plate, 1 or 2 millims. in thickness, of artificial pumice, such as is used by polishers, is packed watertight by means of asbestos. This apparatus is arranged for the purpose required exactly as the funnel in the method of filtration by pressure above described. In order to have a number of these filters in readiness, a pumice-stone cylinder of the required diameter is turned in a lathe, and then the thin plates sawn off by means of a small hand-saw in the small wooden support shown in fig. 6. The upper surfaces of the plates may afterwards be rendered perfectly even by a coarse file.

By the aid of these pumice-stone filters many chemical products may be made, the preparation of which has hitherto been almost impossible. For the sake of example I take the preparation of pure dry chromic anhydrid; in an hour it is easily possible to filter, wash, and dry crystals of this substance an inch in length. A solution of two parts of potassium dichromate in 20 parts of water mixed with ten parts concentrated sulphuric acid, deposits, after standing about 24 hours, numerous brilliant needles of chromic anhydrid. These may be drained from adhering mother-liquor upon the pumice filter by means of the pump, and in a few minutes completely washed by a small quantity of fuming nitric acid free from nitrous acid. A covering of tolerably strong sheet copper provided with two arms, as shown in fig. 5, is then placed around the tube; by hanging lamps upon the arms the tube may be readily heated to about 60° or 80° C.; and by connecting a chlorid-of-calcium tube with the upper end of the glass vessel, a current of dry air may be drawn through the apparatus by means of the pump, and thus in a comparatively short time large and brilliant crystals of chromic anhydrid, perfectly dry and free from all impurity, may be easily obtained.

A single pump of the above description costs, including the

leaden piping, about 8 thalers (24 shillings); and experience has shown that five or six are amply sufficient for a laboratory of fifty or sixty students. The apparatus, as may readily be seen, can be applied in the operation of evaporating in vacuo; if, however, circumstances will not permit of its being adapted to this purpose, then a fall of 10 or 15 feet is sufficient to filter a precipitate according to the above described method, and so far to dry it that it can be immediately ignited in the crucible. It is therefore not absolutely necessary to employ an airpump in this process of filtration; any apparatus producing a difference of pressure amounting to a quarter of an atmosphere is sufficient. The simple arrangement represented in fig. 7 is very useful, and is frequently employed in my laboratory. It consists of two equal-sized bottles, a and a', of from 2 to 4 liters capacity, each of which is provided near the bottom with a small stopcock designed to regulate the flow of water. Suppose a filled with water and placed upon a shelf as high above the ground as possible and a' placed empty on the floor, and the two stopcocks connected by means of caoutchouc tubing c, then on allowing water to flow down the tube the air in the upper bottle becomes somewhat rarefied; and in order to employ the consequent difference in pressure (amounting to a column of mercury about 0.2 meter in height) for the purpose of filtration, it is only necessary to connect the mouth of the upper bottle with the tube of the filter-flask. When the water has ceased to flow, the position of the bottle is reversed, when the operation recommences. So small a pressure as 0.2 meter suffices to render the filter and its contents so far dry that they may be immediately withdrawn from the funnel and ignited without any other preliminary desiccation. The following experiment,made with a portion of the same solution of chromium used in the former determinations, will serve to show the saving of time effected by this simple arrangement :

This amount of chromium sesquioxyd (0-2435 grm.) differs from the mean of the former experiments (0-2436 grm.) by one

tenth of a milligram only, and shows that even by a pressure of 0.184 meter the washing is as complete by the single addition of 26 cub. centims, of water. The duration of the filtering process in the former experiments ranged from 12 to 14 minutes under a difference of pressure amounting to from 0-53 to 0.572 meter; in the last experiment it required 25 minutes under a pressure of 0.184 meter, or about double the length of time. The time needed to analyze potassium chromate in the former case was reduced from 14 hours to 32 minutes; by the latter method the reduction would be from 14 hours to 44 minutes.

The employment of the second method is particularly to be recommended to beginners in qualitative analysis. The experimenter needs only a single funnel, he is obliged to work carefully and cleanly, and the great saving of time and work amply compensates for the little trouble needed to reverse from time to time the position of the bottles.

I believe that the above-described water air-pump will soon become an indispensable piece of apparatus in chemical laboratories. It not only serves as the most convenient method of producing the differences in pressure required to accelerate the process of filtration, and of obtaining the necessary vacuum for evaporation; it is equally adapted to purposes to which neither the mercury nor the ordinary pumps are in any way applicable. By its aid it is possible to calibrate a thermometer with the greatest accuracy, and to estimate the vapor-tension of such corrosive bodies as bromine, chromyl dichlorid, &c., by the simplest method possible, in which the necessary operations require scarcely more time than an ordinary determination of a boiling-point.

I purpose returning to these applications of the instrument in a future communication.

ART. XXIX.-Note upon the origin of the Phosphatic Formation; by C. U. SHEPARD, Sr.

CONCERNING the origin of this extensive formation, several explanations suggest themselves. Among these the best answering the purpose at present is perhaps the supposition, that the great Carolina Eocene bed of shell-marl on which it rests, formerly, and for a long period, protruded many feet above the present sea-level, giving rise to a luxuriant soil (analogous to that now existing over portions of some of the guano islands and shores) and which was then depressed beneath the sea, where it under

went those changes that have resulted in the present formation. For the superabundance of the phosphate of lime in the soil supposed, we would point to the deposition of bird-guano as it is now going on upon the Musquito coast of the Caribbean sea. A layer of such soil, clothed with an abundant vegetation of from three to five feet in thickness, and nearly submerged, and afterwards becoming more or less covered with sand, in the absence of strong tidal action and oceanic currents (and these may have been precluded by shoals and even land in a seaward direction) would give rise to enormous quantities of carbonic acid, whereby the carbonate of lime of the soil and of portions of the upper layer of the marl itself would be withdrawn, and thus permit the segregation of the phosphate of lime into nodules or even into an imperfect stratum. The alumina and oxyd of iron would be precipitated upon, and among the phosphate everywhere as we find them, mingled also with considerable quantities of siliceous matter. The sulphate of lime which occurs intimately distributed through the phosphatic masses, may be supposed to have originated by double decomposition through the meeting of sulphate of soda and chlorid of calcium. The carbonate of lime present in the nodular phosphate, would be looked for, as the residuum of that belonging to the soil and the marl, which the free carbonic acid was inadequate to dissolve. Meanwhile the deposit would be enriched from the precipitation out of the supernatant waters, of the osseous remains of fishes and other marine animals; and to some extent also by those from the land, which the rivers might bring into the estuary.

The

The nodular phosphates from some portions of the formation strikingly resemble in their color, hardness and specific gravity, the stone-guano (pyroclasite) of Mong's island in the Caribbean sea; and like it, decrepitate violently when heated. Caribbean mineral is known to originate in sea birds, as the process of its formation is still in progress at many places on the Musquito coast. It is not unreasonable therefore, to ascribe the origin of our phosphatic formation partly to the same cause, at least under the modifications above pointed out.

A reason why the nodular layer is interrupted in its lateral extent may be, that the original surface, prior to its submersion, was not everywhere sufficiently above the sea-level to give rise to a soil abounding in organic matter. There probably existed lagoons, whose bottoms were free from the phosphatic deposits, which, in some instances, may correspond to the places where these are wanting, as well as to the ferruginous sand-layer, at Port Royal on St. Helena, Pinckney Island, and other localities, where the stratum of the nodular phosphate appears to find its equivalent in this latter formation.

It is certainly a striking fact, that no remains of plant or animal, then in process of growth, have been detected within the nodular layer. The nodules merely contain occasional casts and imprints of the fossils belonging to the Eocene bed on which it rests. The condition of the waters, either as to what they held in solution or from their temperature, appear to have been incompatible with organic life. Even the remains of fishes and sea-tortoises, whose detached bones are so frequent, would seem to have belonged to individuals accidentaly caught within the land-locked area when the changes of level along the coast occurred.

In some instances, where the nodules are small (from two to six inches in diameter), they have evidently been subjected to slight attrition, and assume the appearance of water-worn pebbles, reminding the observer strikingly of diluvial drift. A nearer inspection, however, soon satisfies him that they have merely been subjected to a partial movement, for a slight distance backward and forward, by wave action, without breaking up the general continuity of the layer, or much influencing its thickness in any one place. Nor is this appearance maintained over more than a few acres, when the usually tuberose and deeply pitted surfaces of the masses reoccur. At St. Helena, on the beach, opposite Port Royal city, at half tide level, there occurs a highly interesting patch of fluvio-litoral marl, two or three feet in thickness, containing 45 per cent carbonate of lime, and literally filled with the blanched but perfectly preserved shells of the following genera :-Planorbis, Physa, Auricula, Nassa, with a few Columellas. This formation appears to be more recent than that of the phosphatic nodules, which however is here wholly wanting, unless the outcrop of the ferruginous sand, occurring at a little distance to the south of the marl, at or near low water level, be regarded as the equivalent of the nodules. At any rate, it is clear that the marl has not been subjected to the corrosive or solvent agency so apparent upon the superior layer of the Eocene. The freshwater marl above mentioned has its interest hightened by containing the gigantic skeleton of a mastodon, brought to light about a year ago, by Capt. Charles Boutelle, of the Coast Survey, and by him generously contributed to my geological museum in Amherst College. A further account of its discovery may soon be looked for in this Journal.