11. A GLASS TUBE open at both ends, about two feet in length, and of sufficient width to admit being drawn over the beak of the combustion tube when in use; this tube is leaned against a filtering stand. (Vide Plate XX.)

12. A sheet of GLAZED paper, with cut edges.

13. A LIEBIG'S COMBUSTION FURNACE of sheet iron, with a single and a double screen. The furnace has the form of a long box open at the top and behind. Plate LV. represents the furnace as seen from the top.

PLATE LV.

It is about twenty-four inches long and four inches deep, and the bottom which, by the excision of small slips of the iron sheet, is converted into a grating, has a width of about

three inches. The side walls are inclined slightly outward, so that at the top they are about five inches from one another. The furnace is provided with a series of straight and upright pieces of strong sheet iron of the form illustrated in Plate LVI. B, and which are fastened upon the bottom of the furnace, at intervals of about two inches between each other. These pieces serve to support the combustion tube. They must be of exactly corresponding height, with the round aperture in the front piece of

the furnace.

PLATE LVI.

B

PLATE LVII.

This aperture must be sufficiently large to permit the easy introduction of the combustion tube. Of the two screens, the one has the form represented by Plate LVII., the other that is represented by Plate LVI. A, with the border turned down at the upper edges. The openings cut into the screens must be sufficiently large to receive the combustion tube without diffi

culty. The furnace is placed upon two flat bricks resting upon wood, and is slightly raised at the posterior end, by introducing a piece of wood between the supports, (vide Plate LX.) The apertures of the grating at the anterior extremity of the furnace, must not be blocked up by the supporting brick. In cases where the combustion tubes are of a good quality, the furnace may be raised more advantageously by introducing a little iron staff or a piece of a tile between the furnace and the supporting brick; this arrangement will give the air free access to all the holes of the grating; or the furnace may also be directly placed upon a tripod.

14. OXIDE OF COPPER. A Hessian crucible, capable of holding about three ounces of water, is to be nearly filled with oxide of copper prepared according to § 40, 1; the crucible is then closely covered and heated to very gentle redness; the red-hot crucible is subsequently allowed to cool in some measure, so that the hand can just, but hardly, bear its contact by the time that the oxide of copper is required for use.

15. AN EXHAUSTING SYRINGE WITH CHLORIDE OF CALCIUM TUBE. Vide Plate LIX. For the manner of performing organic elementary analysis without the aid of this apparatus, I refer to § 143.

16. HOT SAND, the temperature of this should exceed 212°, but must not be sufficiently high to singe paper.

17. A wooden trough for the sand. Vide Plate LIX.

II. PERFORMANCE OF THE ANALYTICAL PROCESS.

a. Both the potass apparatus and the chloride of calcium tube must be accurately weighed. From 0.350 to 0.600 grammes (more or less according as the substance under examination is rich or poor in oxygen) are introduced into the little tube (I., 2,) and the latter is then accurately weighed with its contents. The weight of the empty tube being known, it is easy to hit the right

quantity required for the analysis of the substance. The tube is then closed with a smooth cork, under which may be placed a little piece of tinfoil.

b. The sheet of glazed paper (I., 12) is spread upon a clean table, and moderately warm mortar (I., 9) placed upon it. Both the combustion tube-(which is likewise still warm)-and the mortar are then rinsed with a little of the hot oxide of copper, (the oxide which has served for this purpose is put by for the moment). The combustion tube (vide Plate LVIII.) is

now filled up to 6 with oxide of copper, which is transferred directly from the crucible to the tube. A portion of the oxide of copper is now projected from the tube into the mortar, and the whole of the substance intended for analysis is added to it from the small tube in which it has been weighed; this small tube is put by in a secure place, since it is necessary to reweigh it.

The oxide of copper and the substance intended for analysis are now most intimately intermixed in the mortar, taking care to avoid a too energetic pressure upon the pestle; nearly the whole of the rest of the oxide of copper in the tube is then likewise transferred from the latter to the mortar, so that there remains only a layer of about one to one and a half inch in the tube. This fresh portion of oxide of copper is then most intimately mixed with the mass in the mortar. The pestle is now taken out of the mortar, and the minute particles of the mixture which adhere to it are shaken off as much as possible; the mixture is now transferred from the mortar to the tube, which is best effected by applying the mouth of the tube directly to the mixture, turning the tube, and depressing its posterior extremity; the rest of the mixture, which cannot be transferred into the

tube in this way, is poured upon a perfectly smooth card, and finally transferred from the latter into the tube; the mortar is then rinsed with a little fresh oxide of copper, which is subsequently likewise transferred to the tube-(this will fill the latter to the point a or thereabout). The rest of the tube is now finally filled to within about an inch and a half of the mouth with pure oxide of copper from the crucible, and the tube is then temporarily closed with a cork. The reason why the operation of transferring the mixture to the tube is conducted over the sheet of glazed paper is that, should any of the mixture be spilled, this may be readily recovered.

c. A few gentle taps on the table suffice generally to shake together the contents of the tube, so as to free the beak perfectly from oxide of copper, and to leave a free passage above the

mixture, from end to end. Should a few gentle taps not suffice to effect this, as will sometimes happen, owing to malformation of the beak, the object in view may be attained by striking the mouth of the tube several times against the side of a table, the tube being held perfectly straight all the while. The tube is now placed into the wooden trough D, (Plate LIX.,) connected by means of a cork with the chloride of calcium tube B, which latter is again connected with an exhausting syringe. The combustion tube is surrounded in its whole length with the hot sand, (I., 16.) The air is now slowly exhausted, (quick and incautious pumping might cause a portion of the mixture to pass into the chloride of calcium tube). After a short interval, the stopcock a is opened, and new air is admitted, which becomes completely dried in its passage through the chloride of calcium tube; the air is then again exhausted, and fresh air admitted; this process of alternate exhaustion and readmission of air, if repeated ten or twelve times, will ensure the perfect removal of the moisture which the oxide of copper may have absorbed from the air during the operation of mixing.

d. The end of the weighed chloride of calcium tube (I., 5) is now connected with the combustion tube by means of the perforated and dried cork (I., 8); the combustion tube is placed upon the supports in the furnace, (which latter rests likewise upon its supports, and is slightly inclined forward, Plate LX.,) and the end B. of the chloride of calcium tube, connected by means of the little caoutchouc tube with the end m of the potass apparatus, the caoutchouc tube is secured on both sides with silk cord, (I., 7.), taking care to press the balls of the two thumbs close together whilst tightening the cords, since otherwise, should one of the cords happen to give way, the whole apparatus might be broken. The potass apparatus is made to rest upon a folded piece of cloth. Plate LX. illustrates the whole arrangement.

e. The tightness of the junctions is examined in the following manner. A piece of wood of the size of a finger (s) (or a cork or other body of the kind) is placed under the bulb r of