4. Transfer the residue of 2 (dried at 212° F.) to a weighed platinum dish, add the ash of the filter, dry at 320° F., and weigh. The weight expresses the total amount of substances insoluble in water. Moisten the residue now with a little nitric acid, ignite gently, with access of air, until the whole of the organic matter and charcoal is burnt, then weigh again; the difference between the results of the two weighings expresses the quantity of the organic matter.

5. Boil the residue of 4 with dilute hydrochloric acid; after boiling for some time, dilute with water, filter, and dilute the filtrate by means of the washing water to litre; treat the insoluble residue as directed in 7.

6. Of the hydrochloric acid solution obtained in 5, measure off two portions, one of 50, the other of 100 c.c. In the former determine the sulphuric acid, in the latter the phosphate of sesquioxide of iron (if present), lime, magnesia, and phosphoric acid, as directed in 3, 8 and y.

7. Dry, ignite, and weigh the insoluble residue of 5. It generally consists only of sand, clay, and silicic acid. To make quite sure, however, boil with concentrated hydrochloric acid; should some more gypsum be dissolved, determine the amount of this in the solution. Treat the insoluble residue as directed § 235, b, to separate the silicic acid from the clay and sand.

8. Lastly, determine the nitrogen in 0.8-1 grm. of the superphosphate (§ 187). As the nitrogen is already included in the weight of the organic matter, it is simply entered in the report of the results by way of information, and not as an element of the calculation.

9. Should the superphosphate contain an ammonia salt, determine the ammonia as directed § 99, 3, a.

VI. ANALYSIS OF ATMOSPHERIC AIR.

$265.

In the analysis of atmospheric air we usually confine our attention to the following constituents: oxygen, nitrogen, carbonic acid, and aqueous vapor. It is only in exceptional cases that the exceedingly minute quantities of ammonia and other gases (many of which may be assumed to be always present in very minute traces) are also determined.

It does not come within the scope of the present work to describe all the methods which have been employed in the excellent investigations made in the last few years by Brunner, Bunsen, Dumas and Boussingault, Regnault and Reiset, and others, and to which we are indebted for a more accurate knowledge of the composition of our atmosphere. Excellent descriptions of these methods will be found in H. Rose's "Handbuch der analytischen Chemie," vol. ii. p. 853; in Graham's "Chemistry;" in Liebig, Poggendorff, and Wöhler's "Handwörterbuch der Chemie," 2nd edit. vol. ii. p. 431; and Bunsen's "Gasometry," translated by Roscoe. I confine myself to those methods which are found most convenient in the analysis of the air for technical or medical purposes.

A. DETERMINATION OF THE WATER AND CARBONIC ACID.

$266.

The determination of these two constituents of the atmosphere was formerly usually effected by Brunner's method, that is, by slowly drawing,

by means of an aspirator, a measured volume of air, through accurately weighed apparatus filled with substances having the property of retaining the aqueous vapor and the carbonic acid, and estimating these two constituents by the increased weight of the apparatus.

Fig. 147 represents an aspirator, constructed on Regnault's plan.

The vessel V is made of zinc-plated sheet iron, or of sheet zinc; it holds from 50 to 100 litres, and stands upon a strong tripod in a trough of sufficient capacity to contain the whole of the water. At a a brass tube, c, with stopcock is firmly fixed in with cement. Into the aperture b, which serves also to fill the apparatus, a thermometer reaching down to the middle of V is fixed air-tight by means of a perforated cork soaked in wax.

The efflux tube, r, is bent slightly upward, to guard against the least chance of air entering the vessel from below. The capacity of the vessel is ascertained by filling it completely with water, and then accurately measuring the contents in graduated vessels. The end of the tube c is connected air-tight with F, by means of a caoutchouc tube; the tubes A-F are similarly connected. A, B, E, and F are filled with perfectly neutral chloride of calcium; C and D with moist hydrate of potassa.* Finally, A is also connected with a long tube leading to the place from which the air intended for analysis is to be taken. The corks of the tubes are coated over with sealing wax. The tubes A and B are intended to withdraw the moisture from the air; they are weighed together. C, D, and E are also weighed jointly. C and D absorb the carbonic acid; E the aqueous vapors which the air dried in A and B may take up from the

In the last edition I had recommended pumice stone moistened respectively with sulphuric acid and solution of potassa, instead of chloride of calcium and hydrate of potassa, as originally employed by Brunner. But Hlasiwetz ("Chem. Contralbl.," 1856, page 575) having shown that solution of potassa absorbs, besides carbonic acid, also oxygen, a fact remarked also by H. Rose-and that sulphuric acid absorbs not only aqueous vapor, but also carbonic acid, I have returned to Brunner's original practice.

hydrate of potassa. F need not be weighed; it simply serves to protect E against the entrance of the aqueous vapors from V.

The aspirator is completely filled with water; c is then connected with F, and thus with the entire system of tubes of which the apparatus consists; the cock r is opened a little, just sufficient to cause a slow efflux of water. As the height of the column of water in V is continually diminishing, the cock must from time to time be opened a little wider, to maintain as nearly as possible a uniform flow of water. When V is completely emptied, the height of the thermometer and that of the barometer are noted, and the tubes A and B, and C, D, and E weighed again.

As the increase of weight of A and B gives the amount of water, that of C, D, and E, the amount of carbonic acid in the air which has passed through them; and as the volume of the latter (freed from water and carbonic acid) is accurately known from the ascertained capacity of V :* the calculation is in itself very simple; but it requires, at least in very accurate analyses, the following corrections :

a. Reduction of the air in V, which is saturated with aqueous vapor, to dry air; since the air which penetrates through c is dry (see § 198, y). B. Reduction of the volume of dry air so found, to 32° F., and 760 millimetres bar. (§ 198, a and ẞ).

When these calculations have been made, the weight of the air which has penetrated into V is readily found (1000 c.c. of dry air at 32° F., and 760 millimetres bar., weighing 1.29366 grm.); and as the carbonic acid and water have also been weighed, the respective quantities of these constituents of the air may now be expressed in per cents. by weight or, calculating the weight in volumes, in per cents. by measure or volume.

Considering the great weight and size of the absorption apparatus, in comparison to the increase of weight by the process, at least 25,000 c.c. of air must be passed through; the air inside the balance-case must be kept perfectly dry by means of a sufficient quantity of chloride of calcium, and the apparatus left for some time in the balance-case, before proceeding to weigh. Neglect of these precautions would lead to considerable errors, more particularly as regards the carbonic acid, the quantity of which in atmospheric air is, on an average, about 10 times less than that of the aqueous vapor (compare Hlasiwetz, "Chem. Centralbl.," 1856, page 575).

The determination of the carbonic acid in atmospheric air may be effected with much greater accuracy by the following method, recommended by Fr. Mohr, and most carefully tested by H. von Gilm ("Chem. Centralbl.," 1857, page 760). The aspirator employed in this method, holding at least 30 litres, is arranged like that shown in fig. 147, but has, besides a and b, a third aperture, which bears a small manometer. The air is made to pass through a tube, 1 metre long, and about 15 millimetres wide; this tube is drawn out thin at the upper end, and at the lower end bent at an angle of 140-150 degrees. It is more than half filled with coarse fragments of glass and perfectly clear baryta water, and fixed in such a position that the long part of it inclines to the horizontal line at an angle of 8-10 degrees. A narrow glass tube fitted into the lower end of the tube by means of a perforated cork, serves to

Or from the quantity of water which has flown from V, as the experiment may be altered in this way, that a portion only of the water is allowed to run off, and received into a measuring vessel.

admit the air. Two small flasks, filled with baryta water, are placed between the absorption tube and the aspirator; these are intended as a control, to show that the whole of the carbonic acid has been retained. When about 60 litres of air have slowly passed through the absorption tube, the carbonate of baryta formed is filtered off out of contact of air, and the tube as well as the contents of the filter washed, first with distilled water saturated with carbonate of baryta, then with pure boiled water. The carbonate of baryta in the filter and in the tube is then dissolved in dilute hydrochloric acid, the solution evaporated to dryness, the residue gently ignited, the chlorine of the chloride of barium determined as directed § 141, b, a, and 1 equivalent of carbonic acid entered for 1 equivalent of chlorine. As will be readily seen, the baryta in the hydrochloric acid solution may also be determined by precipitating with sulphuric acid. For filtering the carbonate of baryta, Gilm employs a double funnel (see Fig. 148). The inner cork has, besides the perforation through which the neck of the funnel passes, a lateral slit, which serves to establish a communication between the air in the outer funnel and the air in the bottle.

X

Fig. 148.

As, with the absorption apparatus arranged as described, the air has to force its way through a column of fluid, the manometer in the third aperture

of the aspirator is required to determine the actual volume of the air; the height indicated by this instrument being deducted from the barometric pressure observed during the process.

Pettenkofer (Liebig, Poggendorff, and Wöhler's "Handwörterbuch der Chemie," 2nd edition, vol. ii., page 445) has recently recommended the following simple and expeditious method of determining with sufficient accuracy the carbonic acid in atmospheric air. Take a perfectly dry bottle, of about 6 litres capacity, with wellfitting ground glass stopper, and accurately determine the capacity; fill the bottle, by means of a pair of bellows, with the air to be analysed; add 45 c.c. of clear lime-water of known strength, determined by means of standard oxalic acid, and cause the lime-water to spread over the inner surface of the bottle, by turning the latter about, but without much shaking. In the course of about an hour the whole of the carbonic acid is absorbed, Pour the turbid lime-water into a cylinder, close securely, and let deposit; then take out, by means of a pipette, 30 c.c. of the clear supernatant fluid, determine the lime in this by standard oxalic acid, multiply the volume used by 15 (as only 30 c.c. of the original 45 are employed in this experiment), and deduct the result from the amount of lime originally present in the 45 c.c. of lime-water; the difference expresses the quantity of lime converted into carbonate; calculate the carbonic acid from this. If the air is unusually rich in carbonic acid, the quantity of lime-water used is correspondingly increased. Pettenkofer uses a standard oxalic acid containing, at 63.5° F., 2.250 grammes of crystallized oxalic acid in the litre; 1 c.c. of this solution saturates 0.001 grm. of lime. The point of neutralization is recognised most accurately, by letting a drop of the fluid fall upon turmeric paper, and observing whether a brown coloration appears round the drop.

B. DETERMINATION OF THE OXYGEN AND NITROGEN.

$267.

Among the many methods recommended for determining the oxygen of the air, I select the one which I deem best suited for the purposeviz., Liebig's (" Annal. d. Chem. u. Pharm.," 77, 107).

This method is based upon the observation made by Chevreul and Döbereiner, that pyrogallic acid, in alkaline solutions, has a powerful tendency to absorb oxygen.

1. A strong measuring tube, holding 30 c.c. and divided into or c.c., is filled to with the air intended for analysis. The remaining part of the tube is filled with mercury, and confined over that agent in a tall cylinder, widened at the top (Fig. 114, § 184).

2. The volume of air confined is measured (§ 12). If it is intended to determine the carbonic acid-which can be done with sufficient accuracy only if the quantity of the acid amounts to several per cents.-the air is dried by means of a ball of chloride of calcium introduced into it (§ 16), and then again measured. If it is not intended to determine the carbonic acid, this operation is omitted. A quantity of solution of potassa of 1·4 specific gravity (1 part of dry hydrate of potassa to 2 parts of water), amounting to from too of the volume of the air, is then introduced into the measuring tube by means of a pipette with the point bent upward (see Fig. 149), and spread over the entire inner surface of the tube by shaking the latter (§ 184 a a); when no further diminution of volume takes place, the decrease is read off. If the air has been dried previously with chloride of calcium, the diminution of the volume expresses exactly the amount of carbonic acid contained in the air; but if it has not been dried with chloride of calcium, the diminution in the volume cannot afford correct information as to the amount of the carbonic acid, since the strong solution of potassa absorbs also aqueous vapor.

Fig. 149.

3. When the carbonic acid has been determined (or simply removed), a solution of pyrogallic acid, containing 1 gramme of the acid in 5 or 6 c.c. of water,* is introduced into the same measuring tube by means of another pipette, similar to the one used in 2 (Fig. 149); the quantity of pyrogallic acid employed should be half the volume of the solution of potassa used in 2. The mixed fluid (the pyrogallic acid and solution of potassa) is spread over the inner surface of the tube by shaking the latter, and, when no further diminution of volume is observed, the residuary nitrogen is measured.

4. The solution of pyrogallic acid mixing with the solution of potassa, of course dilutes it, causing thus an error from the diminution of its tension; but this error is so trifling that it has no appreciable influence upon the results; it may, besides, be readily corrected, by introducing into the tube, after the absorption of the oxygen gas, a small piece of hydrate of potassa corresponding to the amount of water in the solution of the pyrogallic acid.

5. There is another source of error in this method; viz., on account of

* Liebig has recently described a very advantageous method of preparing pyrogallic acid. See "Annal. d. Chem. u. Pharm.," 101, 47.