degrees of a circle of the plane of polarised light produced by 1 gramme of the substance dissolved in 1 c.c. of liquid examined in a column one decimetre long.

If a

го

7

(a)D

Then (a)D

=

=

rotation observed.

weight in grammes of the substance per cubic centimetre. = length of tube in decimetres.

=

=

specific rotation for light with wave-length corresponding to the D line (sodium flame).

In this formula + indicates that the substance is dextrorotatory, that it is lævorotatory.

If, on the other hand, (a), is known, and we wish to find the value of w, then

This instrument is one in which a spectroscope and polarising apparatus are combined for the purpose of determining the concentration of substances which rotate the plane of polarised light. It was invented by E. v. Fleischl for the estimation of diabetic sugar in urine. Its chief advantage is that no difficulty arises of forming a judgment as to the identity of two coloured

surfaces, as in Soleil's saccharimeter, or of two shades of the same colour, as in Laurent's instrument. The light enters at the right-hand end of the instrument, is polarised by the Nicol's prism b, and then passes through two quartz plates, cc, placed horizontally over each other. One of these plates is dextro-, the other lævo-rotatory, and they are of such a thickness (7·75 mm.)

that the green rays between the E and b lines of the spectrum are circularly polarised through an angle of 90°, the one set passing off through the upper quartz to the left, the other through the lower to the right. The light then continues through a long tube, ff, which contains 15 c.c. of the solution under examination. It then passes through an analysing nicol, d, and finally through a direct vision spectroscope, e. On looking through the instrument, the tube ff being empty, or filled with water or some other optically inert substance, two spectra are seen, one over the other, but each shows a dark band between E and 6 owing to the extinction of these rays by the circular polarisation, produced by the quartz. The analyser can be rotated: a vernier, g, is attached to, and moves with it, round a circular disc (seen in section at h) graduated in degrees. The two bands in the spectra coincide when the zeros of vernier and scale correspond. If now the tube ƒ is filled with an optically active substance like sugar, the bands are shifted, one to the right, the other to the left, according to the direction of rotation of the substance in f. The rotation is corrected by rotating the analyser into such a position that the two bands exactly coincide once more as to vertical position. The number of degrees through which it is thus necessary to move the analyser measures the amount of rotation produced by the substance in ƒ, and is a measure of the concentration of the solution. The degrees marked on the circular scale are not degrees of a circle, but an arbitrary degree of such a length that each corresponds to 1 per cent. of sugar in the given length of the column of fluid in ff (177.2 mm.).

PFLÜGER'S MERCURIAL AIR-PUMP

7 is a large glass bulb filled with mercury; from its lower end a straight glass tube, m, about 3 feet long, extends, which is connected by an indiarubber tube, n, with a reservoir of mercury, o, which can be raised or lowered as required by a simple mechanical arrangement. From the upper end of the bulb, l, a vertical tube passes; above the stopcock, k, this has a horizontal branch, which can be closed by the stopcock, f. The vertical part is continued into the bent tube, which dips under mercury in the trough, h. A stopcock, j, is placed on the course of this tube. Beyond ƒ the horizontal tube leads into a large double glass bulb, a b; a mercurial gauge, e, and a drying-tube, d, filled with pieces of pumice-stone moistened with sulphuric acid being interposed. a is called the blood-bulb, and the blood is brought into it by the tube, c; the gases, as they come off, cause the blood to froth, and the bulb, b, is called the froth-chamber, as it intercepts the froth, preventing it from passing into the rest of the apparatus.

The pump is used in the following way: is filled with mercury, the level in 7 and o being the same; k is closed; o is then lowered, and when it is 30 inches lower than the stopcock k, the mercury in 7 falls also, leaving that bulb empty; j being closed and ƒ open, k is then opened, and the air in a, b, d, &c. rushes into the Torricellian vacuum in l; f is closed and j opened; the reservoir, o, is raised; the mercury in 7 rises also, pushing the air before it, and it bubbles out into the atmosphere through the mercury (the tube, h, is not at this stage in position). When 7 is full of mercury, k and j are once

more closed and o is again lowered; when 7 is thus rendered once more a vacuum, k and ƒ are opened and more of the air remaining in a, b, d, &c. rushes into the vacuum; f is closed, j is opened, and this air is expelled as before. The process is repeated as often as is necessary to make a, b, d, &c.

as complete a vacuum as indicated by the mercury in the gauge, e, as is obtainable.

a being now empty and the stopcock, f, closed, blood is introduced by the tube, c; it froths and gives off all its gases, especially if heated to 40–45° C. In the case of serum, acid has to be added to disengage the more firmly combined carbonic acid.' The bulb, l, is once more rendered a vacuum, and

1 Phosphoric acid is usually employed.

k and f are opened, j being closed. The gas from a and b rushes into the bulb 7, being dried as it passes through d; ƒ is then closed and j opened; the reservoir o is raised, and as the mercury in 7 rises simultaneously, it pushes. the gases into the cylinder, h, which is filled with mercury and inverted over the end of the bent tube. This gas can be subsequently analysed. By alternately raising and lowering o, and regulating the stopcocks in the manner already described, all the gas from the quantity of blood used can be ultimately expelled into h.

A good grease for the stopcocks is a mixture of two parts of vaseline to one of white wax.

Alvergniat's pump has the advantage over Pflüger's of fewer connections, and all of these are surrounded by mercury, which effectually prevents leakage; it has the disadvantage of a rather small bulb in place of 1, and thus it is more labour to obtain a vacuum.

ANALYSIS OF GASES

Waller's modification of Zuntz's more complete apparatus will be found. very useful in performing gas analysis, say, of the expired air: a 100 c.c. measuring-tube graduated in tenths of a cubic centimetre between 75 and 100; a filling bulb and two gas pipettes are connected up as in the diagram.

It

KHO

75

80

85

02

90

It is first charged with acidulated water up to the zero mark by raising the filling bulb, tap 1 being open. is then filled with 100 c.c. of expired air, the filling bulb being lowered till the fluid in the tube has fallen to the 100 mark. Tap 1 is now closed. The amount of carbonic acid in the expired air is next ascertained; tap 2 is opened, and the air is expelled into the gas pipette containing strong caustic potash solution by raising the filling bulb until the fluid has risen to the zero mark of the measuring tube. Tap 2 is closed and the air left in the gas pipette for a few minutes, during which the carbonic acid is absorbed by the potash. Tap 2 is then opened and the air drawn back into the measuring tube by lowering the filling bulb. volume of air (minus the carbonic acid) is read, the filling bulb being adjusted so that its contents are at the same level as the fluid in the measuring tube. The amount of oxygen is next ascertained in a precisely similar manner by sending the air into the other gas pipette, which contains sticks of phosphorus

The

95

100

FIG. 73.-Waller's apparatus for gas

analysis.

in water, and measuring the loss of volume (due to absorption of oxygen) in the air when drawn back into the tube.

KJELDAHL'S METHOD OF ESTIMATING NITROGEN

This simple and accurate method has very largely replaced the older complicated processes.

I take the following account of the method with the modifications proposed by Warington from Sutton's 'Volumetric Analysis.'

From 0.1 to 1 gramme of the dry powdered substance is put into a boiling flask holding about 100-120 c.c. The acid used for the destruction of the organic material is made by mixing 200 c.c. pure oil of vitriol with 50 c.c. Nordhausen oil of vitriol, and 2 grammes of phosphoric acid in sticks; all these must, of course, be free from ammonia : 10-20 c.c. of this mixture is poured over the substance in the flask and heated on wire gauze over a small Bunsen flame. The temperature must be kept below boiling; with prolonged heating the organic matter is gradually destroyed, and the liquid becomes clear and quiet. The nitrogen originally present is thus converted into ammonia, and this may be hastened by adding to the liquid very minute pinches of pure potassium-permanganate. A violent commotion takes place with every addition, but there is no fear of any ammonia being lost. The operation is ended when the mixture becomes permanently greenish (from one to two hours), and moderate heat is continued for a few minutes more. The flask is cooled, some water added, and the contents washed out into a large flask of 700 c.c. capacity with as little water as possible. It is then made alkaline with excess of either pure caustic soda or potash solution (sp. gr. 1·3). A little metallic zinc is added to prevent bumping during the subsequent distillation. The flask is then rapidly closed with a perforated caoutchouc stopper, through which passes an upright tube with two bulbs about an inch in diameter blown upon it: these arrest and carry back any spray of soda from the liquid. The tube above the bulbs is bent over and connected to a condenser, and the delivery end of the condenser leads into a flask containing a measured excess of standard acid. The mixture in the flask is then distilled, the ammonia passes over into the acid. Distillation is continued from forty-five to sixty minutes. The amount of acidity is then determined in the distillate by titration with standard potash or soda, methyl orange being used as the indicator of the end of the reaction. (This gives a pink colour with acid, yellow with alkali.)

Example. Suppose 0.15 gramme of a nitrogenous substance is taken, treated with acid, neutralised, and the ammonia distilled over and received by 100 c.c. of a decinormal solution of hydrochloric acid (= 10 c.c. normal acid). The distillate is then titrated with decinormal soda, and it is found that the neutral point is reached when 60 c.c. of the decinormal soda have been added. The other 40 c.c. must therefore have been neutralised by the ammonia derived from the nitrogenous substance under investigation. This 40 c.c. of decinormal acid 4 c.c. of normal acid = 4 c.c. of normal ammonia = 4 × 0·017 = 0·068 gramme of ammonia: 0·15 gramme of the substance therefore yields 0.068 gramme of ammonia, and this amount contains 0.056