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; ƒ 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 l 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, ƒ, closed, blood is introduced by the tube c; it froths and gives forth 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.1 The bulb, l, is once more rendered a vacuum, and k and ƒ are opened, j being closed. The gas from a and b rushes into the bulb 7, being dried as it passes through d; f 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 7, and thus it is more labour to obtain a vacuum.

B.B.

R

B.B.

b

FIG. 80.-L. Hill's air-pump.

Leonard Hill's pump.- This is a simpler instrument, and is sufficient for most purposes. It consists of three glass bulbs (B.B. in fig. 80), which we will call the blood bulb; this is closed above by a piece of tubing and a clip, a; this is connected by good india-rubber tubing to another bulb, d. Above d, however, there is a stopcock with two ways cut through it: one by means of which B.B. and d may be connected, as in the figure; and another seen in section, which unites d to the tube e, when the stopcock is turned through a right angle. In intermediate positions the stopcock cuts off all communication from d to all parts of the apparatus above it; d is connected by tubing to a receiver, R, which can be raised or lowered at will. At first the whole apparatus is filled with mercury, R being raised. Then, a being closed, R is lowered, and when it is more than the height of the barometer (30 inches) below the top of B.B. the mercury falls and leaves the blood bulb empty; by lowering R still further, d can also be rendered a vacuum. A few drops of

Phosphoric acid is usually employed.

mercury should be left behind in B.B. B.B. is then detached from the rest of the apparatus and weighed, the clips, a and b, being tightly closed. Blood is then introduced into it by connecting the tube with the clip a on it to a cannula filled with blood inserted in an artery or vein of a living animal. Enough blood is withdrawn to fill about half of one of the bulbs. This is defibrinated by shaking it with a few drops of mercury left in the bulb. It is then weighed again; the increase of weight gives the amount of blood which is being investigated. B.B. is then once more attached to the rest of the apparatus, hanging downwards, as in the side drawing in fig. 80, and

the blood gases boiled off; these pass into d, which has been made a vacuum; and then, by raising R again, the mercury rises in d, pushing the gases in front of it through the tube e (the stopcock being turned in the proper direction) into the eudiometer E, which has been filled with, and placed over, mercury. The gas can then be measured and analysed.

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 or blood gases: 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 is first charged with acidulated water up to the zero mark by raising the filling bulb A, tap 1 being open. It 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 filling bulb until

Tap 2 is closed,

B, containing strong caustic potash solution, by raising the the fluid has risen to the zero mark of the measuring tube. 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. The 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 in water, and measuring the loss of volume (due to

absorption of oxygen) in the air when drawn back into the tube. The remaining gas is nitrogen.

KJELDAHL'S METHOD OF ESTIMATING NITROGEN

This simple method can be used in connection with most substances of physiological importance. Briefly, it consists in converting all the nitrogen present into ammonia by means of sulphuric acid; then rendering alkaline with soda, and distilling over the ammonia into standard acid, the diminution in acidity of which measures the amount of ammonia present.

The following modification of the original method is used in this laboratory.

About 1 gramme of the substance under investigation (or in the case of urine when one wishes to make an estimation of total nitrogen, 5 or 10 c.c. of that fluid) is placed in a round bottomed Jena flask of about 250 c.c. capacity, and 20 c.c. of pure sulphuric acid added. Six grammes of potassium sulphate and about half a gramme of copper sulphate are also added. The flask should be provided with a loose balloon stopper, and arranged in a sloping direction over a small flame. The mixture is heated slowly until it boils. In about twenty minutes the fluid becomes nearly colourless; boiling is continued for another forty-five minutes. By this time all the nitrogen will be in combination as ammonia.

After cooling, the fluid is washed into a litre flask of Jena glass (fig. 82, A) and water added until the total volume of the fluid is about 400 c.c. Add then an excess of 40 per cent. caustic soda solution,

a few pieces of granulated zinc to avoid bumping in the subsequent distillation, and immediately fit the glass tube B into the neck of the flask by means of a wellfitting rubber stopper. The other end of B leads into the flask C which contains a measured amount (50 or 100 c.c.) of standard sulphuric acid; normal acid is a convenient strength to use. The bulb D shown in the figure guards against regurgitation, and the end of the tube should dip just below the surface of the acid in C. The mixture in the flask is now boiled for about half an hour when all the ammonia will have distilled over; the use of a condenser around the tube B is unnecessary. The acidity of the standard acid is then determined by titrating with standard alkali, a few drops of methyl orange being added to act as the indicator of the end of the reaction (this gives a pink colour with acid, yellow with alkali).

FIG. 82.Kjeldahl's method: distilling apparatus.

Example. Suppose 1 gramme of a nitrogenous substance is taken, and the ammonia distilled over into 100 c.c. of normal sulphuric acid (= 20 c.c. normal acid). This is then titrated with a corresponding solution of soda, and it is found that the neutral point is reached when 60 c.c. of the soda solution have been added. The other 40 c.c. must therefore have been neutralised by the ammonia derived from the substance under investigation. This 40 c.c. of acid=8 c.c. of normal acid = 8 c.c. of normal ammonia = 8 × 0·017 = 0·136 gramme of ammonia. One gramme of the substance analysed, therefore, yields 0.136 gramme of ammonia, and this contains 0.112

gramme of nitrogen; 100 grammes will therefore contain 11.2 grammes of nitrogen. If the strength of the acid is that just recommended, each c.c. corresponds to 0·0028092 of nitrogen.

The investigations of physical chemists during recent years have given us new conceptions of the meaning of the words that stand at the head of this article. I propose to state what these new conceptions are, and briefly to indicate the bearing they have on the elucidation of physiological problems.

Solutions. Water is the fluid in which soluble materials are usually dissolved, and at ordinary temperatures it is a fluid, the molecules of which are in constant movement; the hotter the water the more active are the movements of its molecules, until, when at last it is converted into steam, the molecular movements become much more energetic. Perfectly pure water consists of molecules with the formula H2O, and these molecules undergo practically no dissociation into their constituent atoms, and it is for this reason that pure water is not a conductor of electricity.

If a substance like sugar is dissolved in the water, the solution still remains incapable of conducting an electrical current. The sugar molecules in solution are still sugar molecules; they do not undergo dissociation.

But if a substance like salt is dissolved in the water, the solution is then capable of conducting electrical currents, and the same is true for most acids, bases, and salts. These substances do undergo dissociation, and the simpler materials into which they are broken up in the water are called ions. Thus if sodium chloride is dissolved in water, a certain number of its molecules become dissociated into sodium ions, which are charged with positive electricity, and chlorine ions, which are charged with negative electricity. Similarly a solution of hydrochloric acid in water contains free hydrogen ions and free chlorine ions. Sulphuric acid is decomposed into hydrogen ions and ions of SO. The term ion is thus not equivalent to atom, for an ion may be a group of atoms, like SO4, in the example just given.

Further, in the case of hydrochloric acid, the negative charge of the chlorine ion is equal to the positive charge of the hydrogen ion; but in the case of the sulphuric acid, the negative charge of the SO, ion is equal to the positive charge of two hydrogen ions. We can thus speak of monovalent, divalent, trivalent, &c. ions.

Ions charged with positive electricity are called kat-ions because they move towards the kathode or negative pole; those which are charged with negative electricity are called an-ions because they move towards the anode or positive pole. The following are some examples of each class :—

Kat-ions. Monovalent :-H, Na, K, NH4, &c.

Divalent: Ca, Ba, Fe (in ferrous salts), &c.

Trivalent: Al, Bi, Sb, Fe (in ferric salts), &c. An-ions. Monovalent: Cl, Br, I, OH, NO3, &c. Divalent:-S, Se, So1, &c.

Roughly speaking, the greater the dilution the more nearly complete is the dissociation, and in a very dilute solution of such a substance as sodium