DRYING

A water-oven at the temperature of 100°, or, better still, an air-bath 10° or 20° higher, and the temperature kept constant by a gas regulator, may be used for drying filter-papers and organic substances generally. Crucibles may be

FIG. 4.--Hot-air oven with gas regulator (G). (Gschleidlen.)

readily and quickly dried by holding them with tongs for a few seconds in a Bunsen flame.

A flask is dried by warming it and then sucking the air from the interior with a long glass tube dipping into it.

COOLING AFTER DRYING

Substances must not be weighed hot, otherwise air currents are set up which disturb a delicate balance. They must not be allowed to dry in the air, or they (especially if hygroscopic) become

moist again. They are generally cooled in an exsiccator-a closed glass vessel containing a tray of sulphuric acid. See figure 5.

A filter is usually allowed to cool and is weighed between two watchglasses clipped together, or in a thin wide-mouthed glass bottle. The bottle or watch-glasses, however, must be dry, and cooled before weighing, in an exsiccator. After weighing any substance that has been dried, it is again heated to 110° for some hours, cooled and weighed as before; the process being repeated until two

successive weighings give the same result--that is, till there is no more loss of

weight from evaporation of water. This is called weighing to constant weight. Exsiccators may be used for drying in vacuo at a low temperature such substances as would be injured by the application of heat. In such cases the top of the bell jar of the exsiccator is connected by a tube to an air-pump. The tube should be fitted with a stop-cock. The air is thus exhausted either by an ordinary air-pump or by a water air-pump constructed on the same principle as that already described (fig. 2). When the vacuum is as complete as possible the stop-cock just mentioned should be closed and the pump can be detached. Moisture rapidly passes off from the substance which is to be dried, and is absorbed by the sulphuric acid.

INCINERATION

The substance to be incinerated must be dry and must not touch the side of the crucible more than is absolutely necessary. A crucible of known weight is placed upon a triangle over a Bunsen flame and at first heated very cautiously, or the contents are apt to froth and be partially lost. The heat is gradually increased, and ultimately the flame is allowed to surround the crucible, which should be tilted. The process, which is a long one with porcelain crucibles, is allowed to continue till the ash is white, when it may be cooled and weighed. Rose's method is better than the preceding, and is as follows:

The dry substance is carefully carbonised in a crucible over a Bunsen flame. After cooling the contents are heated with distilled water again and again to dissolve all soluble salts. The hot aqueous extracts are mixed and filtered through a small filter of known ash. The insoluble matters together with this small filter are dried at 110° and ignited at a red heat; when the residue is white the crucible is cooled and weighed; this gives, subtracting the weights of the crucible and filter ash, the weight of the insoluble salts. Either in the same crucible or in a separate one the aqueous extract is evaporated to dryness, dried at 110°, and ignited at a red heat; the crucible is then cooled and weighed; the increase in weight is the amount of soluble salts.

EVAPORATION

The usual temperature employed is 100°, which is most easily obtained with a water-bath; for lower temperatures, a water-bath is kept at a constant temperature by a gas regulator.

Ethereal or alcoholic solutions must never be evaporated over a naked flame, but over a water-bath. In heating glass vessels it is also advisable to interpose a flat iron plate, or a piece of wire gauze, or asbestos cardboard, or a sand-bath between the glass and the flame.

BOILING

The boiling point of a liquid is that temperature at which the liquid becomes no hotter, but at which any further heat is used up in converting the liquid into vapour. If the liquid, however, be enclosed in a vessel so that no vapour can escape, its temperature will continue to rise. Hence it is possible to heat aqueous solutions above 100° C. by enclosing them in sealed tubes or a Papin's digester, and placing these in a liquid such as oil, which boils at a higher temperature than water.

1 The ash of a filter may be ascertained by incinerating, say, 12 similar filters, weighing the ash, and obtaining the average by dividing by 12.

Sometimes one requires to heat a liquid for a long time without its losing much of its bulk; a long glass tube or a condenser is then attached by a cork with a hole in it to the neck of the flask, the vapour condenses in the tube and runs back into the flask.

DISTILLATION

Some substances are much more volatile-that is, boil at a lower temperature -than others. Advantage is taken of this to separate such substances by a process of distillation.

A distillation apparatus consists essentially of a boiler and a condenser. The boiler may be a flask or a retort closed with a cork through which a tube passes; the tube leads to the condenser, which in the form commonly used (Liebig's) consists of a long tube surrounded by an outer tube; cold water is made to circulate between the two tubes, and thus the vapour which passes along the inner tube is condensed, and the fluid so formed is collected at its far end.

A thermometer is fixed into the retort through the cork at its summit; fractional distillation consists in collecting the substances in separate vessels that distil over at different temperatures.

DIALYSIS

If a solution of albuminous, gelatinous, or mucilaginous substances, mixed with saline and crystalline substances be placed in a dialyser, in distilled water, it will be found that the crystalline substances pass through the parchment membrane into the water, while the proteid or gelatinous substances remain in the dialyser. The substances which pass through membranes in this way are

generally crystallisable, and were termed crystalloids by Thomas Graham.

The

substances which are indiffusible are generally non-crystalline (a striking excep

tion to this rule, however, is hæmoglobin), and were termed colloids by Graham. The distinction is generally stated to be due to the large size of the molecules of colloid materials, rendering them unable to pass through the membrane.

The forms of dialyser employed are depicted in the accompanying figures. That in fig. 7 is the more convenient, as by its use a larger surface is exposed to the action of water, and so the time necessary for diffusion is lessened.

b

FIG. 8.-Dutrochet's Endosmometer. A, glass cylinder constructed so that an organic membrane (piece of bladder) a, b, can be tied over the lower end by the ligature i, i. The tube C is passed

through a cork B, fitting tightly into the upper constricted end of the cylinder. D a scale attachel to C divided into millimetres. (Gschleidlen.)

In dialysing to get rid of salts from organic material, as long a time as four to seven days is generally necessary; different salts vary a good deal as to the rate at which they pass out. The distilled water in the outer vessel should be changed frequently; or, better still, a stream of water running from the tap should be kept continually flowing for three to four days, and then distilled water be used for the last few days of the operation; this should be contained in a large vessel, and changed three or four times a day.

Occasionally one dialyses into other liquids than water; e.g. in Haycraft's method of estimating urea in blood and similar liquids, one dialyses into alcohol.

Taking distilled water as a standard, the rate and amount of diffusibility may be measured by an endosmometer. It will be found that a constant relation exists between the weight of water which passes in one direction, and that of salt which passes in the other. The weight of water necessary to replace by diffusion one gramme of the dissolved substance is called the endosmotic equivalent of that substance, which in its turn depends on

the nature of the substance and its concentration. The following table gives the endosmotic equivalent of certain materials:

Thus 4 grammes of water would pass through the membrane into the endosmometer for 1 gramme of sodium chloride, 11 for 1 gramme of sodium sulphate, and so on. Sometimes negative osmosis occurs, i.e. more of the substance passes out of the osmometer than water passes in; this is the case with acids. The rate of osmosis increases with the concentration of the substance with the temperature of the liquids used. There is also no doubt that the nature of the membrane affects osmotic action; different varieties of dead membrane affect the rate of osmosis; the osmosis that occurs in living membranes is also no doubt very different again, but is a difficult subject to investigate experimentally. A

living membrane is not fixed or stable, but is constantly undergoing processes of building up and breaking down.' Thus the discussion whether the formation of lymph is due to filtration or diffusion of the blood plasma through the vessel walls has not yet received a satisfactory answer. The question is still further complicated in the living body by the fact that the fluids on the two sides of any membrane are almost invariably at different pressures; and in addition it is possible that there may be some kind of attractive

influence exerted by the tissues themselves, analogous to the selective activity of secreting cells.

1 In a recent paper, Prof. Waymouth Reid (Brit. Med. Journ. vol. i. 1890, p. 165) brings out very clearly the difference as regards diffusion between dead and living membranes. The membrane he experimented with was the skin of the frog. As he points out, we have doubtless in a living membrane to deal with an absorptive force dependent on protoplasmic activity, and comparable to the excretive force of a gland cell. This is excited especially to make osmosis take place more readily in one direction than in the other; in the case of the frog's skin from without in.

? A discussion on this subject will be found in Foster's Physiology, vol. ii. p. 503, 5th edit., 1889.