2. To a solution of glycogen add a drop of dilute iodin solution. Observe the reddish-brown color of the liquid. Heat the tube. Note the disappearance of the color. It reappears on cooling. Compare with the behavior of starch.

3. Test a solution of glycogen with Fehling's solution. Does it reduce? Heat the glycogen solution with a drop of acid. Test the reducing power, after neutralizing the solution. What has been formed? Compare with the behavior of starch.

Dextrin is formed by heating starch to 150°-160° C., and also importantly by the action of saliva on starch. It also occurs in several prepared foods, which are made by heating starchy materials to a definite temperature. The dextrins, as found, are scarcely to be considered as chemical individuals. Three forms of dextrins are recognized: erythrodextrin, achroödextrin, and maltodextrin. The first is so called because of the red color which its solutions give with iodin. The second and third yield no distinct coloration with that reagent. Maltodextrin, is formed by the action of malt diastase on starch. The dextrins are the intermediate compounds between the starches and the monosaccharids. All of the dextrins reduce Fehling's solution slightly.

Cellulose forms the framework of practically all plants. It is not found in the higher animals, and its reported occurrence in pathological conditions, in the fibrous growths of tubercular tissue, is still a matter of doubt. It is difficultly soluble in most

solvents. Schweitzer's reagent (an ammoniacal solution of cupric oxid) dissolves it, and it is precipitated unchanged from this solution by water, alcohol, and acids.

Its chief value in the animal economy is to furnish an insoluble bulky material which will encourage peristalsis. One of the most important uses to which cellulose is put in physiological chemistry is the preparation of the parchment paper used in dialysis. Parchment paper is made by immersing ordinary paper in 60 per cent. sulfuric acid for a few seconds, and washing immediately with water till the paper no longer exhibits an acid reaction. Paper so prepared is much tougher than ordinary paper, and does not disintegrate in water. Made up in the form of tubes, it is used for dialyzing, as it allows the salts which are contained in a solution to pass through the very fine interstices of the paper, while substances of high molecular weight, such as the albumins, are prevented from passing through. In this way a solution containing salts and albumins may be freed from the former.

The Polarimeter.-Many of the methods for estimating the carbohydrates quantitatively are based on polarimetric methods.

Each carbohydrate, roughly speaking, has the power of rotating the plane of polarized light either to the right or to the left to a definite extent. If a known quantity of the compound be dissolved in a given weight of water, and be introduced into a tube of definite length, the column of liquid so produced will have the property of rotating the plane

of polarized light through a definite number of degrees. Conversely, if it be found that a column of liquid of the same dimensions be capable of turning the plane of light through a given number of

FIG. 5.-The Schmidt and Haensch polariscope.

degrees, one should be able to calculate the amount of carbohydrate in solution. It is on this principle that many of the methods for the estimation of cane-sugar and of glucose are based.

Although not the most accurate, the most satis

factory polarimeter in use in physiological chemistry, and especially for the estimation of glucose in urine, is the saccharimeter of Schmidt and Haensch. This instrument is so constructed that with a 200 mm. tube the readings on the scale are given as glucose in per cent., and fractions of a per cent. A description of these instruments will be found in any elementary book on physics.

THE FATS.

The fats are found in nearly all the tissues of the body, and are also an important element in the food. Chemically they are the salts, or esters, of oleic, palmitic, and stearic acids, in which the alcohol is glycerol. As this alcohol is triacid and the acids are monobasic, it follows that to form neutral salts 1 molecule of the alcohol is combined with 3 molecules of the acids. As a consequence the fats are often spoken of as triglycerids.

The principal fats and their formulæ are:

While stearin and palmitin when pure are solid at ordinary temperatures, the compound of oleic acid with glycerol is a liquid. At the temperature of the body-37° C.-the mixture of glycerids is liquid, but solidifies when cooled to room temperature. In milk and butter the glycerol is combined with lower fatty acids-caproic, caprylic, and butyric acids. These acids are volatile with steam, while

the higher fatty acids do not come over on distillation with water. It is on this property that the recognition of artificial butter is based, as margarin or "oleomargarin," which is made of animal fats, does not contain the volatile fatty acids which are found in butter.

The higher fatty acids are insoluble in water, sparingly soluble in alcohol, and easily soluble in ether and in chloroform.

The sodium and potassium salts of these acids. are known as the " soaps. When a mineral acid is added to a solution of these soaps the compound is decomposed, and the acid itself is precipitated as a white, tallow-like substance which crystallizes from ether or alcohol in characteristic forms.

On heating the fats with alkalies or with superheated steam they are decomposed, yielding in the former case the alkaline salts of the fatty acids and glycerol, and in the latter case the acids themselves and glycerol. This process is called saponification, and in this way the soaps are made.

The fats themselves are insoluble in water, but under certain conditions, viz., in the presence of small quantities of soaps, they can be finely divided or emulsified. This takes place in the intestine, and in this state the fats are absorbed by the membrane of the duodenum and jejunum, and the process can be imitated in the laboratory. All fats as occurring in nature contain traces of free fatty acids, and these latter in the presence of the alkaline fluids of the bowel are converted into soaps. Through the peristaltic action of the bowel, and