into a mass forming butter. Good milk, when perfectly fresh, is always feebly alkaline; when left to itself, however, it soon becomes sour, and is found to contain lactic acid. The quantity and quality of milk vary. We give below a statement of the composition of cow's-milk, from an analysis made by Haidlen:

566. Gelatine.-Animal membranes, skin, tendons, and even bones, dissolve in water at a high temperature, more or less completely, but with very different degrees of facility, giving solutions which on cooling acquire a soft-solid, tremulous consistence. The substance so produced is called gelatine. It does not preëxist in the animal system, but is generated from the membranous tissue by the action of hot water. Cut into slices and exposed to a current of dry air, it shrinks much in volume, forming a transparent, glassy, brittle mass soluble in hot water. The aqueous solution is precipitated by alcohol, tannin-solution, and many other substances. Gelatine is largely employed as an article of food, and in manufactures as size" and "glue." The cartilages of the joints, the cornea of the eye, and the ribs, yield a gelatine, called, by way of distinction, chondrine.

66

Chitine, C,H,,HO, constitutes the skeletons of insects and crustacea. It is a white substance which retains the

form of the texture from which it is obtained; the word chitine means a mantle.

567. Urea, CH ON,, is one of the chief solid constituents of urine, from which it may be obtained. It is also produced by heating ammonic cyanate (H,N,CNO), with which it is isomeric. Urea crystallizes in transparent colorless prisms soluble in water and alcohol. It is inodorous, and has a cooling, saline taste. By heat it is decomposed into ammonia, ammonic cyanate, and cyanuric acid. This compound is of special interest as the first organic compound artificially produced.

11

568. Creatine, CH12ON,.—Creatine occurs in the juice of flesh. When pure, it forms colorless brilliant prismatic crystals, readily soluble in hot water. The aqueous solution has a slightly bitter and acrid taste, and a neutral reaction. It forms no salts with acids. By the action of strong acids, it is converted into creatinine.

569. Pepsine, is a nitrogenous substance contained in the gastric juice, and has never been perfectly isolated. It is the active agent concerned in digestion. An artificial gastric juice which acts upon albuminoid substances is obtained by digesting the mucous membrane of the stomach (usually of a pig) with a warm, dilute solution of hydric chloride.

570. Hæmoglobine, also called hæmato-crystalline, is made up of carbon, hydrogen, iron, nitrogen, oxygen, and sulphur, and forms the chief part of the red globules of the blood of vertebrate animals. Usually it is amorphous, but from some animals it can be separated in crystalline form. Dilute solutions of this substance may be heated to 160° F. without marked change, but if the heat is continued the hæmoglobine is disorganized and splits up into hæmatine, CHN1,Fe,O,,, and coagulated albumen. Alcohol also decomposes it.

571. Putrefaction.-The exact character of the fermentation which takes place when animal bodies putrefy is but little known. Among the products of these changes are hy

drogen, nitrogen, carbonic dioxide, ammonia, hydrogen carbides, sulphides, and phosphides. The gaseous combinations of sulphur and phosphorus are the chief causes of the offensive odor of putrefying bodies. As the presence of moisture, a favoring temperature, and access of air, are essential conditions of putrefaction, if any of them are withdrawn, the effect is prevented. It is well known that the most perishable organic substances, both vegetable and animal, may be indefinitely preserved by drying. Cold checks decomposition, and it is entirely arrested by freezing. So, if the prime inciter of change, the air with its floating organic germs, is excluded, putrefaction cannot take place. This fact is illustrated by the general practice of preserving all kinds of alimentary substances, meat, fruits, and vegetables, in vessels which exclude the air. It is not enough, however, to exclude these agents from the surface of fermentable bodies, the germs which have already been absorbed must also be destroyed. This may be done by a sufficient. elevation of temperature. In some cases boiling is effectual, in others a much higher temperature is required.

572. Ferment-Diseases. The foul accumulations of neglected towns, and the decomposing organic matter of many swampy districts, give off invisible emanations known as miasms and malaria, which fill the air, and often occasion fatal epidemics. Of their composition, nature, or mode of action, nothing very definite is known, but it has been held that the effects produced by them are due to the presence of a large quantity of ferment-germs which, being inhaled, develop and induce a condition somewhat similar to fermentation in the living system. Intermittent fever, typhoid fever, and cholera, have been ascribed to this cause. Various other diseases, as small-pox, hydrophobia, etc., have also with much reason been considered as consisting in processes of fermentation running their course in the living organism.

9 =

10 =

=

18=

26=

23

40 =

|| || || || || || || || || || || || || || || || || || || || || || || | || || || || || | || || || || ||

+ 21.2 19.4

=

17.6

15.8

14

=

12.2

=

10 4

3.2

0.4

2.2

4

5.8

7.6

9.4

11.2

13

14.8

16.6

18.4 20.2

22

23.8

25.6

27.4

29.2

31

32.8

34.6

36.4

38.2

40

It is often necessary to convert temperatures expressed on the Fahrenheit scale into the corresponding temperatures on the Centigrade scale, and the following rules will assist the pupil in transforming one scale into the other:

(1.) To convert Fahrenheit Degrees into Centigrade Degrees.-Subtract 32 from the number of degrees, and multiply the remainder by ž (or 0.5).

(2.) To convert Centigrade Degrees into Fahrenheit Degrees.-Multiply the number of degrees by (or 1.8), and add 32 to the product.