The lactic acid formed causes the separation of the casein, thus giving the well-known appearance of sour milk. ~Isomeric compounds.~ It will be observed that cane sugar and milk sugar have the same formulas. Their difference in properties is due to the different arrangement of the atoms in the molecule. Such compounds are said to be isomeric. Dextrose and levulose are also isomeric. ~Dextrose~ (_grape sugar, glucose_) (C_{6}H_{12}O_{6}). This sugar is present in many fruits and is commonly called grape sugar because of its presence in grape juice. It can be obtained by heating cane sugar with dilute acids, as explained above; also by heating starch with dilute acids, the change being as follows: C_{6}H_{10}6_{5} + H_{2}O = C_{6}H_{12}O_{6}. Pure dextrose is a white crystalline solid, readily soluble in water, and is not so sweet as cane sugar. In the presence of yeast it undergoes alcoholic fermentation. It is prepared from starch in large quantities, and being less expensive than cane sugar, is used as a substitute for it in the manufacture of jellies, jams, molasses, candy, and other sweets. The product commonly sold under the name of _glucose_ contains about 45% of dextrose. ~Levulose~ _(fruit sugar)_(C_{6}H_{12}O_{6}). This sugar is a white solid which occurs along with dextrose in fruits and honey. It undergoes alcoholic fermentation in the presence of yeast. ~Cellulose~ (C_{6}H_{10}O_{5}). This forms the basis of all woody fibers. Cotton and linen are nearly pure cellulose. It is insoluble in water, alcohol, and dilute acids. Sulphuric acid slowly converts it into dextrose. Nitric acid forms nitrates similar to nitroglycerin in composition and explosive properties. These nitrates are variously known as nitrocellulose, pyroxylin, and gun cotton. When exploded they yield only colorless gases; hence they are used especially in the manufacture of smokeless gunpowder. _Collodion_ is a solution of nitrocellulose in a mixture of alcohol and ether. _Celluloid_ is a mixture of nitrocellulose and camphor. _Paper_ consists mainly of cellulose, the finer grades being made from linen and cotton rags, and the cheaper grades from straw and wood. ~Starch~ (C_{6}H_{10}O_{5}). This is by far the most abundant carbohydrate found in nature, being present especially in seeds and tubers. In the United States it is obtained chiefly from corn, nearly 80% of which is starch. In Europe it is obtained principally from the potato. It consists of minute granules and is practically insoluble in cold water. These granules differ somewhat in appearance, according to the source of the starch, so that it is often possible to determine from what plant the starch was obtained. When heated with water the granules burst and the starch partially dissolves. Dilute acids, as well as certain enzymes, convert it into dextrose or similar sugars. When seeds germinate the starch present is converted into soluble sugars, which are used as food for the growing plant.