sand as the unit of measure, ranges from 120 to 4,000 pounds; and the size of pulleys range

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tions: (1) To expand and rarify the air so that the coarser particles will fall out of the current and allow it to deliver the impalpable powder at the discharge spout. (2) When a large output of the finely ground material is required per hour, the apparatus should be capable of using an amount of air sufficient to lift the total weight of the charge of raw material. (3) When using a large volume of air, sufficient room should be provided for its expansion and rarefication so as to obtain a current light enough to carry away the impalable

FIG. 12.

from 24 x 4 inches to 36 x 12 inches in diameter. These machines require from one to eighteen horsepower to drive them in drygrinding, and from one-half to nine horsepower, in wet-grinding, at cylinder speeds ranging from 44 to 13 revolutions per minute.

Fig. 10 illustrates a tube mill of the ordinary type. The general principle of grinding employed is similar to that of the pebble mills, but with the difference that the material to be ground in the tube mill is fed at one end, and delivered as a finished product at the other end, its fineness being regulated simply by the speed at which the material is fed into the machine. As every particle of the material must pass under the grinding action of the entire charge of pebbles, a thorough and uniform grinding is the result, and the use of sieves is unneces

sary.

Fig. 1 shows a vertical cross-section of an "Abbe" ball mill.

Jar-mills, consisting of porcelain jars in which pebbles are rotated together with the material to be ground, are extensively used for the grinding of paints, ink, and other chemical compounds. They are composed of one or more grinding jars. Fig. 12 shows a machine of the single jar type. Its outside dimensions are about 12 x 13 inches; it uses a charge of porcelain balls weighing about 22 pounds; and is capable of grinding up to 15 pounds at a charge, when running at 50 revolutions per minute.

Impact Pulverizing Machines are represented by the "Max" mills and various forms of "Raymond" pulverizers. The principle employed is that of percussion, the working device consisting of a vertical shaft attached to a carrier provided with hardened steel beaters. As the material is fed into the mill it is thrown by centrifugal force against a hard iron plate, the particles at the same time being thrown into violent contact with each other. Equipped with vacuum air separators they are extensively and economically used for the reduction of all kinds of dry colors and chemicals, and for the threshing, cleaning, and separating of tobacco stems.

The mechanical apparatus required to effectively apply this system of air separation should be capable of satisfying the following condi

powder only. (4) The apparatus should be so constructed that the coarse particles will fall out of the light air current by gravity into the contracted portion of the separator where the blast is stronger, and thus pass out through the tailing spout or into the pulverizer to be reground, without being accompanied by any of the fine powder. (5) The air space within the apparatus should approach as near as possible to a perfect vacuum.

As the fineness of the product depends upon the size of the separator, almost any limit of grinding is readily obtained by making the apparatus of sufficient size to produce the proper relative expansion and rarefication of the air. When it is desired that the finished product should be an extremely fine powder, the use of

chines with fan and dust collector attachments are the most effective.

Fig. 13 illustrates a Raymond impact pulverizer and vacuum separating plant as erected for the cleaning and separating of tobacco stems. The material is fed to the machine by the operator in a manner similar to the feeding of a threshing machine. The perforations of the screen surrounding the pulverizing cylinder vary in size in proportion to the fineness desired. As the stems come in contact with the rapidly revolving beaters, the particles of the leaf are liberated therefrom, and passing through the screens with the stems are caught by the air current and drawn up into the separator, which expands the air so that all the light pieces of stems drop back and are discharged through the opening at the bottom into the drag elevator. The particles of leaf drop into the centre cone of the separator and discharge from the spout, while the dust is carried through the fan into the dust collector and discharged therefrom.

Fig. 14 shows the general arrangement of a type of pulverizing machinery called shredders, which are extensively used in tanneries, paper mills, chemical works, tobacco factories, and extract plants. The shredding of the material is accomplished by the wearing action of the hammers attached to the revolving shaft, against the teeth on the inner surface of the cylinder. For the separation of material reduced to a fine powder an exhaust fan carries the material through the outlet to the receiving receptacle.

Arrastra Machinery.- Arrastra plants are

employed for the reduction of metallic ores into a fine pulp which is subsequently treated by the patio or some other process and the pure metal separated from the accompanying impurities. The bottom of the arrastra is paved with stones 10 x 10 inches square, made of the best granite and set vertically to the depth of about three feet in the ground. The outside is constructed of flat stones which project from eighteen to thirty inches above the floor. Line shafts driven by engine power extend over this floor and operate poles or arms which revolve and move the "dragging stones" over the granite floor and thus accomplish the reduction of the ore fed into the arrastra to a fine pulp. These stones are made of the best granite, and range in weight from 11,000 to 25,000 pounds. The ore is first prepared by being passed through the jaw-crushers of a primary crushing plant. From these machines it passes through a set of crushing rolls and is dropped automatically into revolving screens which separate the fine pulp from the coarser grindings. The last-named material is sent back and passed again through the crushing rolls until all of the pulp is fine enough to be finally treated in the arrastra. Each arrastra requires about six horsepower for its operation, and range in crushing capacity from six to ten tons each, per twenty-four hours, according to the character of the ore.

For other forms of crushing and grinding machinery, and their special application, see articles under the titles BRICK MAKING MACHINERY, COAL MINING MACHINERY, and MINING AND MILLING MACHINERY, in this Encyclopædia.

WILLIAM MOREY, JR., C. E., Consulting Civil and Mechanical Engineer, New York City.

Crusius, Christian August, German theologian and philosopher: b. Leuna, near Merseburg, 10 Jan. 1715; d. Leipsic 18 Oct. 1775. He received his education at Leipsic, where later he became professor of theology, a position which he held until his death. He vigorously attacked the philosophy of Leibnitz and Wolf as being incompatible with the dogmas of the Christian Church, and at the same time put forth his own philosophy, which he claimed to be perfectly orthodox. His philosophy subordinated the scholastic principle of contradiction to that of conceptibility; gave to the soul faculties and liberties almost as limitless as those of the Deity; propounded psychology as the basis for logic; and regarded time and space as modes of divine existence and not as substances. Among his works the most important are: 'Entwurf der nothwendigen Vernunftwahrheiten) (1745); Logik, oder weg zur Gewissheit und Zuverlässigkeit der menschlichen Erkenntniss (1747); Anleitung über natürliche Begebenheiten ordentlich und vorsichtig zu denken' (1774).

Hanover 20 Dec. 1857. He has been a professor Crusius, Otto, German philologist: b. in the University of Heidelberg and has written: Untersuchungen zu den Mimiamben des Heron'Zur griechischen Religiousgechichte) (1886); das) (1892); and has also edited Philologus: Herondas' (1901); 'Fabeln des Babrius' (1897); Griechische Lyriker) (1897); etc.

Crustacea, krus-tā'shě-a, a primary group (phylum) of animals represented by the bar

nacle, lobster, crayfish, shrimp, and crab. Crustacea differ from other arthropod animals. The body consists of about 20 segments which in the more specialized forms are grouped into two regions, the head-thorax (cephalothorax) and hind-body or abdomen. The segments of the cephalothorax are fused together so that the limits between the segments are lost, and the whole mass is protected by the shield or carapace. The skin is thick and rendered solid by the deposition of lime (carbonate and phosphate), so that the integument forms a dense crust, hence the name Crustacea. They differ from trilobites and king crabs (qq.v.) in having two pairs of antennæ, while they breathe by means of gills attached to the legs. Like the other marine arthropods named, they have legs which are divided into two divisions, an outer (exopodite) and an inner (endopodite). Crustacea differ from the Paleopoda also (trilobites, merostomes and arachnids) in the high degree of specialization of their appendages, there being from three to six kinds, with corresponding functions, while in the trilobites, so far as we know, the single pair of antennæ are succeeded by numerous (over 20) pairs of legs, all of the same shape and functions. In the head-thorax, besides the antennæ, there is on each side of the mouth a pair of mandibles, each with a palpus, two pairs of maxillæ or accessory jaws, which are flat, divided into lobes, and of unequal size; three pairs of footjaws (maxillipedes), which differ from the maxillæ in having gills like those on the five following pairs of legs. There are thus 13 pairs of cephalothoracic appendages, indicating that there are 13 corresponding segments; these, with the seven abdominal segments, indicate that there are 20 segments in a typical crustacean. There are six pairs of swimming legs (swimmerets), the last very broad in the lobster and shrimp, with the telson forming the "tail-fin."

The Crustacea as a rule respire by gills. These, as in the lobster and crab, are composed of a series of little filaments into which the blood flows to be aërated. The filaments branch out from a common stalk which grows out of the basal joint of the five pairs of legs and the three pairs of foot-jaws. These gills are folded up toward the back, and are contained in a sort of chamber made in part by the carapace. In shrimps, lobsters, and crabs the sea-water passing into the cavity between the body and the free edge of the carapace is afterward scooped out through an opening or passage on each side of the head by the movements of membranous flaps called "gill-bailers." The digestive organs are well developed, especially the fore stomach, in the hinder part of which are several very large calcareous teeth for crushing the food, serving, when closed together, as a trainer through which the partly digested food presses into the long slender straight intestine, which ends in the telson. The liver is very large, as in all marine arthropods, or in such terrestrial tyres as the scorpions and spiders, which are derived from the king crabs. The brain of the higher Crustacea is very complex, corresponding with the complicated reflex movements of an animal composed of so many segments, and bearing such a complicated series of appendages devoted to so great a variety of functions. The eyes are usually compound or many-faceted, and are mounted on freely mov

able stalks. The ear is a sac in the basal joint of the smaller or second pair of antennæ. The organs of smell are usually well developed, as Crustacea mainly depend on this sense in finding their food. These consist of minute delicate sensory rods on the smaller antennæ. The hairs fringing the mouth-parts and legs are often delicate tactile organs. The green glands in the head function as kidneys, and open out at the base of the larger antennæ,

With only a single known exception (Squilla), Crustacea carry their eggs about attached to the swimming or other legs. The eggs of some crabs (Neptunus) are minute and excessively numerous, their number amounting to millions, while the lobster may produce from 20,000 to 80.000 eggs. Crustacea pass through a well-marked metamorphosis, nearly all (except the amphipods and isopods) hatch from the egg as a larva called a "nauplius," which has an oval non-segmented body, with three pairs of appendages, by which it swims about at the surface of the sea. After a series of molts, at each of which new segments with their appendages arise, they finally reach maturity. The shrimps and crabs hatch in a more advanced larval stage called the zoca, the nauplius stage being partly suppressed and thrust back into the embryo period. The zoea has a head and abdomen, but no thorax: this, however, is developed later, and after a series of molts the parent form is attained.

The process of molting is a precarious one, not infrequently resulting in death. The crust being too solid to admit of a continuous growth, and increase in size being rapid, frequent sheddings of the skin are necessary. In the lobster, the old skin being detached from the under cellular layer by the secretion of the new cuticle beneath, it ruptures between the thorax and abdomen, and the lobster draws itself out of the rent, shedding not only the entire skin and every hair, but also the lining of the mouth, throat, and fore stomach, and likewise the end of the intestine. In about three weeks after the casting of the shell the new one becomes solid and hard. In the crayfish the old skin is loosened and pushed away from the cellular layer beneath by the growth of temporary, short stiff hairs, which disappear after the skin is shed.

The Crustacea are a very ancient type. The earliest remains are found in the Cambrian rocks, but are very scanty compared with the trilobites. They comprise traces of barnacles, Ostracoda or small shelled forms, Phyllocarida, and an obscure form supposed to be allied to the modern freshwater Apus. In the Devonian Period shelled phyllopods (Estheria) appeared, while in the Carboniferous arose an order (Syncarida) represented by an ancient form (Anaspides) still living in a lake in Tasmania. From this group the existing Schizopoda or "opossum shrimps" (Mysis), the Squilla, and the ordinary shrimps and crabs, are supposed to have descended. Isopoda also appeared as early as the Devonian. A shrimp-like Crustacean occurs in the Devonian, and true crabs date from the Jurassic.

The Crustacea are divided into II orders, the Branchiopoda, Phyllopoda, Ostracoda, Copepoda, Cirrepedia or barnacles, Arthrostraca, Cumacea, Phyllocarida, Syncarida, Schizopoda, Stomatopoda, and Decapoda, There are over