CHALKS 



71 



3. Dragon's blood in powder. 



4. Dragon's blood in the tear (Sanguis Draconis in granis). 



5. Lump Dragoris blood (Sanguis Draconis in massis). 



Besides these, there are Dragon's blood in cakes and False Dragon's blood in oval 

 masses. 



DRAINING TILES. Burnt clay tiles, generally shaped in section like a 

 horse-shoe, about one foot long, and two or three inches broad. These are much used 

 in agricultural draining. See STONE-WAKE. 



DRAWING CHAIiKS. Chalks or crayons are frequently nothing more than 

 the natural production reduced to a convenient form : they are, however, sometimes 

 prepared artificially ; a few of these manufactures are named. 



The Brothers Joel, in Paris, employ as crayon cement the following composition : 

 6 parts of shellac, 4 parts of spirit of wine, 2 parts of turpentine, 12 parts of a 

 colouring powder, such as Prussian-blue, orpiment, white lead, vermilion, &c., and 

 12 parts of blue clay. The clay being elutriated, passed through a hair sieve, and 

 dried, is to bo well incorporated by trituration with the solution of the shellac in the 

 spirit of wine, the turpentine, and the pigment ; and the doughy mass is to be pressed 

 into proper moulds, so as to acquire the desired shape. They are then dried by a 

 stove heat. 



In order to make cylindrical crayons, a copper cylinder is employed, about 2 inches in 

 diameter and 1^ inch long, open at one end, and closed at the other with a perforated 

 plate, containing holes corresponding to the sizes of the crayons. The paste is 

 introduced into the open end, and forced through the holes of the bottom by a piston 

 moved by a strong press. The vermicular pieces that pass through are cut to the 

 proper lengths, and dried. As the quality of the crayons depends entirely upon the 

 fineness of the paste, mechanical means must be resorted to for effecting this object in 

 the best manner. The following machine has been found to answer the purpose 

 exceedingly well. 



Fig. 638 is a vertical section through the centre of the crayon mill. Fig. 639 is a 

 view of the mill from above. A, the mill tub, whose bottom B must be a hard flat 

 plate of cast iron ; the sides A being of wood or iron at pleasure. In the centre of the 

 bottom there is a pivot c, screwed into a socket cast upon the bottom, and which may 

 be strengthened by two cross bars D, made fast to the frame E. F, the millstone of 



641 



cast iron, concave, whose diameter is considerably smaller 639 



than that of the vessel A ; it is furnished within with a 



circular basin of wood G, which receives the materials to be 



ground, and directs them to the holes H, which allow them 



to pass down between the under part of the mullor and the 



bottom of the tub, to undergo trituration. 



By the centrifugal motion, the paste is driven towards the 

 sides of the vessel, rises over the sides of the muller, and 

 comes again through the holes H, so as to be repeatedly 

 subjected to the grinding operations. This millstone is 

 mounted upon an upright shaft i, which receives a rotatory motion from the bevel- 

 work K. driven by the winch L. 



The furnace in which some kinds of crayons, and especially the factitious black- 

 lead pencils, are baked, is represented in Jig. 640, in a front elevation ; and in Jig 641, 

 which is a vertical section through the middle of the chimney. 



A A, six tubes of greater or less size, according as the substance of the crayons is a 



