CHAPTER VI. SUBLIMATION. SUBLIMATION is the process of distilling volatile solids. The product is termed a sublimate. The objects of sublimation are-1, to purify volatile solids from admixed and fixed impurities, and, 2, to provide a convenient means of collecting volatile solids resulting from chemical reaction at high temperatures. The retorts or apparatus used may either be of iron, or of glass or stone-ware if the degree of heat necessary will admit of the use of the latter. Sublimation is almost exclusively confined to operations which are conducted by manufacturers on the large scale. A process was formerly official for the sublimation of benzoic acid. It consisted in introducing benzoin into a shallow tinned-iron pan, and pasting over the top a sheet of filtering-paper. A pasteboard hood, shaped like a hatbox, was then fitted to the pan and tied or pasted with paper so that a tight joint was made; the apparatus was placed on an iron plate and subjected to a low but uniform heat: the vapors of benzoic acid passed through the pores of the filtering-paper, were separated from impurities, and, coming in contact with a cooler atmosphere in the hood, slowly condensed, often forming crystals of great beauty. The temperature at which the condensation of the vapor is effected in sublimation has a very important influence in determining the physical character of the sublimate, and two kinds of sublimates are produced: 1. Cake sublimates. 2. Powder sublimates. Cake Sublimates.-If the temperature of the condensing surface and of the air in contact is but slightly below that at which the volatile body is capable of subliming, the particles will be deposited in compact masses, like corrosive sublimate, commercial sal ammoniac, or carbonate of ammonia. Fig. 185 shows a simple apparatus for obtaining FIG. 185. Subliming apparatus. sublimates in cakes or masses. A shallow sheet-iron dish, having its upper edge turned out so that it forms a flat ring, is provided with an earthen-ware cover (it will be usually found more convenient to have the iron dish made to fit the cover than the reverse), through which a hole is drilled to permit the escape of air (this may be done with a three-cornered file). After the substance which is to be sublimed has been placed in the iron dish, a piece of asbestos twine, slightly longer than the circumference of the ring, is laid upon it, and this is covered with a lute composed of equal parts of potters' clay and flaxseed-meal with sufficient water, the earthen-ware cover is pressed upon it until it adheres, and, when necessary, iron clamps are used to make a tight joint. The aperture in the cover is loosely covered with a cone of cardboard, the dish is placed in a sand-bath and gradually heated; the aperture is kept open during the sublimation by occasionally probing it with a glass rod; after cooling, the sublimate will be found adhering to the earthenware top in one cake or mass, and may be removed by a spatula. The earthen-ware top may be replaced by a sheet-iron one in cases in which the former would be likely to be fractured by excessive heat. Powder Sublimates.-If the apparatus for conducting sublimation is so contrived that there is a marked difference between the temperature of the air in contact with the vapor and the subliming-point of the volatile body, the sublimate will be deposited very rapidly and in small particles, like calomel, sulphur, etc. Fig. 186 shows a convenient apparatus for subliming camphor in powder. It is well adapted also for a lecture-room illustration of the FIG. 186. process. A wooden case has two openings made in the sides to admit sheets of glass, which are secured in place by putty in the usual manner. One of the sides has a hinged door, which fits the frame snugly; the opposite side has a tapering circular aperture, which admits the shortened beak of a retort, as shown in the cut. Camphor is placed in the retort, a safety-tube is adjusted in the tubulure, and the retort is then placed deeply in a sand-bath on a good gas stove. Care must be observed in heating at first, and a Bunsen burner should be at hand to heat occasionally those portions of the top of the retort and the beak upon which the sublimate is forming. When the boiling-point is reached, the camphor vapor passes over rapidly, and at once falls in the form of powder upon coming in contact with the cold air in the chamber. The especial points to be observed are care in heating, and watchfulness that the beak of the retort does not become clogged with the sublimate. A judicious use of the Bunsen flame will soon melt the obstruction. Subliming camphor. The most important, and in practice the most difficult, part of the operation of sublimation is the regulation of the heat. The temperature of the condensing surface should always be below the fusing-point of the substance if distinct crystals or crusts are expected. QUESTIONS ON CHAPTERS V. AND VI. DISTILLATION AND SUBLIMATION. DISTILLATION. What are the elements of distillation? How many times its weight of water at 20° C. (68° F.) are required to codense steam at 100° C. (212° F.)? What two forms of apparatus are used in distillation? What is the form of an alembic? What is the body of it called? What is the form of a retort? Has a retort any advantages over an alembic, and if so, what are they? What is a tubulated retort? Of what materials are retorts made? For what purposes are the various kinds used? What are the essential qualities of a good retort? What are the advantages of using a flask for distillation? What is the best shape for a flask? How may glass tubes be cut? How may glass tubes be bent? What is a cork-borer, and how is it used? How may rubber corks be cut? What is the advantage of rubber corks? What substitute for rubber corks may be made? How may a satisfactory lute for closing joints be made? How may bladder be used to join tubes? How may paper be used to join tubes? Is rubber superior to bladder for such purposes? If so, why? How is it used? What are receivers ? What are tubulated and quilled receivers ? What are adapters, and how are they used? What is a Welter's safety-tube? For what purposes are retort-stands used? How may funnels or percolators be protected from the breakage due to contact with the iron rings of the ordinary retort-stand? What is meant by bumping in distillation? How may it be prevented or lessened? What is a Liebig's condenser? Describe the pinchcock contrived by Dr. Squibb. Describe Mohr's spring pinchcock. Describe Hoffman's screw pinchcock. How may vapors be condensed when the ordinary condensers and a steady supply of water are not at hand? What is a condensing worm? What is the best metal to use for making it? What objection is there to copper? What objection is there to iron? What objection is there to tinned iron ? What objection is there to earthen-ware? What is the best material to use for pharmaceutical stills? Describe Procter's still. Describe Wiegand's still. Describe Curtman's still. Describe Prentiss's still. Describe Rice's still and condenser. What is the disadvantage of Liebig's condenser? What is the disadvantage of a worm condenser? Describe Remington's still and condenser. What is meant by fractional distillation? What is meant by destructive distillation? What objection is there to using glass vessels in destructive distillation? SUBLIMATION. What is sublimation? What is the product called? What are the objects of sublimation? Of what material are the retorts or apparatus usually made? What effect does the temperature of the condensation point of the vapors of solids have upon the character of the sublimate? What is the difference between a cake sublimate and a powder sublimate? How are powder sublimates obtained? What is the most important point to be observed in the operation of sublimation? CHAPTER VII. DESICCATION. DESICCATION is the process of depriving solid substances of moisture, and in pharmacy should be effected at as low temperatures as possible. (See Exsiccation.) The objects of desiccating medicinal substances are threefold: 1. To aid in their preservation. 2. To reduce their bulk. 3. To facilitate their comminution. 1. To Aid in their Preservation.-Chemical salts frequently contain water either chemically or mechanically combined with them. An elevation in the temperature, or the absorption of water from moisture present in the air, will in some instances cause deliquescence, whilst in others contact with a dry atmosphere will cause efflorescence, due to the evaporation of chemically-combined water; hence such salts in their natural condition are unstable; they are much more permanent when dried. Vegetable drugs soon decompose or become mouldy if allowed to remain in a moist condition, and desiccation is absolutely necessary to preserve them. 2. To Reduce their Bulk.-If desiccation is performed successfully, -i.e., at properly-regulated temperatures under certain precautions,the substance is merely deprived of water without suffering any loss of medicinal activity, and the reduction in bulk that follows is a practical advantage which results in adding to the strength of the medicinal substance. 3. To Facilitate their Comminution.-The presence of water gives to drugs an elasticity and ability to resist disintegration, which in some cases interfere greatly with the process of bruising, grinding, or reducing the drug to particles. One of the first steps in comminution is to dry the substance thoroughly in order to make it brittle or crisp. The apparatus employed in desiccation is frequently of the simplest character, and the heat is usually not especially created for the purpose, for it is most economical to use the waste heat from kitchen fires or cellar furnaces or the diffused heat in lofts or unused attics. There can be no objection to this if care is taken to provide protection for the substance from dust, light, and injury during desiccation. Herbs may be dried by tying them in bunches and suspending them to the attic ceiling or to the rafters of a barn during summer weather, and this is an excellent method usually, notwithstanding its slowness, because there is no danger of the heat being strong enough to cause loss of valuable volatile principles. Roots, barks, and leaves may be dried by spreading them out upon clean tables or floors in a dry room and turning them repeatedly, so as to |