86 MARINE PRODUCTS OF COMMERCE 



Antibiotics, sulfa compounds, vitamins, and other medicaments are sometimes 

 enclosed in an agar capsule to effect a slow release of the therapeutic agent. An 

 agar coating or capsule remains intact longer than one composed of carrageenin 

 or gelatin. Thus, the release of a drug to a point beyond the stomach can be de- 

 layed as with the use of lactic acid in an agar vehicle to inhibit undesirable in- 

 testinal bacteria, or to administer gentian violet for intestinal parasites. Agar 

 particles distributed in paraffin constitute a pill coating that results in a delayed 

 release of its contents. The slow swelling of the agar particles ultimately ruptures 

 the coating (Miller, 1935). 



Bacteriological Culture Media. Agar in bacteriological culture media serves to 

 solidify nutrient broth so that an inoculum containing bacteria can be spread on 

 the agar surface. Bacteria thus scattered about cannot move but will form colonies 

 composed of cells derived from one original cell or clump of cells of the inoculum. 

 Agar is ideal for this purpose because of its gel strength, clarity, temperature of 

 gelation and melting, and low viscosity when melted. 



Dental Impression Compounds. Agar is often the principal ingredient in a com- 

 pound used by dentists to make impressions for dental crowns and plates. In im- 

 pression compounds agar is present in relatively high concentration so that a very 

 strong and somewhat elastic gel is produced. Since it gels at a temperature just 

 above that of the human body, 38 to 40° C (100 to 104° F), the melted com- 

 pound can be poured into the mold at a few degrees above its temperature of 

 gelation, and solidification will occur with only a few degrees drop in temperature. 

 The elasticity of the semisolid mass permits removal from undercuts without break- 

 ing or distortion. Impression compounds are used also in prosthetic work. 



Laboratonj Procedures. Agar gel in the form of tiny blocks has been used as 

 a vehicle for carrying plant hormones and for applying these hormones to various 

 parts of plants to study the effect. This procedure is known as the Avena test 

 method. Agar has been used in electric pH apparatus to solidffy the potassium 

 chloride bridge. The sensitivity of dry agar to changes in humidity has been em- 

 ployed in the more accurate hygrographs to activate the indicator. It is said to be 

 more accurate than human hair, a frequently used substance. 



In preparing very small pieces of plants or animals for permanent microscope 

 slides the tissue is often embedded in agar so that it will not be lost in passage 

 through a series of solutions. Biological materials are sometimes embedded in agar 

 to facilitate the cutting of thin sections with a freezing microtome. In chemical 

 laboratories agar is added in very small amounts to speed the precipitation of 

 barium sulfate. 



Agar and related phycocolloids have been important as a source of information 

 on the physical and chemical phenomena of hydrocoUoids. The expanding uses 

 and value of phycocolloids, as well as the many new types, will stimulate con- 

 siderable research in this field. 



Miscellaneous Uses. The addition of small quantities of agar or Irish moss 

 extractive increases the effectiveness of insect sprays. In some cases the amount 

 of nicotine can be reduced to one-third without loss of toxicity. 



Granules of humus or peat are used as the vehicle in nitrogen-fixing bacteria 

 for inoculating legumes. If this material is coated with agar, the moisture content 

 is more readily controlled and there is a much slower loss in the viability of the 

 bacteria. 



