38 GENERAL MICROBIOLOGY 



during which process the vegetable jelly was extracted. 

 The solution so obtained was poured off, leaving the useless 

 sediment behind. This jelly is the same in composition 

 as that existing in the vegetable tissues; it has not been 

 changed chemically, as is collagen in the preparation of 

 gelatin. The commercial agar is most probably prepared 

 by evaporating this solution to dryness by different means. 



Agar usually comes into the hands of the bacteriologist 

 as long, slender, grayish-white strips, or as blocks, or more 

 especially in recent years, in the form of a gray-white pow- 

 der of European manufacture. 



Agar, in contrast with gelatin, is a carbohydrate, i.e., 

 it consists of a combination of carbon, hydrogen and oxygen 

 only. Traces of nitrogen are present as impurities. The 

 above qualitative determinations of its elementary constit- 

 uents were made by Payen, by Parumbaru and by Hueppe, 

 who made their determinations on agar from different 

 sources. As far as can be ascertained, its empirical formula 

 has not yet been investigated to any extent. 



Like gelatin, however, agar is a reversible colloid. It 

 soaks up in cold water, dissolves in hot water after a long 

 boiling to a tasteless and odorless clear solution, and solid- 

 ifies upon cooling to a more or less opaque jelly. Its 

 watery solution is neutral or nearly neutral to phenol- 

 phthalein; still, a drop or two of twentieth normal sodium 

 hydrate is sufficient to make the pink color perceptible. 



The colloidal properties of agar are not destroyed by a 

 long-continued heating at a high temperature, nor by the 

 action of ordinary microorganisms as are those of gelatin. 

 The above properties, however, are influenced and may 

 be wholly impaired by the reaction of the liquid in which 

 the agar is dissolved. 



The reaction of the liquid, i.e., whether it is acid or 

 alkaline, influences the agar as to its solubility, solidity, 

 color, transparency, filterability and amount of condensa- 

 tion water. If agar is dissolved in a liquid of an acidity 



