228 REPORTS ON THE STATE OF SCIENCE. 



exudations, which thickened and hardened on the surface of the plant, 

 and thus it has been applied not only to substances which have an affinity 

 for water, but also to resins and caoutchouc. As the latter are very 

 different in their composition and properties from the substances of the 

 class described above, it is most convenient not to include them with 

 these. 



One of the earliest recorded uses of the word is by Herodotus, who 

 described, about 450 B.C., how in Egypt the embalmers swathed the 

 corpse in strips of linen smeared with gum, which, he adds, the Egyptians 

 generally use instead of glue ; he employs the word k-o^z/xi, from which 

 the word ' gum ' is descended. The gums have been familiar substances 

 in European literature from that time to the present, being designated by 

 some form or other of the word Kuj-ipt, which itself was not a native Greek 

 word, but was of foreign origin. 



The views held regarding the chemistry of the gums since the year 

 1774, which may be regarded as the period of the dawn of modern science, 

 have passed through three phases. 



In the first phase they were regarded as being among the proximate 

 or immediate principles of plants, such a proximate principle being defined 

 as a distinct compound existing ready formed in plants. They were 

 accordingly placed in the lists of such principles, which at that time were 

 not very extensive. It was then imagined that the chemistry of animal 

 and vegetable products was far simpler than we now know it to be. It 

 was known that the gums were composed of carbon, hydrogen and oxygen, 

 with pos.'sibly some nitrogen. 



In the second phase, as analysis appeared to show that the hydrogen 

 and oxygen were in the same proportions as in water, they were con- 

 sidered to be carbohydrates ; and when, by studying the action of reagents 

 on them, it was found that on hydrolysis they yielded various sugars, they 

 were classed as polysaccharides — that is, substances formed of two or more 

 sugar residues united together, and differing from the sugars by one or 

 more molecules of water. 



In the third phase, by a more careful and systematic series of fractiona- 

 tions and hydrolyses, several of the gum substances have been shown to 

 be built up of the residues of sugar molecules, united by an ethereal 

 oxygen attachment to an organic acid, which is different in different 

 gums, and which may be regarded as the nucleus of the particular gum. 

 In other words, they are glycosides of certain organic acids. The number 

 of these sugar residues in a gum compound is considerable, and the natural 

 gum is often a mixture of several gum compounds, diSering from one 

 another in the number of sugar residues in their molecules. As the 

 attachment is ethereal, and not like that of an ester, the gum compounds 

 possess acid properties, since the acid groups are not neutralised by the 

 sugar residues. 



This is the most modern view, and doubtless many, if not all, of the 

 gums will be found to be of this nature when fresh examination of them 

 has been made. 



It must be understood, however, that these phases are not sharply 

 marked off from one another in sequence of time, the germs of the later 

 idea being often found in the records of earlier investigations. 



In the early part of the nineteenth century a good many gums were 

 known, the most familiar of which were gum arable, gum tragacanth, 

 gum Bassora, gums from the genus Prunus, such as cherry-tree gum, 



