880 



SCIENCE 



[N. S. Vol. XXXVI. No. 939 



the application of highly theoretical knowl- 



A further, but less widely appreciated, 

 example of the same thing is afforded by 

 the present subject, namely, the control of 

 the certain ripening processes in bitter, or 

 rather astringent fruits. We shall see, as 

 the details are unfolded, that here too em- 

 piricism has preceded science, but that sci- 

 ence is at the moment endeavoring to ex- 

 plain experience, with the result that, 

 while problems of very great practical im- 

 portance are being solved, new facts of 

 still greater theoretical weight are being 

 discovered. 



Astringency in fruits and other plant 

 parts is due to the presence of tannin in 

 some form, very frequently tannic acid. It 

 will suffice here to speak of it simply as 

 tannin. Certain kinds of bark and other 

 parts of many plants contain this substance 

 in such quantities that they are used for 

 the tanning of hides and for the extraction 

 of tannin for other purposes, some of which 

 are medicinal. The usefulness of tannin 

 in the arts depends on its peculiar property 

 of being able to enter into combination 

 with many other substances, such as pro-<. 

 teins, gelatins, mucilages and the like, to 

 form complexes (or compounds in a loose 

 sense) which have in common the peculiar 

 property of resisting agents of decay. This 

 fact is perfectly well known and tannin 

 has been made use of since time immemorial 

 as a preservative of material exposed to 

 conditions favorable to decay, such as 

 sails, fishing tackle and the like. 



But if one wishes to have a more vivid 

 impression of this peculiarity of tannin 

 seen in its ready combination with colloidal 

 materials, one needs but to take into the 

 mouth a few drops of a solution of tannin, 

 or to bite into an astringent fruit, such as 

 the persimmon, or an unripe banana. The 

 tannin in these will quickly attack and 



combine with the skin of the mouth, and 

 produce the drawing or puckering effect 

 familiar to all. The same experience is had 

 on drinking tea which has been too long 

 standing. What happens in the mouth 

 may be. accurately and simply described as 

 a rapid manufacture of a thin coating of 

 leather over the mucous membranes. 



Another peculiarity of these tannin com- 

 plexes, such as leather and the like which 

 it is necessary to mention briefly, is this, 

 namely, that one may easily extract with 

 water a certain amount of the tannin with- 

 out any obvious change in the physical 

 character of the complex. If we soak 

 leather in water, we can show that some of 

 the tannin has come out. We may do this 

 repeatedly, and always get some but again 

 always, a decreasing amount of tannin. 

 An analogous example is afforded by cer- 

 tain dyes which stain the cotton fiber but 

 which may be extracted in repeatedly di- 

 minishing quantities by water. Such com- 

 plexes are said to arise by adsorption ; they 

 are not, at any rate, chemical compounds in 

 the ordinary sense. In this connection our 

 attention should be drawn to another fact 

 of prime importance in what follows. It is 

 that dyes are held more tenaciously by 

 coagulable siibstances, such as the white of 

 egg, when they are in a state of coagula- 

 tion than when not, so that, given a certain 

 amount of dye and a certain amount of the 

 albumin (or whatever other substance may 

 be used), a less amount of the dye can be 

 extracted by water if the albumin has been 

 coagulated by heat — as happens when an 

 egg is cooked — than otherwise. I have 

 shown that tannin in its relation with a 

 coagulable substance in the persimmon, 

 analogous to the white of egg, acts in the 

 same way. The significance of this in re- 

 lation to non-astringency of fruits will be 

 indicated as we proceed. 



The term colloid has already been used. 



