Decembek 27, 1912] 



SCIENCE 



881 



Inasmuch as the living body, whether of the 

 plant or of the animal, is made np, aside 

 from the water content, very largely of col- 

 loids, I must venture, at the risk of appear- 

 ing to dwell overmuch upon very elemen- 

 tary matters, to draw attention to certain 

 of their characteristics. In negative fash- 

 ion, a colloid may be described as a sub- 

 stance which does not crystallize, and this 

 feature serves to contrast it with other sub- 

 stances, such as salts, sugars, etc., which, 

 upon going out of solution, assume geo- 

 metrical forms. It is more difficult to de- 

 fine colloid in positive terms, but for- 

 tunately we are all of us familiar enough 

 with them so that we do not need a formal 

 description. Glue, gelatin, mucilage are 

 examples. Tannin, which claims our espe- 

 cial notice at this time, is also a colloid. 

 When colloids are dissolved in water, they 

 break up into particles which are far too 

 small to be seen with the naked eye, but 

 which are very much larger than the par- 

 ticles in a solution of a crystalloid. These 

 may be identical with the molecules, or stUl 

 smaller, when they represent the ions, or 

 grosser components of the molecule small 

 beyond even the strongest powers of the 

 microscope. Colloids, however, in many 

 cases, may in their dissolved condition be 

 seen by means of the ultramicroseope, when 

 they appear as minute brilliantly illumi- 

 nated particles (suspensoids). One may 

 understand this by recalling that a very 

 small mirror at a great distance can be seen 

 when it is caused to reflect the sunlight 

 into the eye of the observer. The ultra- 

 microscope therefore enables us in many 

 instances to see what goes on in solutions 

 of colloids. For example, it makes it pos- 

 sible to watch the process of coagulation 

 in those colloids in which coagulation is 

 possible. Thus, if we examine a weak 

 casein solution — we can make such by 

 thinning skimmed milk with water — ^we 



see a very pandemonium of dancing il- 

 lumined particles. These remain in con- 

 stant motion, flying hither and yon at a 

 rate of speed too great to follow with the 

 eye. If now we add a minute amount of 

 an acid, the particles may be observed to 

 hit one another and to remain in contact, 

 so forming a continuous mass or apparently 

 continuous, since we know that water is 

 held within the coagulum. Quite similar 

 appearances may be had by adding a solu- 

 tion of tannin to one of gelatin. 



If, however, before adding acid to the 

 casein solution, we add a little mucilage, 

 the coagulation may be prevented. This 

 illustrates the principle of colloidal protec- 

 tion — in this instance the mucilage pro- 

 tects the casein from the action of the acid. 

 I have shown that a similar relation exists 

 between tannin and mucilage as against 

 alkaloids. Tannin immediately precipi- 

 tates an alkaloid, such as antipyrin, in so- 

 lution in water. When, however, a muci- 

 lage has previously been added, the pre- 

 cipitation is prevented. If, therefore, on 

 adding an alkaloid to a solution of tannin 

 we get no precipitate, we must argue that 

 there is a third substance present which 

 protects the tannin. Such a condition will 

 be shown to occur in the fruits with which 

 we concern ourselves to-day. 



We now turn to examine typical ex- 

 amples of fruits which, before entering the 

 condition regarded as edible, are highly 

 astringent, but which, when ' ' ripe, ' ' appear 

 to be entirely devoid of the astringent 

 principle, tannin. I use this wording ad- 

 visedly, since the fact is that such fruits 

 contain quite as much tannin when non- 

 astringent as before. What has become of 

 the tannin I propose to show you. The ex- 

 amples in question are the date of Arabia 

 and Africa, the staple food product of the 

 Arab, and the persimmons of eastern Asia 

 and of North America. 



