324 SCIENCE AND FRUIT GROWING 



flocculation. The further addition of acid, etc., then produced 

 no further increase in the volume, at any rate, till other changes 

 supervened, into which it is not necessary to enter here. 



Various theories had been put forward to explain flocculation, 

 but none of them had satisfied all the facts of the case : 1 amongst 

 these the question of the combination of the acid, etc., with the 

 clay had been considered, but had been dismissed, owing to the 

 absence of evidence that any such combination did occur, or 

 owing to the smallness of the proportion in which it must occur 

 if there were any combination at all. But work done in connec- 

 tion with the Woburn Farm placed the occurrence of such com- 

 bination beyond doubt ; for, with acids, it was found that the clay 

 actually removed nearly the whole of the acid from the liquid, 

 carrying it down with it to the bottom, and, moreover, that the 

 combination occurred in more or less definite proportions, for 

 the clay would remove a certain quantity of acid, and that quan- 

 tity only ; if any excess were added beyond that limit, it remained 

 unaffected in the liquid. The total proportion of acid thus 

 combining was, however, small, only one-fiftieth to one two- 

 hundredth of the clay, and if it were simply a question of the acid 

 itself combining with the clay, an addition such as that could 

 not account for an increase of 100 to 200 per cent, in the size of 

 the particles. But the acid does not exist in the liquid as such, 

 it is combined with a large proportion of water in the form of 

 hydrates, which contain several hundred molecular proportions 

 of water to each one of acid, and the union of these bulky hydrates 

 with the clay particles would be quite sufficient to account for 

 the composite particle being two or three times larger than the 

 solid, uncombined clay particles. 



In these composite particles we have, 'therefore, a solid nucleus 

 enclosed in a liquid envelope of weak acid, the whole forming a 

 sphere floating in the rest of the liquid, which consists of nearly 

 pure water, and there must, therefore, be what is known as surface 

 tension between the outside of this sphere and the water, just as 

 there is in the case of the spheres of oil in an emulsion, or in that 

 of the bubbles of air in a froth. When the surface tension is 

 not great, the same thing will happen in all these cases ; several 

 of the spheres, or bubbles, will gradually coalesce, to form larger, 

 irregular, masses, and if, as in the case of the clay, each of the 

 spheres contains a solid nucleus, these solid particles will, when 

 several spheres do coalesce, congregate towards the centre of the 

 resulting mass of liquid, and give the impression of there having 



1 Hall and Morison, Journ. Agric. Science, ii, p. 244. 



