EFFECTS OF TENSION 45 



into threads or bridges, which finally break through in the 

 middle, the broken halves of these threads certainly contract 

 to some extent, but they do not gradually flow back per- 

 fectly, as they ought to do in a fluid froth. 



Any one who has followed out the gradual thickening of 

 the oil used will allow that in this process a sharp limit 

 between a firm and a fluid condition can be drawn just as 

 little as in the drying up of a solution of gum, but rather 

 that both conditions pass gradually one into the other. 

 Hence, for the foams just spoken of the consistence cannot 

 be stated with absolute definiteness. 



If such foams, the microscopic character of which is on 

 the whole just the same as in the case of the more fluid 

 foams, be subjected to pressure or tension, e.g. by rapidly 

 drawing off the fluid under the cover glass, they naturally 

 become stretched and drawn out in certain places or regions. 

 Here and there portions of the foam are forced apart ; be- 

 tween the portions coarser or finer threads are stretched 

 like bridges. The very variable appearances that may 

 chance to be formed in this way scarcely require more de- 

 tailed description. It can be seen in them, however, that 

 wherever the action of such tensions makes itself felt, 

 fibrous structures appear (see Plate VI. Fig. 2, a, ft). In 

 the stretched threads, which are spread out like bridges, it is 

 naturally at once seen that the direction of the fibres 

 coincides with the direction of the pull to which they are 

 subjected. A more thorough microscopic investigation 

 leads, as was to be expected, to the result that the fibrous 

 appearance solely depends upon the extension and stretching 

 of the meshes in certain directions. Thus, for example, 

 one can often discern plainly in the fibrous bridges stretched 

 between two portions of froth, how the drawn out 

 fibrous framework of the bridge passes into the usual 

 irregular framework of the portions that are not stretched. 

 Not infrequently also, in the larger portions of froth pressed 

 out in this way, one remarks quite irregular and tangled 

 fibrous structures, which can easily be explained by the fact 

 that at these spots simultaneous or successive strains took 

 effect in different directions. 



