4 U. S. BUKEAU OF FISHERIES. 



leads; this rubbing slowly abrades the threads and wears them out. 

 Actual breakage by being caught on snags or by the struggles of 

 large fish and the like must be reckoned with; such breakings, of 

 course, become more numerous when the net becomes weaker by rot- 

 ting. A factor in the destruction of nets that is sometimes over- 

 looked may also be mentioned here : That when the threads are coated 

 with thick tars and the like the total weight of the net is greatly 

 increased and the preservative action of the tar is thus in part neu- 

 tralized, the pull of the extra weight being added to other strains 

 that may break the threads. 



BACTERIAL. DECOMPOSITION. 



But of more importance than mechanical wear and tear in shorten- 

 ing the life of nets is the action — chemical in nature — of water and 

 air, aided by other agencies, particularly bacteria, in causing a weak- 

 ening and rotting of the fibers. This rotting deserves very careful 

 study, for it destroys nets long before they would otherwise be 

 destroyed by mechanical wear and tear, and its nature, while exceed- 

 ingly complex, must be understood, at least in part, before remedial 

 measures can be taken intelligently. 



Up to this point chemistry has made some progress, and some 

 valuable work has been done on the chemistry of preserving mate- 

 rials, but as a whole the fishing industry has never taken very 

 kindly to chemistry, and most of the work that bears useful fruit 

 for the net preserver has been incidental to work in other important 

 fields — the tanning of leather, bacteriology, and textile science. We 

 must admit frankly that we know very little directly about the inti- 

 mate nature of the rotting of nets. 



In the discussion of bacterial decomposition let it be assumed that 

 cotton nets are being considered, because it is chiefly nets of this 

 material that need preserving. They consist of fine fibers of almost 

 pure cellulose, a substance very close to starch in composition, differ- 

 ing from it chiefly in being insoluble in water under all ordinary 

 conditions, while starch, by simple treatment, may be made to dis- 

 solve. Cellulose can be made soluble in different liquids by severe 

 chemical treatments. As bacteria are also capable of dissolving or 

 rotting the fibers some detailed consideration will be given to their 

 mode of action. 



Bacteria are plants, so very small that they can be seen only by 

 the aid of a microscope ; they are not provided with many parts, as 

 are the large plants of our acquaintance, as they have no roots, leaves, 

 bark, or flowers; they are simple little rods or spheres, movable, 

 reproducing under favorable conditions with the greatest rapidity. 

 They have no claws, mouth, jaws, teeth, or any other offensive 

 weapons. How, then, do they do the great damages that they are 

 accused of? In a most interesting and important way. AVe know 

 that land plants take nourishment from the ground and air by im- 

 bibing it and that they have no mouths or offensive weapons. So 

 do bacteria, by absorbing or soaking up the food on Avhich they live. 

 But anything to be absorbed must first be made liquid ; solids can 

 not be soaked up, and it happens that very few natural foods are 

 liquid. So the bacterium must secrete a digestive juice that causes 



