FOULING OF SHIPS' BOTTOMS 



231 



That copper poisons are especially efficacious in preventing the attachment of 

 the young larvae of these forms, provided the paint has been applied recently, is 

 acknowledged generally. Many copper and mercury salts are extremely toxic to 

 most animal and many plant organisms. It would be supposed, naturally, that 

 these would be effective against those organisms that cause fouling, although no 

 experiments to prove such a contention have been tried, as far as the author can 

 learn, on any of the organisms as they exist at the time of attachment. Recently, 

 Bray (1923) studied the resistance of the earliest larval stage of a single barnacle 

 (Balanus eburneus) to various poisons, the results of which study will be considered 

 below. 



That the efficacy of poisons has been doubted by many is indicated by the 

 following quotation from Lewes (1889), of the Royal Naval College of Great Britain: 



On examining the conditions under which a vessel is put when coated with a composition 

 which relies for its antifouling powers on metallic poisons only, we at once see the reasons which 

 must make such a coating of little or no avail. In the composition we have drastic mineral poisons — 

 probably salts of copper, mercury, or arsenic — which have been worked into a paint by admixture 

 with varnishes of varying composition, and each article of poison is protected from the action of 

 sea water by being entirely coated with this mixture; that this must be so is evident, or the com- 

 position would not have sufficient cohesive power to stick on the ship. As a rule, care is taken 

 to select fairly good varnishes, which will resist the action of sea water for, perhaps, two or three 

 months before they get sufficiently disintegrated to allow the sea water to dissolve any of the 

 poison; whilst even with the accidental or intentional use of inferior varnishes, three or four weeks 

 will pass before any solution can take place and any poison liberated to attack the germs. A ship 

 is dry-docked, cleaned, and her antifouling composition having been put on, she goes probably into 

 the basin to take on cargo. Here she is at rest and, with no skin friction or other disturbing causes 

 to prevent it, a slimy deposit of dirt from the water takes place, and this, as a rule, is rich in the ova 

 and germs of all kinds of growth whilst the poisons in her coating are locked up in their restraining 

 varnish and are rendered inactive at the only period during which they could be of any use. 



After a more or less protracted period the ship puts to sea, and the varnish being aided by 

 friction of the water the poisonous salts begin to dissolve or wash out of the composition; but the 

 germs have already got a foothold, and with a vessel sweeping at a rate of 10 to 12 knots through 

 the water the amount of poison which can come in contact with their breathing and absorbing 

 organs is evidently so infinitesimally minute that it would be impossible to imagine it having any 

 effect whatever upon their growth. If the poison is soluble, it is at once washed away as it dissolves; 

 if it is insoluble, then it is also washed away, but there is just a chance that a grain or two may 

 become entangled in the organs of some of the forms of life and cause them discomfort. As the 

 surface varnish perishes, the impact of the water during the rapid passage of the vessel through the 

 water quickly dissolves out or washes off the poisonous salts and leaves a perished and porous, but 

 still cohesive, coating of resinous matter, which forms an admirable lodgment for anything that can 

 cling to it; and by the time the vessel lays-to in foreign waters, teeming with every kind of life, the 

 poison which would now again have been of some use is probably all washed away, and a fresh crop 

 of germs is acquired, to be developed on the homeward voyage, and a "bad ship" is reported by the 

 person who looks after her docking. It is evident that a poison, even if it had the power of killing 

 animal and vegetable life in all stages, could only act with the vessel at rest, unless it were of so 

 active a nature as to burn off the roots and attachments of the life rooted to it, and if it did this, 

 what, may I ask, would become of the protective composition and the plates of the vessel? And 

 I think it is also evident that any poison so used must be under conditions in which it is very unlikely 

 to be in a position to act when it might do good. 



The practical proof, given by experience, that poisons alone are unable to secure a clean bottom 

 soon led many inquirers to the conviction that it was exfoliation in the case of copper which had 

 acted in giving fairly good results, and in many compositions the attempt has been made to provide 

 a coating which will slowly wash off, and, by losing its original surface, shall at the same time clear 



