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X.— EXPERIMENTS IN FEEDING WITH METALLIC SALTS. 



With the object of trying to produce in the laboratory results similar to those 

 observed in Nature, and with the view of testing the part which the absorption of 

 metallic salts might play in colouring the oyster, and in possibly rendering it unfit for 

 human food, we have made, during the last couple of years, a large number of experi- 

 ments by keeping oysters in tanks in which definite quantities of various salts of iron 

 and copper had been added to the sea-water.* 



In all cases we first of all ascertained that our oysters, both Americans and 

 Natives, were healthy and colourless, so far as we could judge by observing them 

 through the opening between the shells when in the expanded condition. We could 

 make sure at any rate that the mantle and the gills were not green, and we always 

 opened a few specimens of the same batch as a control. 



In the first place, we tried the effect of pieces of copper, copper filings, and 

 copper dust lying in the bottom of the aquarium ; and similarly, of steel filings, old 

 rusty nails, and other fragments of iron. We also kept oysters for some time in an 

 old copper vessel, and along with copper pyrites and other ores of copper. None of 

 these gave any definite result. 



We then, with Dr. Kohn's assistance, tried measured quantities and strengths of 

 various metallic salts. On Feb. 5th, 1896, we added 0.2 per cent, of sulphate of copper 

 to 10 litres of sea- water in which a few oysters were placed ; while another set of 

 oysters had the same quantity of ferric ammonium citrate added to their 10 litres of 

 water. From these we got no definite results. The oysters lived better in the iron water 

 than in the copper. The only staining was a deposition on the shell and in the mucus 

 of the mantle edge, and somt post-mortem colouration in the case of those that died. 



On Feb. 27th we started three colourless American oysters in each of three 

 aquaria containing insoluble salts, viz., sulphide of iron, ferric hydrate, and copper oxide, 

 in each case 50 grains being added to one gallon of sea-water. By March 5th, those 

 in the copper oxide fluid were evidently sickly, while the others seemed healthy. On 

 March 12th, those in the copper oxide no longer reacted under stimulation, and were 

 evidently just dead. On being opened, they had all the appearance of ordinary yellow 

 oysters, the gills and palps were normal, and there was no trace of staining in any 

 part. On March 21st, one from each of the other aquaria was opened. They were 



* Recently Carazzi has stated that oysters fed with similar dilute iron solutions acquire a pale yellowish 

 colour in certain parts (branchial epithelium and the oesophageal mucous membrane), and that in these parts micro- 

 scopic tests show the presence of granules of iron. The actual meaning of these results can hardly be recognised 

 without comparative and quantitative data. 



