354 PRESERVATION OF MARINE ORGANISMS 



volumes of water equal to the difference between the 

 percentage desired and 90. Thus, to make 70 per 

 cent., take 70 volumes of 90 per cent., and add 20 of 

 water ; to make 50 per cent., take 50 volumes of 90 per 

 cent., and add 40 of water ; and so on. 



All solutions of alcohol should be made with pure 

 fresh water (ordinary ship's water should be filtered 

 first, even if distilled). Water loaded with mineral 

 salts gives a precipitate when mixed with alcohol, and 

 unless the precipitate is removed by filtration or 

 decantation the specimens generally become coated 

 with a very fine dirty sediment. 



Alcohol by itself is not good for fixing purposes, 

 but is generally used in combination with other 

 chemicals, as it increases their penetrating power. It 

 is a good hardening agent, but unfortunately it pro- 

 duces a considerable amount of shrinkage in the tissues. 



As a preservative it is excellent, and the safest of 

 all preservatives for specimens intended for permanent 

 collections. Though formalin is its strong rival, still 

 formalin is a newly discovered fluid and a rather un- 

 stable one, and we have yet to find out how long its 

 preserving qualities are going to last. Hydroids and 

 Medusae which have been in formalin 10 per cent, for 

 twelve years show no visible signs of deterioration. 



The strength of the alcohol commonly used for the 

 preservation of animals lies between 70 and 80 per 

 cent. The minimum strength should be rigidly fixed 

 at 70 per cent. A slight increase over 70 per cent. — say 

 75 per cent. — is an advantage, because it provides a 

 margin against evaporation. 



Delicate, soft-bodied animals should not be placed 

 straight into 70 per cent., but first into 30 per cent, 

 alcohol for a few hours, and then in 50 per cent, for 

 several hours, and each solution should be changed at 



