PRESERVATION 325 



therefore occupies a great deal more space than forma- 

 lin, of which a 5 per cent, solution of the ordinary 

 commercial 40 per cent, solution {i.e., a 2 per cent, 

 solution of pure formaldehyde) is sufficiently strong 

 for all ordinary purposes. 



Formalin is unsuitable for specimens which are 

 required for osteological work, as, after a time, it 

 softens the bony tissues. 



Both preservatives rather rapidly extract the blue 

 and bluish-green colours of many fishes ; neither has 

 any great effect on dark pigments, and alcohol dissolves 

 red and yellow pigments more rapidly than formalin. 

 In any case, the colours of fishes — and especially the 

 paler colours — can only be properly studied after pre- 

 servation if the specimens are kept in light-proof 

 vessels or, at any rate, wrapped in muslin. 



Probably the most satisfactory course is to preserve 

 fishes in formalin, and subsequently to transfer to alcohol 

 those which it is desired to retain permanently as 

 museum specimens ; in effecting such a transfer, the usual 

 precaution of dehydrating the specimens gradually and 

 successively in 30, 50, and 75 per cent, alcohol, should be 

 observed . In practice, many of the fishes which we handle 

 have been transferred after a few weeks preservation in 

 formalin to a mixture of equal parts of 95 per cent, 

 alcohol and the 2 per cent, solution of pure formalde- 

 hyde above mentioned ; this seems to make an admir- 

 able preservative, but, as it has not yet stood the test 

 of time, we can say nothing as to its permanent value. 



For preserving larvae the methods above suggested 

 will be found quite satisfactory ; preserving solutions 

 containing osmic acid or corrosive sublimate should on 

 no account be used. 



