ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 875 



not suffice to give the proper depth of colour. Small and delicate 

 ohjects, on the other hand, may be most successfully treated with a 

 solution which has been diluted with 70 per cent, alcohol, or one 

 which has been weakened by previous use. It is always necessary to 

 free the tissues, after staining, from the surplus dye ; and this may be 

 done by washing in 70 per cent, alcohol, which must be changed until 

 it shows no colour. This process requires, for larger objects, con- 

 siderable time and alcohol, but may be hastened by using the alcohol 

 slightly warm. 



The colour ultimately assumed by objects treated with cochineal 

 tincture varies much, and depends partly on the reaction of the tissues 

 themselves, partly on, the presence or absence of certain salts. It is 

 certainly one of the best recommendations of this staining agent that 

 varying with the nature of the object and its mode of treatment both 

 before and after staining, it gives such an extraordinary diversity of 

 results. On account of the great variety of substances contained in 

 the dried dye-stuff, it is evident that the composition of the tincture 

 must vary according to the strength of the alcohol employed as a 

 solvent. Solutions in 90 per cent, or 100 per cent, alcohol have a 

 light red colour, and stain too diffusely to have any practical value. 

 The weaker the alcohol the stronger the tincture, and the stronger the 

 alcohol the more easily it penetrates objects ; the grade of alcohol 

 may therefore be selected with reference to two points, depth of colour 

 and readiness of penetration ; 70 per cent, or 60 per cent, is recom- 

 mended by Dr. Mayer as combining both these qualities in a very 

 favourable degree. It is important to remember that whatever be the 

 strength of the solution, a precipitate will always be produced if an 

 alcohol of a different grade, whether higher or lower, be mixed with 

 it. It is evident, then, that a tincture of any given strength contains 

 substances that are insoluble in any other grade of alcohol, and this 

 explains why superfluous colouring matter can only be removed from 

 objects by the aid of alcohol of precisely the same degree as that of 

 the tincture. 



Over-staining, which seldom occurs, may be easily corrected by 

 the aid of acid alcohol ( T 1 ^ per cent, hydrochloric acid, or 1 per cent, 

 acetic acid). Acid makes the tincture lighter, more yellowish-red, 

 while the addition of ammonia and other caustic alkalies changes it 

 to deep purple. Still more important is the fact that salts soluble in 

 alcohol give a blue-grey, green-grey, or blue-black precipitate. For 

 example, if a piece of cloth that has been dyed in cochineal and 

 washed be treated with an alcoholic solution of a ferric or a calcic 

 salt, it will assume a more or less deep blue colour. 



As the salts present in the living organism are seldom, if ever, 

 fully removed by preservative fluids, but in some cases even increased, 

 it will often happen that an object, though put in the red fluid, comes 

 out blue, precisely as when stained with hasmatoxylin. Such a result 

 cannot, however, be obtained where the tissue is in the presence of 

 acids, or free from inorganic salts ; under these conditions the colour 

 is always red. It is not possible, therefore, to know what colour an 

 object will ultimately present. 



