FIXING AND HARDENING AGENTS. 55 



BENEDEN lias recommended a saturated solution in 25 per 

 cent, acetic acid, and Lo BIANCO (Mitth. Zool. Stat. Neapel, 

 ix, 1890, p. 443) a mixture of 2 parts saturated solution with 

 1 part of 49 per cent, acetic acid. 



It is sometimes advisable to take the most concentrated 

 solution obtainable. The cold saturated aqueous solution 

 will suffice in most cases ; but for some very contractile 

 forms (coral polypes, Planaria), a concentrated solution in 

 warm or even boiling water should be employed. For 

 Arthropoda alcoholic solutions are frequently indicated. 

 Delicate objects, however, may require treatment with weak 

 solutions. 



Objects should in all cases be removed from the fixing bath 

 as soon as fixed, that is, in other words, as soon as they are 

 seen to have become opaqiie throughout, which is practically 

 as soon as they are penetrated by the liquid. Small objects 

 are fixed in a few minutes. I have found that a " salivary J 

 gland of the larva of Chironomus is thoroughly fixed in three 

 seconds. 



Wash out with water or with alcohol. I consider alcohol 

 almost always preferable. Alcohol of about 70 per cent, may 

 be taken. The extraction of the sublimate is hastened by the 

 addition of a little camphor to the alcohol. Or, much better 

 (MAYER, Intern. Munafxxch-r. An at. Phi/s., iv, 1887, p. 43), 

 a little tincture of iodine may be added to the liquid, either 

 alcohol or water, used for washing, enough to make it of a 

 good port wine colour, and the liquid be changed until it no 

 longer becomes discoloured by the objects. APATHY (Mikro- 

 technik, p. 148) takes a 0'5 per cent, solution of iodine in 

 strong alcohol, leaves the objects in it (suspended) until they 

 have become of about the colour of the solution, and then 

 washes for twenty- four hours in pure alcohol. 



In obstinate cases solution of iodine in iodide of potassium may be taken. 

 MATER (Zeit. f. iviss. Mik., xiv, 1897, p. 28) makes it by dissolving 

 5 grammes of iodide of potassium in 5 c.c. of distilled water and mixing 

 this with a solution of Oo gramme of iodine in 45 c.c. of 90 per cent, 

 alcohol, but seldom uses the mixture concentrated, merely adding as much 

 of it as is required to the alcohol or water containing the objects. The 

 iodine may be washed out in obstinate cases with magnesia water. Simi- 

 larly APATHY, Mitth. Zool. Stat. Neapel, xii, 1897, pp. 729, 730. 



It has been objected to this process that iodine in potassic iodide precipi- 



