436 JOTTINGS FROM SYDNEY UNIVERSITY BIOLOGICAL LABORATORY. 



Fluid 2 thus enters into the mixing bottle at an extremely slow 

 rate of flow, and becomes completely diffused, at first in extremely 

 minute quantity, through fluid 1. The fluid from the mixing 

 bottle is meanwhile entering bottle b at the same extremely slow 

 rate, and it is obvious that with two fluids that readily mix, fluid 

 1 may be made to replace fluid 2 in bottle b with the required 

 excessive slowness and regularity. 



In the case of some of the liquids used in fixing and preserving, 

 it is not necessary to use such a precaution as this. We may 

 substitute saturated solution of corrosive sublimate for sea-water 

 without the least risk of damage to the most delicate structures — 

 the specific gravity of the two being very nearly the same. 



Similarly distilled water may be at once substituted for osmic 

 acid solution, or 1% chromic acid, or other fluid that does not differ 

 at all widely from water in specific gravity. But with certain 

 fluids the gradual substitution is necessary, and it is above all 

 necessary in replacing water or a watery solution by alcohol, and 

 this in the case of large specimens intended for museum purposes 

 as well as smaller objects, can very conveniently be carried out by 

 the simple apparatus I have described above. 



Another method of effecting this substitution is the one devised 

 by Schultze ; and this seems to possess some decided advantages, 

 at least for small objects. Schultze places the objects which he 

 wishes to transfer from water to alcohol in a tube full of water, 

 plugged at one end, and closed at the other by a diaphragm of 

 chamois skin. The tube is placed in a vessel of alcohol and left 

 there until by a process of diffusion through the diaphragm the 

 water in the tube becomes completely replaced by alcohol. The 

 same material being used for the diaphragm, the time which will 

 be occupied before complete substitution takes place will vary 

 with the capacity of the tube and the diameter of its orifice ; and 

 a series of experiments and calculations would have to be made 

 before this method could be used with the assurance of good 

 results. Should it be desired to have the specimens in absolute 

 alcohol at the end of the process, some calcined sulphate of copper 

 may be placed in the outer vessel. 



