212 THE AMERICAN MONTHLY [July 



fully, and to observe the selective action of the reagent. 

 If stained previously, picro-carmine or hematoxylin are 

 to be preferred. I shall have more to say about this 

 part of the process later. 



I 7i filtration. — The objects may now be immersed direct 

 from the alcohol to the celloidin solution, using the thin- 

 ner of the two, in which they may remain any length of 

 time ; the longer the better. 



Specimen tubes 3in. by lin. are useful, and need not be 

 kept tightly corked, for the gradual evaporation of the 

 solvents will thicken the celliodin, and bring it nearer the 

 desired consistency. See that all air-spaces in the speci- 

 men are opened, and, as the solution diminishes by evap- 

 oration, add a little of the thickest grade until the whole 

 is of the consistency of treacle, As to the time that the 

 specimen should remain in the celloidin bath, everything 

 depends upon its nature. Small flies, insect-heads (for 

 preparation of the eyes), and spiders, from a week to a 

 month ; muscle, skin, liver, kidney, lung, embryo of chick, 

 amphioxus, marine worms, three to five days ; very small 

 or easily-penetrated objects, such as marine sponges, 

 twenty-four hours. 



Congealing the Mass. — While infiltration has been pro- 

 gressing, prepare a number of discs of close, hard-grain- 

 ed wood, from Jin. to fin. diameter, and Jin. thick. Soak 

 these thoroughly in the celloidin solution — in fact, treat- 

 ing them exactly as described in the foregoing paragraph, 

 and then allowing them to dry in the air. Take a slip of 

 ordinary thin writing-paper and make a thimble, having 

 a wood disc for a closed end, and gumming the overlap 

 with a narrow strip of gum arabic solution, but do not 

 gum it to the block in any way, so that it can afterwards 

 be rolled off without difficulty. When the gum is dry, 

 dip the thimble in thin celloidin, and pour enough thick 

 solution in to cover the bottom for the depth of sin. Half- 

 fill one of the specimen tubes with rectified spirits, or 



