ON STAINING FLUIDS, AND STAINING. Ill 



principle of the methods of this second class is 

 either, (like that of the first,) that by the immersion 

 of the object to be cut in some material that is 

 liquid whilst warm, and solid when cold, all the 

 parts of the object may be duly surrounded by the 

 supporting mass (the second class differing from the 

 first chiefly in the employment of materials possess- 

 ing greater power of penetration whilst liquid, in 

 longer immersion in the liquid mass, and in such 

 previous preparation of the object, by soaking in 

 some liquid that is a solvent of the embedding 

 material, as makes it more readily susceptible of 

 infiltration by the latter) ; or the processes may 

 be based on another principle, namely, that of the 

 employment of substances which whilst in solution 

 are suniciently fluid to penetrate the object to 

 be embedded, whilst at the same time after the 

 evaporation or removal by other means of their 

 solvent, they acquire and impart to the embedded 

 object sufficient firmness for the purpose of cutting. 

 The collodion process suniciently exemplifies this 

 principle. If a piece of soft tissue be dehydrated, 

 and soaked first in ether and then in collodion, and 

 if the ether be allowed slowly to evaporate, the 

 tissue and surrounding mass of collodion will 

 acquire a consistency such as to admit of thin 

 sections being cut from them." 



Embedding in Carrot. This is a very 

 primitive and almost exploded process, but it has 

 its uses and advantages when cylindrical specimens, 

 for instance, vegetable stems, are to be embedded ; 

 it is also available for such tissues as can be 

 rolled up and so enclosed in the centre of the 

 carrot. A " well " microtome is, of course, neces- 



