168 PRACTICAL HISTOLOGY. 



307. Small quantities of finely powdered ice and salt 

 are placed alternately with the aid of a small spoon in the 

 freezing box, and are stirred round the well, the tube H 

 being carefully kept open to allow the water to escape from 

 the melting ice. When snow can be obtained, it is, of 

 course, used instead of ice. The powdering of the ice is 

 rather tedious, but it may be readily done in that form of 

 sausage machine where two cylinders with spiral grooves are 

 rolled against one another. Before putting the ice into 

 this apparatus it should be wrapped in a thick cloth and 

 broken with a wooden mallet. The freezing process is 

 really very simple, and can be fully carried out in from ten 

 to twenty minutes. Of course the period is shorter the 

 colder the surrounding air. 



308. It is not necessary to wet the knife, for this is 

 thoroughly done by the thawing gum. The sections often 

 contain a number of air bubbles ; these, however, together 

 with the gum, are soon removed by placing them in dilute 

 alcohol (rectified or methylated spirit i, water 2 parts). 

 If the sections are to be preserved for a time until they are 

 examined or mounted, they are transferred from the dilute 

 alcohol to rectified spirit. When sections are apt to be 

 spoilt by manipulation, they should be placed on the slide before 

 they thaw. They can then be readily washed by making a 

 pool of dilute alcohol around them, which may be changed 

 two or three times. 



309. The more one employs the freezing microtome, 

 the more clearly does one perceive the great assistance 

 which it offers to the histologist A number of tissues may 

 be frozen and cut at the same time. Delicate organs, such 

 as the retina, the embryo, villi of the intestine, lung, trachea 

 with its ciliated epithelium, may all be readily cut without 

 fear of their being destroyed by the imbedding agent, as is 

 apt to be the case when paraffin, pith, and carrot are em- 

 ployed. The cases to which freezing is applicable have 

 already been detailed in 282. It must be carefully borne 

 in mind, however, that in the case of a tissue saturated with 

 water spicules of ice are formed, and after section various 

 crevices may sometimes be seen which have resulted from 



