ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



149 



Fig. 27. 



others 50 c.cm. The large vessel is to be filled with absolute alcohol 



and the tubes with distilled water (or alcohol of different strengths) 



and the preparation dropped in the latter (or 



attached to the hooks on the stoppers). An 



exchange then takes place between the alcohol 



and the water, the latter falling to the bottom 



of the vessel and absolute alcohol replacing it 



in the tubes. From time to time the dilute 



alcohol at the bottom of the vessel can be drawn 



off through the tap, first taking out the stopper 



in the cover. "When the alcohol has sunk to 



the level of the points of the tubes the vessel 



should be filled up again through a funnel 



reaching half-way down and having a bent 



point. By regulating the sizes of the apertures 



at the bottom of the tubes the time required 



for the exchange of the alcohol and the water 



can be varied— from several days to a few 



hours — so as to suit all requirements. The 



tubes should be numbered and a table made showing their different 



periods. 



Imbedding in ParaflEln by means of a Vacuum.*— P. Francotte 

 has improved on Hoffmann's apparatus described Vol. IV. (1884) 

 p. 820, which requires for producing the vacuum either a pressure of 

 water or a very long aspirator. The former is often wanting and the 

 latter very inconvenient. 



Dr. Francotte at first attempted to obviate these inconveniences 

 by boiling and then cooling ether (which requires only a temperature 

 of 40°), but he now uses by preference steam. The vessel R (fig. 28) 



Fig. 28. 



holding about half a litre, has a barometer tube B, which passes into 

 a vessel of mercury F, and another communicating with the flask P, 

 containing the melted parafiin, heated by a water bath M. It is 

 useful though not necessary to interpose a flask N to collect the 

 steam if it is formed in too great quantity. 



The water is boiled by a spirit-lamp and the air escapes by the 

 * Bull. Soc. Belg. Micr., xi. (1884) pp. 45-8 (1 pi.). 



