OF MICROSCOPIC OBJECTS. 63 



put upon it, and heated until it begins to steam. If small bubbles 

 appear, a puff of breath removes them. The slide being held slight- 

 ly inclined from the operator, and the drop of balsam becoming 

 convex at its lower edge, the cover is brought in contact with it 

 at that point, gradually laid down, pressed with the forceps, and 

 brought to its central position. When cool the superfluous balsam 

 (if any) is removed with a heated knife-blade, the slide cleaned 

 with a little turpentine, and finished by washing in a hand-basin 

 with soap and water. In this process there is no delay if the bal- 

 sam be sufficiently thick, as the slide may be cleaned off almost 

 before it is cold. 



It is now well known that from common chalk it is an easy 

 matter to obtain interesting specimens of Foraminifera. Scrape a 

 small quantity of chalk from the mass and shake it in water ; 

 leave this a few minutes, pour the water away and add a fresh 

 quantity, shake up as before, and repeat two or three times. Take 

 a little of the residue, and spread it upon the slide, and when quite 

 dry add a little turpentine. When viewed with a power of two 

 hundred and fifty diameters this will generally show the organisms 

 very well. If it is desired to preserve the slides, they may be then 

 mounted in Canada balsam. Mr. Guyon, in " Recreative Science, " 

 observes that the accumulation of the powder, by the action of the 

 rain or exposure to the atmospheric action, at the foot or any pro 

 jection of the chalk cliffs, will afford us better specimens than that 

 which is " scraped," as the organisms are less broken in the former. 



When the Foraminifera arc of a larger size, though transparent 

 enough to be mounted in balsam, the air must be first expelled 

 from the interior, otherwise the objects will be altogether unsatis- 

 factory. To accomplish this they must be immersed in turpentii e 

 and submitted to the action of the air-pump. So difficult is it to 

 get rid of this enemy that it is often necessary to employ three or 

 four exhaustions, leaving them for some time under each. Wlien 

 all air has given place to the turpentine, they must be mounted in 

 the ordinary way. 



Of all objects which are commonly met with, few are such gene- 

 ral favourites as the POLYCYSTIN^E, and deservedly so. Their 

 forms are most beautiful, and often peculiar stars varying in de- 

 sign, others closely resembling crowns ; the Astromma Aristotelis 



