56 VIEWS OF THE MICROSCOPIC WORLD. 



raalcules, and when mixed with the ground bark of trees is used by the inhabi- 

 tants as an article of food in times of scarcity. The same kind of earth occurs 

 in San Fiora in Tuscany, and also near Egra in Bohemia, about three feet below 

 the surface of the ground. To the eye it appears when dry like pure magnesia ; 

 but when examined by the microscope, it is seen to consist entirely of a richly 

 figured species of Infusorial shell, which is called the Campilodiscus. A specimen 

 Fig. si. f rora tms locality, very highly magnified, is delineated in figure 81. 

 Its natural size varies from one-four hundred and thirtieth to 

 one-two hundred and fortieth of an inch. In the province of 

 Luneberg, in Saxony, a layer of eatable Infusorial earth also 

 occurs, twenty-eight feet in thickness, which is the greatest deposit 

 that has yet been discovered : and similar strata have been found 

 in Africa, Asia, and the South Sea Islands. On the banks of the Amazon, in 

 South America, an Infusorial clay occurs of the same nature. It is not a recent 

 deposit from the swelling of the river ; but is an ancient bed whose age is unde- 

 termined, and exists as an elevated and extensive plain, shaded with woods and 

 the thick foliage of forests. 



FOSSIL ANIMALCULES OF CHALK AND FLINT. Chalk consists in a great 

 measure of fossil Infusoria, together with minute shells, so exceedingly small that 

 a million distinct structures are computed by Ehrenberg to be contained in the 

 space of a cubic inch. These organic remains constitute nearly half the bulk of 

 the chalk of Northern Europe, and exceed this proportion in that of Southern 

 Europe. The portion of these chalk formations that is not organized was origi- 

 nally shells, which having become decomposed, now form a cement for the organic 

 remains, uniting them together in one compact mass. The larger shells are per- 

 ceived, when the sediment obtained by brushing chalk into water is closely ex- 

 amined ; but in order to detect the true microscopic structures, the following pro- 

 cess must be adopted, which has been pursued by Ehrenberg. A drop of water 

 is first placed upon a thin slip of glass, and then upon the water as much scraped 

 chalk must be spread as will cover the fine point of a knife. After leaving the 

 chalk to rest for a few seconds, the finest particles suspended in the water must 

 be withdrawn, together with most of the liquid ; and the remainder suffered to 

 become perfectly dry. This sediment must now be covered with Canadian bal-' 

 sam, and the glass held over a spirit lamp until the balsam becomes slightly fluid 

 without froth or air bubbles. In this state it is kept for a short time, until the 

 balsam thoroughly penetrates every part of the sediment, flowing into the cham- 

 bers and cavities of the microscopic shells, and causing their structure to be 

 more readily detected. When a preparation thus made is magnified three hun- 

 dred times, the chalk is seen teeming with minute organic forms, the peculiarities 

 of which are so clearly revealed, that the observer is enabled to arrange and 

 classify them with the utmost ease. Flint to a large extent has also been proved to 

 be of animal orgin ; and a distinguished English naturalist has observed, that 

 masses of flint, or nodules as they are termed, are almost entirely composed of 



