24 Transactions of the American Institute. 



names are mere sounds, which serve ns for symbols in making com- 

 putations, but to which it is impossible to attach any clear uotions. 

 Let me try another illustration. Draftsmen, and persons who have 

 occasion to make use of divided rules, are aware that a division finer 

 than about 150 to the inch is with difficulty discerned. Few eyea 

 Avill distinguish lines closer than 200 to the inch. Divisions so fine 

 as 1,000 to the inch defy the keenest vision to separate them at all. 

 A cube, therefore, having its sides only equal to the one-thousandth 

 part of an inch, is an object invisible to the unaided human eye. But 

 such a cube is large enough to hold not less than 2,000 of the minuter 

 monadin. 



The monads are organisms of the last degree of simplicity of 

 structure. They interest by their extreme minuteness, and by the 

 fact that they are living things ; but neither in their forms nor their 

 integuments do they present any remarkable degree of beauty or 

 variety. In this resj)ect the loricated vegetable cells called diatoms, 

 are of much higher interest. Of these the greater portion are more 

 or less elongated and bear considerable resemblance, in a lateral view, 

 to a weaver's shuttle. Others are circular or disk-shaped ; others 

 still square, triangular, wedge-shaped, or having the form of simple 

 rods. The cell walls of all these objects are silicious, though the 

 silex is interpenetrated by organic matter, which remains and pre- 

 serves its form after the mineral portion has been dissolved by hydro- 

 fluoric acid. These walls are marked with the delicate and beautifully 

 varied patterns of which I have spoken, and being indestructible by 

 ordinary causes of decay, they persist after the life of the organism 

 lias perished, and become accumulated in vast quantities wherever 

 these forms of life abound. Diatoms are found in great numbers in 

 all waters which favor at the same time the growth of the higher 

 orders of vegetation, aud in the ooze which settles at tlie bottom of 

 all lakes and seas, their silicious shells form a conspicuous part, con- 

 stituting often, much the larger portion of the deposit. Some of 

 them are of considerable dimensions, measuring the 100th or the 

 200th part of an inch in length or in diameter, many much less. 

 Of the elongated forms, the cross section hardly exceeds on an average, 

 the 500,000tli part of a square inch, and is sometimes as small as one 

 5,000,000th. The slate of Bilin, in Bohemia, which is pulverized 

 and sold extensively as a polishing powder, is made up almost exclu- 

 sively of these fossil remains, of which 40,000,000,000 of individual 

 shells are contained iu a single cubic inch. 



