96 ]\rULTUM E PARVO. 



Let us look now at another class of labourers by wLora 

 miglity deeds are performed, though the performers them- 

 selves are so inconceivably minute, that to say they bear 

 the same relation to the coral polype that a mouse does to 

 an elephant, would be greatly to overrate their dimensions. 

 They are, in fact, invisible to the sharpest sight, except 

 when a2f2freixated toojether. I refer to the Diatomacece. 



Of late years the attention of microscopic observers 

 has been largely and increasingly occupied by a tribe of 

 organic beings which are found to exist in all parts of 

 the world, in fresh and salt waters chiefly, and present a 

 great variety of species as well as of form and markings. 

 They consist of a glassy shell, formed of flint, inclosing a 

 soft coloured substance, generally of a golden yellow or 

 brown hue. This is called the endoc/irome, and the shell 

 is called the frustide. The latter has a determinate form, 

 which often assumes extraordinary elegance, and is usually 

 marked with series of specks, which are either knobs or 

 pits, arranged in the most varied and exquisite j)atterns. 

 They may exist either as isolated forms, or, more com- 

 monly, as united into long chains, or other connected 

 figures. These are called Diatoms. They have spontan- 

 eous movements, and hence they were considered, when 

 first discovered, to be animals ; but the 02)inion now 

 generally prevails, that they are plants of a very low 

 grade. 



The influence of these tiny atoms upon this world in 

 which we live is almost beyond belief. "The whole 

 bottom of the ocean," observes Dr Barclay Montgomery, 

 " seems to be in a great measure made up of these bodies. 



