DIATOMS PROTOCOCCUS. 259 



little chlorate of potash has been added ; and it is still advisable 

 after this to lay the material for two days in ammonia, and after- 

 wards transfer to nitric acid. 



A very simple method of preparation, by which the cell- 

 contents of the diatoms are dissolved, and good valves as a rule 

 obtained, is treatment by potassium permanganate. Crystals of 

 this salt are added to the fresh diatomaceous material, in the 

 proportions of about 1 part of the salt to 10 parts water, and 

 the mixture exposed in an evaporating dish of about 100 c.c. 

 capacity either on the hot plate of an oven or in the sun, and from 

 time to time stirred. The vessel is then half-filled with water 

 and about 50 eg. calcined magnesia is stirred in. After from 

 two to three hours about 1 cc. hydrochloric acid is poured 

 in each ten minutes, the operation being complete when the 

 contents of the vessel are decolorised. 



The richest diatom- gatherings can be made in spring and 

 autumn. Rusty- looking and yellow- brown slimy coatings upon 

 earth and stones, in relatively clear standing and flowing waters, 

 in mill-sluices, and on mill-wheels, indicate diatoms. They cling 

 to water-plants, and a compact turf of moss upon damp rocks will 

 conceal them in great numbers. 



On account of the often exceedingly delicate and regular 

 structural relations of their cell-walls, diatoms are much used as 

 test objects, in guaging the quality of the more powerful micro- 

 scopic objectives. Especially used are the valves of Pleurosigma 

 angnlatum, which, with sufficiently strong magnification, show 

 regularly-arranged hexagons. 



Protococcus viridis. In order to become acquainted with one 

 of the simplest possible forms of the unicellular green algae we 

 will examine a Protococcus. To this group belong in the main 

 all the green incrustations which are found on the stems of trees, 

 damp boards (e.g., wood palings, etc.), walls, and other similar 

 places ; but we must note that it is quite uncertain whether our 

 Protococcus is an independent species, or is to be considered as 

 a stage in the development of another alga. The form (Fig. 98) 

 which we take from an old tree trunk comes under the name 

 Protococcus viridis (Pleurococcus vulgar is). We examine it with 

 a strong magnifying power, and find it composed of globular cells, 

 isolated or united into small groups (Fig. 98, A to F). The 

 contents of the cells are bright green, but the whole protoplasm 

 is not uniformly coloured, for, as sufficiently strong magnification 



