78 PREPARATION AND MOUNTING 



shells and zoophytes of a large size, and will well repay the trou- 

 ble of searching for. Many of the Sertularidae are very beautiful 

 with polarized light, and, indeed, no ramble upon the seaside need 

 be fruitless in this direction. 



The different starches are quite a study in themselves, and are 

 peculiarly connected with polarized light. They are found in the 

 cellular tissue of almost every plant in small white grains which 

 vary considerably in size ; that from the potato averages one-three- 

 hundredth of an inch in diameter, and that from arrow-root about 

 one-six-hundredth. To procure starch from any plant, the tex- 

 ture must first be broken up or ground coarsely ; the mass of 

 matter must be then well washed in gently-flowing water, and, as 

 all starch is totally insoluble in cold water, the grains are carried 

 off by the current and deposited where this is stayed. In procur- 

 ing it from the potato, as well as many other vegetables, it is but 

 necessary to reduce the substance to a coarse pulp by the aid of 

 a culinary "grater;" the pulp should then be well agitated in 

 water, and allowed to rest a short time, when the starch will be 

 found at the bottom, its lighter colour rendering it easily distin- 

 guishable from the pulp. It should, however, be washed through 

 two or three waters to render it perfectly clean. 



These grains have no crystalline structure, but present a very 

 peculiar appearance when examined with polarized light. Each 

 grain shows a dark cross whose lines meet at the point where it 

 was attached to the plant, called the hilum. Round the grain, 

 also, a series of lines are seen, as though it were put together in 

 plates. This is more distinctly visible in some kinds than others. 



As to the mounting of these starches there is little to be said. 

 If the grains are laid upon the slide, and as small a portion as pos- 

 sible of the balsam diluted with turpentine, as before mentioned, 

 be applied, they will cling to the glass and allow the pure balsam 

 to flow readily over them without being so liable to imprison air- 

 bubbles when the thin glass is put upon them. 



The raphides, which were fully described in Chapter II., when 

 required for use with polarized light, must be mounted in balsam, 

 and many are found which give beautiful colours. They require 

 no peculiar treatment, but must be washed quite clean before put- 

 ting up. 



