70 



EECEEATIYE SCIENCE. 



get over a space equal to an inch, in about 

 tkree minutes, and tlie slowest in about an 

 hour ; but such convey a very erroneous im- 

 pression, since the minute size of the objects 

 is left out of the account, and the above dis- 

 tances should be multiplied by three or four 

 hundred to obtain a true idea of the rate of 

 speed as seen with the microscope. The rea- 

 sons for, and way in which, these motions are 

 effected are involved in some obscurity. The 

 most probable supposition is that they are 

 connected with vital processes, the formation 

 of oxygen, and growth of the minute organism. 

 That there are external organs for producing 

 it has been asserted, and lately revived, but 

 the most careful and trustworthy observers 

 agree that with the best glasses they have 

 never seen any such ; and the very delicate 

 mycelioid filaments with which some species 

 especially are liable to be infested have no 

 motion, and can therefore have nothing to 

 do with it. Erustides detached from fila- 

 ments or their stalks, where they have such, 

 will occasionally move, but in a more languid 

 and irregular way than those which live free. 



To ascertain readQy the exact characters 

 of a Diatom which may appear new or doubt- 

 ful, the best way is to put a little on talc, 

 and expose to a gentle red-heat ; the pecu- 

 liarities of shape, the angles, the markings, 

 then come out with great sharpness, and may 

 be studied with ease and satisfaction. The 

 simple way of drying on talc presents advan- 

 tages for preservation in an herbarium, for 

 interchange with brother (or sister) collectors 

 through the post, and so on. 



To mount for the cabinet, however, most 

 will prefer them put up with Canada balsam, 

 on glass slides. Filamentous forms we have 

 seen well and readily prepared by careful heat- 

 ing on a slide, and then covered up. There is 

 so little satisfaction in mounting in fluid, and 

 30 much risk of loss, that we do not recom- 

 mend this mode of preservation. The usual 

 way of obtaining Diatoms in a fit state for 

 mounting, is to put some of the material into 



a little porcelain or platina cup, to pour 

 nitric acid over it, and then to boil for a short 

 time ; what remains should then be carefully 

 washed with pure water several times in suc- 

 cession, in a precipitating glass, allowing the 

 fine white powdery sediment, composed of 

 Diatoms, to fall between each time of washing. 

 Yery ingenious ways of obtaining those of 

 different sizes, as large, middling, and small, 

 have been adopted, on the principle of allow- 

 ing different periods to elapse for the 

 sediment to faU. The advantage of these 

 will be best appreciated in working with 

 fossil, or semi- fossil gatherings. It has been 

 asserted that the acid begins to act upon the 

 Diatoms injuriously after more than a minute 

 or so of boiling, but we have met with only 

 one or two instances, and those somewhat 

 doubtful, where such an effect appeared to 

 have been produced, and, on the other hand, 

 have known them boiled by the half-hour 

 together without any harm resulting. Caus- 

 tic potash and soda, however, will very 

 rapidly destroy them, and form with them 

 soluble silicates. It would be interesting to 

 ascertain whether ammonia wiU do the same, 

 and under what circumstances : that it will 

 in certain conditions, appears extremely pro- 

 bable, or what becomes of the beautiful theory 

 by which the formation of flints out of 

 sponges is accounted for ? 



When the acid has been thoroughly 

 washed away, the Diatomacean sediment may 

 be kept for any length of time in small labelled 

 bottles, with a little pure water; and to mount 

 them, nothing more is required than to shake 

 the bottle, take up a small portion of the 

 milky fluid, put it on a slide, and when quite 

 dry, add Canada balsam, and a thin covering- 

 glass. 



It is sometimes advisable, especially when 

 working with new and rich gatherings, to 

 look carefully over a slide before the balsam 

 is hardened ; different views of a rare form, 

 such as could hardly otherwise, may thus be 

 obtained, on moving it by gentle pressure on 



